I'm still working my way through the latest IEE regs (it's many years since
I last had to do it) but I haven't yet seen anything about loading design.
If the CU has a 100A isolator must I ensure that the sum of the MCB trip
levels is less than 100A or can I use a statistical method (root of some of
squares) to combine the max loads on the basis that they won't all be
maximum at the same time?
TIA

Dave

In article ,
"Dave" writes:
I'm still working my way through the latest IEE regs (it's many years since
I last had to do it) but I haven't yet seen anything about loading design.
If the CU has a 100A isolator must I ensure that the sum of the MCB trip
levels is less than 100A or can I use a statistical method (root of some of
squares) to combine the max loads on the basis that they won't all be
maximum at the same time?

They can add up to more than 100A, and there's a formula.
This is covered in the On-Site Guide, Appendix 1, Table 1B.
Note the figures are a guideline only based on typical usage. If you
have some situation where you know the figures are unsuitable, you
must adjust for that situation.

--
Andrew Gabriel

"Andrew Gabriel" wrote in message
...
In article ,
"Dave" writes:
I'm still working my way through the latest IEE regs (it's many years
since
I last had to do it) but I haven't yet seen anything about loading
design.
If the CU has a 100A isolator must I ensure that the sum of the MCB trip
levels is less than 100A or can I use a statistical method (root of some
of
squares) to combine the max loads on the basis that they won't all be
maximum at the same time?

They can add up to more than 100A, and there's a formula.
This is covered in the On-Site Guide, Appendix 1, Table 1B.
Note the figures are a guideline only based on typical usage. If you
have some situation where you know the figures are unsuitable, you
must adjust for that situation.

--
Andrew Gabriel

Thanks Andy, I've borrowed the guide now and all makes sense - I'll read it
over the weekend.

On a slightly different tack ...
The new CU will be mounted on the house-side of a cavity wall, the other
side is the garage. I propose to run the cables down the garage side of the
wall and through the wall (in a piece of plastic pipe) straight into the
back of the CU - 'any problems with this? I assume the tails and the load
cables should be in separate "ducts"(?).
I'm still not sure whether to run the tails to the gge CU in parallel with
the house CU or to run them from a MCB in the house CU - any suggestions?

Dave

On Fri, 1 Oct 2004 16:55:40 +0100, Dave wrote:

The new CU will be mounted on the house-side of a cavity wall, the
other side is the garage. I propose to run the cables down the
garage side of the wall and through the wall (in a piece of plastic
pipe) straight into the back of the CU - 'any problems with this?

With due regard to cable grouping, ie don't squeeze 'em close together
through that pipe leave plenty of space and don't bundle them up all
neatly together. I'd probably keep the pairs for rings together but
otherwsie distribute them through out the pipe.

I assume the tails and the load cables should be in separate
"ducts"(?).

Seems sensible.

I'm still not sure whether to run the tails to the gge CU in
parallel with the house CU or to run them from a MCB in the house CU

What load is the garage CU likely to carry? Any big machine tools or
heating? Just a light and a couple of 13A sockets for power hand tools
I'd buy a bigger house CU and have it all in there. RCBO for the
garage sockets or individual RCD sockets.

--
Cheers
Dave. pam is missing e-mail

Dave brought next idea :
or can I use a statistical method (root of some of
squares) to combine the max loads on the basis that they won't all be
maximum at the same time?
TIA

The word you are looking for is 'diversity'. Not all loads will be on
at the same time, nor will all the loads be taking their maximum
current.

--


--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org

"Dave Liquorice" wrote in message
ll.com...
On Fri, 1 Oct 2004 16:55:40 +0100, Dave wrote:

The new CU will be mounted on the house-side of a cavity wall, the
other side is the garage. I propose to run the cables down the
garage side of the wall and through the wall (in a piece of plastic
pipe) straight into the back of the CU - 'any problems with this?

With due regard to cable grouping, ie don't squeeze 'em close together
through that pipe leave plenty of space and don't bundle them up all
neatly together. I'd probably keep the pairs for rings together but
otherwsie distribute them through out the pipe.

I assume the tails and the load cables should be in separate
"ducts"(?).

Seems sensible.

I'm still not sure whether to run the tails to the gge CU in
parallel with the house CU or to run them from a MCB in the house CU

What load is the garage CU likely to carry? Any big machine tools or
heating? Just a light and a couple of 13A sockets for power hand tools
I'd buy a bigger house CU and have it all in there. RCBO for the
garage sockets or individual RCD sockets.

--
Cheers
Dave. pam is missing e-mail

The gge will have:
- a 3kW rotary 3-phase converter (probably needs a C or D rated 32A MCB for
starting),
- a 2kW 3-phase inverter (16A MCB),
- a regular ring (32A RCD),
- a couple of non-RCD sockets (16A MCB)
- and lights (6A MCB).
I'm keen to have the gge MCBs in the gge so I need a separate CU, I guess
the load is sufficiently large that it's worth having the 2 CUs in parallel
(?)

Dave

On Fri, 1 Oct 2004 20:56:59 +0100, Dave wrote:

I'm keen to have the gge MCBs in the gge so I need a separate CU, I
guess the load is sufficiently large that it's worth having the 2
CUs in parallel (?)

7kW or more, yes feed 'em in parallel with the henley blocks close to
the meter.

--
Cheers
Dave. pam is missing e-mail

The gge will have:
- a 3kW rotary 3-phase converter (probably needs a C or D rated 32A MCB for
starting),
- a 2kW 3-phase inverter (16A MCB),
- a regular ring (32A RCD),
- a couple of non-RCD sockets (16A MCB)
- and lights (6A MCB).
I'm keen to have the gge MCBs in the gge so I need a separate CU, I guess
the load is sufficiently large that it's worth having the 2 CUs in parallel
(?)

Ahh - this is a "serious" garage installation, then ;-) Reasonable to
reckon you could be pulling 7-8kW, being some 32A or more. For this
amount of "juice" you want to think of the feed to the garage as a
"submain": I'd want a 32A or 40A MCB in the CU, with the supply to the
garage in an Appropriately Rated cable; design calculations might allow
6mmsq, depending on length and circumstances of run, but you want to
keep voltage drop low on this submain and consider splashing out on
10mmsq. Depending on the physical run, it's just possible that PVC T&E
would do (if the gge is attached to the house), but SWA is prolly more
what you're looking at. The MCB in the main CU is only protecting the
cable to the gge CU; from there, protection is provided for each
individual circuit by the MCBs in each CU. You need to do the calcs on
earth loop impedance quite carefully to size this cable, especially if
you're going to use a Type C MCB in the main CU - since it takes longer
to trip, the permissible cable run is lower (to make sure the earth loop
impedance is low enough so it trips in sufficient time to protect the
cable thermally and limit the voltage while an L-to-E fault persists).
Basics of the calcs are well explained in the "standard" electrician's
bibles; but this is territory beyond the Which? Guide To Electrics ;-)

HTH - Stefek

"Stefek Zaba" wrote in message
...


The gge will have:
- a 3kW rotary 3-phase converter (probably needs a C or D rated 32A MCB
for
starting),
- a 2kW 3-phase inverter (16A MCB),
- a regular ring (32A RCD),
- a couple of non-RCD sockets (16A MCB)
- and lights (6A MCB).
I'm keen to have the gge MCBs in the gge so I need a separate CU, I
guess
the load is sufficiently large that it's worth having the 2 CUs in
parallel
(?)

Ahh - this is a "serious" garage installation, then ;-) Reasonable to
reckon you could be pulling 7-8kW, being some 32A or more. For this
amount of "juice" you want to think of the feed to the garage as a
"submain": I'd want a 32A or 40A MCB in the CU, with the supply to the
garage in an Appropriately Rated cable; design calculations might allow
6mmsq, depending on length and circumstances of run, but you want to
keep voltage drop low on this submain and consider splashing out on
10mmsq. Depending on the physical run, it's just possible that PVC T&E
would do (if the gge is attached to the house), but SWA is prolly more
what you're looking at. The MCB in the main CU is only protecting the
cable to the gge CU; from there, protection is provided for each
individual circuit by the MCBs in each CU. You need to do the calcs on
earth loop impedance quite carefully to size this cable, especially if
you're going to use a Type C MCB in the main CU - since it takes longer
to trip, the permissible cable run is lower (to make sure the earth loop
impedance is low enough so it trips in sufficient time to protect the
cable thermally and limit the voltage while an L-to-E fault persists).
Basics of the calcs are well explained in the "standard" electrician's
bibles; but this is territory beyond the Which? Guide To Electrics ;-)

HTH - Stefek

Thanks to all. I've had a slight rethink and will probably go for one CU in
the garage (part of house) rather than 2 separate CUs. The only range I can
find that will give me the mix I need (10 unprotected and 5 protected) is
Proteus from CEF - any views on this range or suggestions for others?

Dave

Thanks to all. I've had a slight rethink and will probably go for one CU in
the garage (part of house) rather than 2 separate CUs. The only range I can
find that will give me the mix I need (10 unprotected and 5 protected) is
Proteus from CEF - any views on this range or suggestions for others?

If you're in the garage, you probably aren't short of room on the wall.
In which case, you'll get most flexibility by using two CUs - say a
12way or 15way with 100A incomer switch for your 10 unprotected final
circuits, and a 6way with an 80A RCD incomer for the 5 circs you want on
an RCD.

As they're close together, it's fine to run them "in parallel" by using
a Henley Block (overgrown junction box) to take the meter tails in and
feed out tails to both CUs; you only need the "submain" thing with its
own short-circuit and overcurrent protection if the second CU is "some
distance" from the main incoming supply fuse, where "some distance" is
a quite short piece of string, around 3m as a typical guideline - actual
limits depend both on Real Engineering Calculations about how long it'll
take the fuse to blow, how high a current will flow until it does, and
thus how hot the cable will get while the fuse is deciding to blow.

Not that the Proteus is a bad brand, or anything, just that you won't
be tied to one particular make and have it filled up (or nearly so)
right from the start.

Hope that's still helpful, rather than being a massive goalpost tour
ending up where we first started!

Stefek