On Friday, February 5, 2016 at 9:25:40 AM UTC-5, Texas Kingsnake wrote:
I have to confess, although I've read a lot and watched a number of videos,
I'm still not very "up" on - not even sure what to call it - multiphase
circuits. As in why doesn't the neutral from the pole have to be larger
than either hot feed because it carries the return curent from both hots?
Not necessary that I do understand since I am just a helper, but it would be
nice to know.
The neutral doesn't carry all the current. It only carries the unbalanced
portion. If you have 30A flowing on one hot, 20A on the other, the
service neutral has 10A. If instead both legs have 20A, then the current
in the neutral is zero.
Draw a simple circuit with two 120V batteries in series that drive
two 3 ohm resistors in series. Connect the midpoint of the batteries with
the midpoint of the resistors. That's essentially what you have with
a balanced load. 20A is flowing, but nothing in the wire between
the midpoint of the batteries and the midpoint of the resistors. Now
change the resistors so one is 4 ohms, the other 2 ohms. Now the load
is unbalanced and you have current flowing in the midpoint (neutral)
connection.
This may sound like a stupid question but why are the breakers
staggered? I
thought the A hot coming in on the left fed all the breakers on the left
and
the B hot of the right fed all the breakers on the right. Apparently
not.
Why?
So that you can put in a 240V breaker, which needs to connect to both
hots.
Is that why the 240 breakers appear to be double-size - they're contacting
two different places on the internal power buss?
Bingo.
AFCI for all the breakers that are on the old, suspect run.
I did spec 4 AFCI's but for the bedrooms and that wire's ironically in the
best of shape. I would have done the whole panel with AFCI's until my buddy
told me what that would cost. Didn't seem worth it but I'll discuss it
again. If it's that much of a safety advantage it might be worth it. I
might want to try to find out how many arc fault fires there are in the US
each year to gage the odds.
I don't know what current code is on AFCI requirements, but
AFAIK,
it's beyond just bedrooms now. I think most living areas require it,
but Gfre here can give you the correct answer. Anyplace with old,
suspect wiring, I'd put them on all the circuits that have that old
wiring.
Would this be a good time to check the house's grounding system?
Yes.
How do you
measure the quality of the house's ground?
IDK, good question. In most cases, I think they just go with what
works. For example, if you have a metal cold water line going to
a well or city water, that works for me. Current code requires an
additional electrode, typically a ground rod.
I've read about grounds called Ufers (one site claims that a direct
lightning hit can destroy your foundation - I am assuming that's a Ufer
ground built into a new home's foundation. It sounds like I would bury a
Ufer close to the building. Still researching that.
I ufer is a concrete encased electrode that uses the cement pour of
the building during new construction. You need a simple ground rod.
Ironically the replacement is because the 100A breaker in the old panel was
heat damaged. If you recall, the hot on one side had lost its Noalox and
began arcing and the heat (measured at over 200F) damaged the breaker. My
electrician buddy pulled the feed wire, cleaned it (no apparent heat
discoloration), gooped it up with Noalox and then tightened it back down
when the set screw stripped.
Thanks in advance for any advice . . .
TKS
Is the existing feed adequate? How many amps? Any new loads, eg AC,
in your future? Now is the time to make sure you have enough capacity.
I've been measuring the loads with a clamp meter. Maximum load hovers at 50
to 60 amps. I assume that 200A service (what he says I have) is good enough
for that kind of load. That was with everything I could think of turned on
and two space heaters running at 15A each.
So, new panel is 200A then?
Thanks for all your help. Are you an electrician? You sound like one!
This sort of feedback will allow me ask smarter questions of my electrician
buddy.
TKS
I'm an electrical engineer.