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  #1   Report Post  
Chief McGee
 
Posts: n/a
Default single phase from 3-ph

This was mentioned a few days ago but would someone please clarify it for
me.
I am installing 3-ph to the shop. It is 200A delta wound, 2
transformers on pole, 3 hot wires 1 neutral from pole, ground rod driven
into the dirt and hooked to the neutral buss bar. Drawing with the meter
box shows the neutral wire coming from the middle of one side of the
triangle that forms the delta. Is this what is called "center tapped"?
I think I want to run 5 wires to each machine. 3 hots, 1 neutral, and
one ground from the machine base all the way back to the neutral buss bar in
the sub panel. Is this right?
I also want to have a 120 outlet and 120 work light on each machine. AT
THE MACHINE, I am going to pick one of the hot legs(NOT the power/stinger
leg) and the neutral. Will connect the ground leg in the light to the base
of the machine. Is this right or do I have to go all the way to the sub
panel with the wires?
Next, I have a 5-hp 220V single phase air compressor. For that I
think I need only 4 wires. 2 hots(NOT the power leg) 1 neutral, and one
ground from chassis all the way back to the neutral buss bar in the sub
panel. Is this correct?
I want this to be safe and legal. Thanks for your time, Chief.



  #2   Report Post  
Don Murray
 
Posts: n/a
Default single phase from 3-ph



Chief McGee wrote:

This was mentioned a few days ago but would someone please clarify it for
me.
I am installing 3-ph to the shop. It is 200A delta wound, 2
transformers on pole, 3 hot wires 1 neutral from pole, ground rod driven
into the dirt and hooked to the neutral buss bar. Drawing with the meter
box shows the neutral wire coming from the middle of one side of the
triangle that forms the delta. Is this what is called "center tapped"?
I think I want to run 5 wires to each machine. 3 hots, 1 neutral, and
one ground from the machine base all the way back to the neutral buss bar in
the sub panel. Is this right?
I also want to have a 120 outlet and 120 work light on each machine. AT
THE MACHINE, I am going to pick one of the hot legs(NOT the power/stinger
leg) and the neutral. Will connect the ground leg in the light to the base
of the machine. Is this right or do I have to go all the way to the sub
panel with the wires?
Next, I have a 5-hp 220V single phase air compressor. For that I
think I need only 4 wires. 2 hots(NOT the power leg) 1 neutral, and one
ground from chassis all the way back to the neutral buss bar in the sub
panel. Is this correct?
I want this to be safe and legal. Thanks for your time, Chief.


Chief,

What you have is an open delta. You can have a look at a wiring diagram
of the bank of transformers at

http://murrayranch.com/Electricity.htm

Take a look at the diagram that is open wye-open delta. Don't concern
yourself with the primary or open wye. But the secondary open delta is
just what you have.

I can't answer your question as to the number of wires to each machine
as I don't know the inside code, my field is as a
lineman/troubleshooter.

Hope this helps,
Don
  #3   Report Post  
Jon Elson
 
Posts: n/a
Default single phase from 3-ph



Chief McGee wrote:

This was mentioned a few days ago but would someone please clarify it for
me.
I am installing 3-ph to the shop. It is 200A delta wound, 2
transformers on pole, 3 hot wires 1 neutral from pole, ground rod driven
into the dirt and hooked to the neutral buss bar. Drawing with the meter
box shows the neutral wire coming from the middle of one side of the
triangle that forms the delta. Is this what is called "center tapped"?

That side of the triangle, with the center tap, is identical to standard
120/240
single-phase service. The two hots are 2 of your 3 phase line
terminals, as well.
The additional transformer provices the 3rd phase line terminal.

I think I want to run 5 wires to each machine. 3 hots, 1 neutral, and
one ground from the machine base all the way back to the neutral buss bar in
the sub panel. Is this right?

Yes, that sounds right, so you can have 120 at each machine location.
The safety
ground should actually have a separate ground bus bar in the panel, but some
installations don't separate the neutral and the ground.

I also want to have a 120 outlet and 120 work light on each machine. AT
THE MACHINE, I am going to pick one of the hot legs(NOT the power/stinger
leg) and the neutral. Will connect the ground leg in the light to the base
of the machine. Is this right or do I have to go all the way to the sub
panel with the wires?

Mostly. It seems OK to me to use the same ground conductor for the machine
AND the light/socket. But, you can't use ANY line terminal for the 120!
You must use one of the two that are on the center-tapped side. Going from
Neutral to the 3rd line terminal will give you roughly 208 V, as this is not
a center-grounded (balanced) Wye system.

Next, I have a 5-hp 220V single phase air compressor. For that I
think I need only 4 wires. 2 hots(NOT the power leg) 1 neutral, and one
ground from chassis all the way back to the neutral buss bar in the sub
panel. Is this correct?

Maybe. If it is entirely 220, including the contactor/motor starter,
you only
need 3 wires. Any two hots (but, maybe best to run it off the extra
transformer)
to reduce load on the center-tapped one that is going to get a lot of
single-phase loads.
Just be sure to get it across one of the transformers. IE, if the tapped
transformer
is supplying phases A and B, and the extra transformer is wired from B to C,
don't connect the compressor from A to C. This L-L circuit will have more
variation from 240 than the other 2 possible connections. If you need 120 V
there for the contactor, then use A-B, and run the neutral as well.

I want this to be safe and legal. Thanks for your time, Chief.

I believe the recent NEC has changed requirements, and grounding through the
conduit alone is no longer permitted.

Jon




  #4   Report Post  
Randy H.
 
Posts: n/a
Default single phase from 3-ph

If you run your wire thru conduit, that is your ground leg. Your breaker box
and all is grounded to the ground rod. You should only need to four wires in
the conduit.

Please note I am not an electrician. But I done a lot of my own wiring. (no
fires yet!)

Randy


"Don Murray" wrote in message
...


Chief McGee wrote:

This was mentioned a few days ago but would someone please clarify it

for
me.
I am installing 3-ph to the shop. It is 200A delta wound, 2
transformers on pole, 3 hot wires 1 neutral from pole, ground rod

driven
into the dirt and hooked to the neutral buss bar. Drawing with the

meter
box shows the neutral wire coming from the middle of one side of the
triangle that forms the delta. Is this what is called "center tapped"?
I think I want to run 5 wires to each machine. 3 hots, 1 neutral,

and
one ground from the machine base all the way back to the neutral buss

bar in
the sub panel. Is this right?
I also want to have a 120 outlet and 120 work light on each

machine. AT
THE MACHINE, I am going to pick one of the hot legs(NOT the

power/stinger
leg) and the neutral. Will connect the ground leg in the light to the

base
of the machine. Is this right or do I have to go all the way to the sub
panel with the wires?
Next, I have a 5-hp 220V single phase air compressor. For that I
think I need only 4 wires. 2 hots(NOT the power leg) 1 neutral, and one
ground from chassis all the way back to the neutral buss bar in the sub
panel. Is this correct?
I want this to be safe and legal. Thanks for your time, Chief.


Chief,

What you have is an open delta. You can have a look at a wiring diagram
of the bank of transformers at

http://murrayranch.com/Electricity.htm

Take a look at the diagram that is open wye-open delta. Don't concern
yourself with the primary or open wye. But the secondary open delta is
just what you have.

I can't answer your question as to the number of wires to each machine
as I don't know the inside code, my field is as a
lineman/troubleshooter.

Hope this helps,
Don



  #5   Report Post  
Marty Escarcega
 
Posts: n/a
Default single phase from 3-ph

"Chief McGee" wrote in
news:eQTNc.204404$Oq2.30164@attbi_s52:

This was mentioned a few days ago but would someone please clarify it
for me.
I am installing 3-ph to the shop. It is 200A delta wound, 2
transformers on pole, 3 hot wires 1 neutral from pole, ground rod
driven into the dirt and hooked to the neutral buss bar. Drawing with
the meter box shows the neutral wire coming from the middle of one
side of the triangle that forms the delta. Is this what is called
"center tapped"?


Sounds like 120/208 to me...(though I'm 15 years rusty, I work on Traffic
Signals these days)

I think I want to run 5 wires to each machine. 3 hots, 1
neutral, and
one ground from the machine base all the way back to the neutral buss
bar in the sub panel. Is this right?


Nope. Run a 3 phase circuit for your machine, 3 hots and a ground. Fed
off the proper size 3 phase breaker.

Run a seperate circuit in a seperate conduit off say a 15 amp or 20 amp
single pole circuit breaker. Yes, some machines have a built in
transformer for a couple reasons, one to reduce the higher voltage to
single phase voltage for control contactors, single phase coolant pumps
and even lighting but you will find that there is a fuse usually on
either the primary or secondary side, sometimes both to protect the lower
current (amp) circuits. Put a light and related wiring on a 30amp 3 phase
breaker, and something goes wrong, the wiring will likely fry before a
breaker trips. That's why I suggest you run a dedicated circuit just for
your machine lighting.


I also want to have a 120 outlet and 120 work light on each
machine. AT
THE MACHINE, I am going to pick one of the hot legs(NOT the
power/stinger leg) and the neutral. Will connect the ground leg in
the light to the base of the machine. Is this right or do I have to
go all the way to the sub panel with the wires?


See the above, Also, I believe Jon Elson to be correct. the NEC requires
a seperate ground conductor. You can't rely on the conduit itself for a
ground. Why? Because they have found that electricians sometimes forget
to tighten the screws or lock nuts to keep the ground continuous, conduit
pulls apart, guess what, no safety ground.

Next, I have a 5-hp 220V single phase air compressor. For that
I
think I need only 4 wires. 2 hots(NOT the power leg) 1 neutral, and
one ground from chassis all the way back to the neutral buss bar in
the sub panel. Is this correct?


Yup for reasons states above, need a seperate ground now, you should be
running flexible metallic conduit to the compressor, you need the
seperate ground in that for the compressor, you could run heavy SO cord
too. As long as it was 4 conductor.


I want this to be safe and legal. Thanks for your time, Chief.


You have doubts, you should run the conduit, and call an electrician to
finish the job. Running the conduit is a good bit of work, let him pull
and terminate the wire. My disclaimer, my advice may or may not be
accurate, consult a licensed electrician for help....gawd I hate that....

Marty



  #6   Report Post  
 
Posts: n/a
Default single phase from 3-ph

In alt.engineering.electrical Don Murray wrote:

| What you have is an open delta. You can have a look at a wiring diagram
| of the bank of transformers at
|
| http://murrayranch.com/Electricity.htm
|
| Take a look at the diagram that is open wye-open delta. Don't concern
| yourself with the primary or open wye. But the secondary open delta is
| just what you have.

Not necessarily. He could have the Scott 4 wire.

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------
  #7   Report Post  
Don Young
 
Posts: n/a
Default single phase from 3-ph

You do not need the neutral for the air compressor or three phase loads. The
neutral is needed only for 120volt lighting and convenience outlets. Ground
conductors are needed for everything and must be kept seperate from the
neutral all the way back to the main service entrance panel.
Don Young
"Marty Escarcega" wrote in message
...
"Chief McGee" wrote in
news:eQTNc.204404$Oq2.30164@attbi_s52:

This was mentioned a few days ago but would someone please clarify it
for me.
I am installing 3-ph to the shop. It is 200A delta wound, 2
transformers on pole, 3 hot wires 1 neutral from pole, ground rod
driven into the dirt and hooked to the neutral buss bar. Drawing with
the meter box shows the neutral wire coming from the middle of one
side of the triangle that forms the delta. Is this what is called
"center tapped"?


Sounds like 120/208 to me...(though I'm 15 years rusty, I work on Traffic
Signals these days)

I think I want to run 5 wires to each machine. 3 hots, 1
neutral, and
one ground from the machine base all the way back to the neutral buss
bar in the sub panel. Is this right?


Nope. Run a 3 phase circuit for your machine, 3 hots and a ground. Fed
off the proper size 3 phase breaker.

Run a seperate circuit in a seperate conduit off say a 15 amp or 20 amp
single pole circuit breaker. Yes, some machines have a built in
transformer for a couple reasons, one to reduce the higher voltage to
single phase voltage for control contactors, single phase coolant pumps
and even lighting but you will find that there is a fuse usually on
either the primary or secondary side, sometimes both to protect the lower
current (amp) circuits. Put a light and related wiring on a 30amp 3 phase
breaker, and something goes wrong, the wiring will likely fry before a
breaker trips. That's why I suggest you run a dedicated circuit just for
your machine lighting.


I also want to have a 120 outlet and 120 work light on each
machine. AT
THE MACHINE, I am going to pick one of the hot legs(NOT the
power/stinger leg) and the neutral. Will connect the ground leg in
the light to the base of the machine. Is this right or do I have to
go all the way to the sub panel with the wires?


See the above, Also, I believe Jon Elson to be correct. the NEC requires
a seperate ground conductor. You can't rely on the conduit itself for a
ground. Why? Because they have found that electricians sometimes forget
to tighten the screws or lock nuts to keep the ground continuous, conduit
pulls apart, guess what, no safety ground.

Next, I have a 5-hp 220V single phase air compressor. For that
I
think I need only 4 wires. 2 hots(NOT the power leg) 1 neutral, and
one ground from chassis all the way back to the neutral buss bar in
the sub panel. Is this correct?


Yup for reasons states above, need a seperate ground now, you should be
running flexible metallic conduit to the compressor, you need the
seperate ground in that for the compressor, you could run heavy SO cord
too. As long as it was 4 conductor.


I want this to be safe and legal. Thanks for your time, Chief.


You have doubts, you should run the conduit, and call an electrician to
finish the job. Running the conduit is a good bit of work, let him pull
and terminate the wire. My disclaimer, my advice may or may not be
accurate, consult a licensed electrician for help....gawd I hate that....

Marty



  #8   Report Post  
Bruce L. Bergman
 
Posts: n/a
Default single phase from 3-ph

On Wed, 28 Jul 2004 15:06:48 -0700, "Randy H."
wrote:

If you run your wire thru conduit, that is your ground leg. Your breaker box
and all is grounded to the ground rod. You should only need to four wires in
the conduit.

Please note I am not an electrician. But I done a lot of my own wiring. (no
fires yet!)

Randy


Don't worry, with that attitude you'll have your big fire any day
now. :-/

Using the conduit as the sole source of ground has not been legal
under NEC for many years - but it /used/ to be legal (since they
didn't really worry about branch circuit grounding at all), so many
people assume it still is. We've since learned that conduit joints
get loose or sprung (and breaks the ground circuit) and nobody sees it
or fixes it.

All power circuits that you run or re-run now should have a green
safety ground wire run along with the power wires - if you are
changing a section of wire in the middle of an old conduit run, bond
your new ground wire to the boxes at each end, using a self-drilling
sheet metal screw if the box doesn't have provisions for a grounding
screw. And leave a pigtail of spare ground wire so you can connect to
it when you rewire the circuit further.

If you take the green wire back to the main panel where the neutral
and safety ground are bonded together, you can connect ground wires to
a vacant hole on the Main neutral bar. If it is a sub-panel, you
can't - you need to bond to the can, or add a separate grounding bar.

You can use a reduced gauge ground wire as compared to the power
wires (like a #12 ground wire with #10 or #8 circuit wires) but it
still has to be large enough to trip the breaker or blow the fuse on a
ground fault without melting in the process.

-- Bruce --
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.
  #9   Report Post  
Bruce L. Bergman
 
Posts: n/a
Default single phase from 3-ph

On Wed, 28 Jul 2004 23:08:44 -0500, "Don Young"
wrote:

You do not need the neutral for the air compressor or three phase loads. The
neutral is needed only for 120volt lighting and convenience outlets. Ground
conductors are needed for everything and must be kept seperate from the
neutral all the way back to the main service entrance panel.
Don Young


True - but putting in one or two extra wires is far cheaper than
repulling it all later. Stick wires for all three phases and a
neutral in that conduit now, and you future-proof yourself.

"Work Smarter, Not Harder."

If you need to get a bigger 3-phase air compressor later - or get a
mega big one and reuse that old power line for something else, and
that something else needs 3-phase and/or a neutral, you're ready.

And if not, in 30 years when they tear down the building you take
the copper in for recycling. It'll be worth more as scrap than you
paid new by then.

-- Bruce --
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.
  #10   Report Post  
Rick
 
Posts: n/a
Default single phase from 3-ph


"Bruce L. Bergman" wrote in message
...
On Wed, 28 Jul 2004 15:06:48 -0700, "Randy H."
wrote:

If you run your wire thru conduit, that is your ground leg. Your breaker

box
and all is grounded to the ground rod. You should only need to four wires

in
the conduit.

Please note I am not an electrician. But I done a lot of my own wiring.

(no
fires yet!)

Randy


Don't worry, with that attitude you'll have your big fire any day
now. :-/

Using the conduit as the sole source of ground has not been legal
under NEC for many years - but it /used/ to be legal (since they
didn't really worry about branch circuit grounding at all), so many
people assume it still is. We've since learned that conduit joints
get loose or sprung (and breaks the ground circuit) and nobody sees it
or fixes it.

All power circuits that you run or re-run now should have a green
safety ground wire run along with the power wires - if you are
changing a section of wire in the middle of an old conduit run, bond
your new ground wire to the boxes at each end, using a self-drilling
sheet metal screw if the box doesn't have provisions for a grounding
screw. And leave a pigtail of spare ground wire so you can connect to
it when you rewire the circuit further.

If you take the green wire back to the main panel where the neutral
and safety ground are bonded together, you can connect ground wires to
a vacant hole on the Main neutral bar. If it is a sub-panel, you
can't - you need to bond to the can, or add a separate grounding bar.

You can use a reduced gauge ground wire as compared to the power
wires (like a #12 ground wire with #10 or #8 circuit wires) but it
still has to be large enough to trip the breaker or blow the fuse on a
ground fault without melting in the process.

-- Bruce --
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.


I agree with the practice of running an equipment grounding conductor in the
conduit for the same reasons you mention, but could you please cite the code
article which prohibits the use of a conduit system as the sole equipment
grounding conductor?




  #11   Report Post  
Don Murray
 
Posts: n/a
Default single phase from 3-ph



wrote:

In alt.engineering.electrical Don Murray wrote:

| What you have is an open delta. You can have a look at a wiring diagram
| of the bank of transformers at
|
|
http://murrayranch.com/Electricity.htm
|
| Take a look at the diagram that is open wye-open delta. Don't concern
| yourself with the primary or open wye. But the secondary open delta is
| just what you have.

Not necessarily. He could have the Scott 4 wire.

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------


Phil,

The only Scott connections I know about are to get 2 phase power from 3
phase. If you look at the bottom of this page

http://murrayranch.com/Electricity.htm

you can see diagrams of three, four and five wire Scott connections.

If you know of any Scott connections which provide 3 phase power, I
would like to learn about them.

Don
  #12   Report Post  
Michael Moroney
 
Posts: n/a
Default single phase from 3-ph

Don Murray writes:

| Take a look at the diagram that is open wye-open delta. Don't concern
| yourself with the primary or open wye. But the secondary open delta is
| just what you have.

Not necessarily. He could have the Scott 4 wire.


Does any utility *ever* provide Scott two-phase power, except for a few
rare cases of feeding very old stuff?

Phil,


The only Scott connections I know about are to get 2 phase power from 3
phase. If you look at the bottom of this page


http://murrayranch.com/Electricity.htm


you can see diagrams of three, four and five wire Scott connections.


If you know of any Scott connections which provide 3 phase power, I
would like to learn about them.


You could configure two transformers with both primaries and secondaries
in a Scott configuration and wind up with what amounts to 3 to 2 to 3
phase transformation. I have never heard of this being done with pole
pigs even though you only need two of them, but someone here mentioned
pad transformers are sometimes built like this internally.
--
-Mike
  #13   Report Post  
Robert Swinney
 
Posts: n/a
Default single phase from 3-ph

Don sez: "Not necessarily. He could have the Scott 4 wire."

Yep! Seems like this could be true -- Scott 2 single-phase circuits derived
from 3-phase. How about this Peter H. What say you?

Bob Swinney


"Don Murray" wrote in message
...


wrote:

In alt.engineering.electrical Don Murray wrote:

| What you have is an open delta. You can have a look at a wiring

diagram
| of the bank of transformers at
|
|
http://murrayranch.com/Electricity.htm
|
| Take a look at the diagram that is open wye-open delta. Don't concern
| yourself with the primary or open wye. But the secondary open delta is
| just what you have.


--


--------------------------------------------------------------------------

---
| Phil Howard KA9WGN | http://linuxhomepage.com/

http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/

http://ka9wgn.ham.org/ |

--------------------------------------------------------------------------

---

Phil,

The only Scott connections I know about are to get 2 phase power from 3
phase. If you look at the bottom of this page

http://murrayranch.com/Electricity.htm

you can see diagrams of three, four and five wire Scott connections.

If you know of any Scott connections which provide 3 phase power, I
would like to learn about them.

Don



  #14   Report Post  
Randy H.
 
Posts: n/a
Default single phase from 3-ph

Thanks guys for the info. I can see why they made the changes in the code as
a good idea. I will take my comment as not having any fires as being lucky
on my part.

My building was built in 1954. I'm sure I will have a lot to change
around.

Thanks again,

Randy Hansen
SC Glass Tech
Scam Diego, Comi-fornia






  #15   Report Post  
 
Posts: n/a
Default single phase from 3-ph

In alt.engineering.electrical Michael Moroney wrote:

| Does any utility *ever* provide Scott two-phase power, except for a few
| rare cases of feeding very old stuff?

I've seen them. It was a big (probably 200 kVA) can doing the 120/240
volt part and a small (probably 10 kVA) doing the 208 volt part. They
probably needed a "little bit of three phase" and a lot of 120/240.


| You could configure two transformers with both primaries and secondaries
| in a Scott configuration and wind up with what amounts to 3 to 2 to 3
| phase transformation. I have never heard of this being done with pole
| pigs even though you only need two of them, but someone here mentioned
| pad transformers are sometimes built like this internally.

I've seen a number of 2 pole pig setups. I did not check out most of them.
But in one case where I did, they were using a three phase motor for a pump.

I've seen 2 core Scott-T in dry-type as well.

http://www.marcustransformer.com/techspecs.htm
See at bottom. They say it cancels triplen harmonics magnetically.
I haven't confirmed that.

http://www.marcustransformer.com/images/27rev.pdf
Just a picture

http://www.dongan.com/products/pg26-33.pdf
See pages 27, 30, and 32 (diagrams 1 and 4)

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------


  #16   Report Post  
Bruce L. Bergman
 
Posts: n/a
Default single phase from 3-ph

On Sat, 31 Jul 2004 00:11:53 -0700, Checkmate
wrote:
On Thu, 29 Jul 2004 16:09:01 GMT, Bruce L. Bergman put forth the notion
that...
On Wed, 28 Jul 2004 15:06:48 -0700, "Randy H."
wrote:


If you run your wire thru conduit, that is your ground leg. Your breaker box
and all is grounded to the ground rod. You should only need to four wires in
the conduit.

Please note I am not an electrician. But I done a lot of my own wiring. (no
fires yet!)

Randy


Don't worry, with that attitude you'll have your big fire any day
now. :-/
Using the conduit as the sole source of ground has not been legal
under NEC for many years


Oh really? I've been licensed since 1974, and that's news to me.


Okay, that was slightly overstated. ;-) Nothing outright, but
there are a bunch of restrictions in Section 250 on using conduit for
ground that you have to consider (and handymen never do) like the
restrictions on using liquidtight and aluminum flexible conduit as the
grounding conductor (conduit length and circuit ampacities).

But the more important part is that you really can NOT trust the
conduit on old installations to be made up properly and a reliable
grounding path, unless you want to crawl the attic and open the walls
to inspect every inch of it... Fittings get loose, couplings get
sprung inside walls where they can't be seen, "handyman" installers
get creative when they don't have the right transition couplings
(using plastic parts and big wads of tape)...

When in doubt, take a green grounding conductor back to the main
with your other new conductors, and bond it to the boxes on your way -
it's cheap insurance. And they'll rarely argue if you go a bit above
and beyond the minimums that the codes call for.

Especially on very old houses that started out with an A-base meter,
30A fused disconnect and knob-and-tube through the attic, and have
been upgraded piecemeal over the years with steel rigid, steel flex,
alflex, Wiremold, XO breakers, Pushmatic breakers, Zinsco breakers...
I've had lots of fun on those Electrical Archaeology projects.

Even on newer installations within the last 20 years I've had fun
with ground faults and bad conduit grounds. Especially with cast zinc
fittings that started a bit loose and burned themselves way loose -
you wiggle the fitting and get a big spark across the gap...

(There's a /reason/ I listen to a portable AM radio at work, the
static will let you pick up on stuff like that - If you wiggle the
fitting and the radio goes staticky, start digging...)

-- Bruce --
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.
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