Conductor Sizing

Seven-Delta-FortyOne

Seven-Delta-FortyOne

Goin’ Down In Flames........
Location
Humboldt
Occupation
EC and GC
I run into this quite often, where someone, often more knowledgeable than myself, will tell me that they always use a certain conductor size for a specific amperage, and the way I read it, they are one size too small.

Rep from PG&E I was talking to today about a couple upcoming projects I have, one of them being a multi unit meter stack, with 4 individual 100A breakers and a 400A rated buss. I have it sized for 600 mcm for the service conductors, PG&E guy told me “everyone always uses 500’s for those 400A.”

It’s happened a few more times recently, like guys using 2/0 for 200A commercial feeders or services. Commercial can’t take advantage of 310.12

I believe that if the load Calc comes out to some amount that would allow the smaller conductor, then the next size up standard size breaker could be used, however, that doesn’t really work out for 2/0 as 175A Breakers are common.

Is there something I’m missing here?
 
See it a lot. Common thinking (i think) is, using the resi rules when it doesn't apply, using next size up ( sometimes ok of course), calling 500 cu "close enough for 400", using. Next size up over 800A, using 90 degree column.

Remember where u have multiple service disconnects, the conductors can be sized to the load, which probably explains your first scenario.
 
you must understand, that the feeder/service size IS NOT the size of the main breaker. the feeder/service size is the CALCULATED size, that it must be based upon load. once that calculated size (which many electricians don't bother calculating) is figured, conductor sizes, and then OCPDs are selected. but i will say again, the service size IS NOT the size of the MCB.

for example, if you calculate that your system needs a 380a capacity, you will use table 310.16 and recognize that 500mcm cu is good for 380a as long as you have 75°C terminations. the overcurrent device protecting these conductors is allowed to be rounded up under 240.4(B). so we install a 400a MCB. this DOES NOT mean, this is a 400a service. it is a 380a service.

does this help?
 
Master Nater said:
you must understand, that the feeder/service size IS NOT the size of the main breaker. the feeder/service size is the CALCULATED size, that it must be based upon load. once that calculated size (which many electricians don't bother calculating) is figured, conductor sizes, and then OCPDs are selected. but i will say again, the service size IS NOT the size of the MCB.

for example, if you calculate that your system needs a 380a capacity, you will use table 310.16 and recognize that 500mcm cu is good for 380a as long as you have 75°C terminations. the overcurrent device protecting these conductors is allowed to be rounded up under 240.4(B). so we install a 400a MCB. this DOES NOT mean, this is a 400a service. it is a 380a service.

does this help?
Click to expand...

Oh yeah I understand that. I happen to know though, that on a lot of these no one is doing a load Calc.

Perhaps they are just going off a lot of previous similar situations.
 
Master Nater said:
for example, if you calculate that your system needs a 380a capacity, you will use table 310.16 and recognize that 500mcm cu is good for 380a as long as you have 75°C terminations. the overcurrent device protecting these conductors is allowed to be rounded up under 240.4(B). so we install a 400a MCB. this DOES NOT mean, this is a 400a service. it is a 380a service.
Click to expand...
So if the load calc calls for a 200 amp service for a dwelling and the conductors are sized according to Table 310.12 does that mean that if someone uses #2/0 cu for 200 amps it's a only a 175 amp service?
 
Dennis Alwon said:
Is PG&E a power company? If so they can do what they want. They are not under the nec rules
Click to expand...

This is for the se conductors in the riser, which we install. PG&E (power and gas) connects their service drop to the se conductors outside the weatherhead, which we supply and install, but then the service point is at the meter.

So while the riser is considered on the utility side of the service point, we supply and install it and have to follow NEC on it.
 
Seven-Delta-FortyOne said:
Oh yeah I understand that. I happen to know though, that on a lot of these no one is doing a load Calc.

Perhaps they are just going off a lot of previous similar situations.
Click to expand...
If they are not doing a load calc's they are wasting money on wire. If the individual units are under 100 amps there is no way the service load is anywhere near 400 amps. Probably 200 amps or less.
 
So thinking about this some more.

Here in actual one I have going on. 4 apartments, one is empty, one has been converted to a storage room, not sure what future plans are. I was just called in because the existing meter stack caught on fire.

I’m installing a 4 meter stack with a 400A bussing, no main disconnect, and 4-100A breakers.

My opinion is I need to run 600’s for the SE conductors, as that is the buss rating.
 
Seven-Delta-FortyOne said:
I’m installing a 4 meter stack with a 400A bussing, no main disconnect, and 4-100A breakers.

My opinion is I need to run 600’s for the SE conductors, as that is the buss rating.
Click to expand...
The conductors need only be as large as the load calculation for combined total of the 4 service disconnects.
 
Seven-Delta-FortyOne said:
I’m installing a 4 meter stack with a 400A bussing, no main disconnect, and 4-100A breakers.
Click to expand...
Not to take this OT. But what are your using for a meter panel? As far as I know Siemens is the only manufacture making compliant versions.
PG&E has been allowing some jobs to include a service disconnect ahead of the meter panel for 6 or less meters. If you install a single main breaker ahead of the meters the conductors would need to match the breaker rating.
 
infinity said:
So if the load calc calls for a 200 amp service for a dwelling and the conductors are sized according to Table 310.12 does that mean that if someone uses #2/0 cu for 200 amps it's a only a 175 amp service?
Click to expand...
i have NEVER seen a load calculation come to exactly 200a. i have, however seen folks just randomly decide to install a 200a service (conductors and MCB), without doing any load calculations, in which case any NEC-based argument is generally thrown out the window. i have also seen folks do their load calculations and come up with a 175a(ish) load, and they simply decided to upsize the entire service to a 200a, because it's a standard size from the utility company.

i suppose the simple answer to your question of whether 2/0 cu on a 200a MCB, is that it would depend upon if it were a dwelling and the conductors qualified for the 310.12 dwelling derating.

perhaps i should modify my statement to say that the rating of the service is the rating of the conductors, not the MCB?

my point is simple. there are always instances, especially in larger/non-dwelling services/feeders where folks just randomly call a service this or that, without any justification, whatsoever. the OP and his 500mcm being a 400a service, is one of these. 500mcm ran to a MCB of 400a does NOT make it a 400a service. it is a 380a service, on a 400a MCB. this becomes very clear, when the feeder/service size goes above 800a, and rounding down is no longer permitted.
 
Seven-Delta-FortyOne said:
So thinking about this some more.

Here in actual one I have going on. 4 apartments, one is empty, one has been converted to a storage room, not sure what future plans are. I was just called in because the existing meter stack caught on fire.

I’m installing a 4 meter stack with a 400A bussing, no main disconnect, and 4-100A breakers.

My opinion is I need to run 600’s for the SE conductors, as that is the buss rating.
Click to expand...
600mcm aluminum, correct? that would be the minimum size required for 400a service based upon the dwelling ratings of table 310.12. much cheaper than 400mcm copper.

depending on the mood i'm in, i either giggle or get very red in the face, when the NEC requires us to run these large conductors, and then watch the utility only hook 4/0 or 300mcm up to them.
 
curt swartz said:
Not to take this OT. But what are your using for a meter panel? As far as I know Siemens is the only manufacture making compliant versions.
PG&E has been allowing some jobs to include a service disconnect ahead of the meter panel for 6 or less meters. If you install a single main breaker ahead of the meters the conductors would need to match the breaker rating.
Click to expand...

Eaton 1MP4124

It requires a barrier kit from Eaton to comply with 230.71

Originally PG&E wanted a disco ahead of the meter stack, but when I showed them that one they approved it.
 
Master Nater said:
600mcm aluminum, correct? that would be the minimum size required for 400a service based upon the dwelling ratings of table 310.12. much cheaper than 400mcm copper.

depending on the mood i'm in, i either giggle or get very red in the face, when the NEC requires us to run these large conductors, and then watch the utility only hook 4/0 or 300mcm up to them.
Click to expand...

600cu.

310.12 won’t apply to this, as per my understanding.
 
Seven-Delta-FortyOne said:
600cu.

310.12 won’t apply to this, as per my understanding.
Click to expand...
why woudn't it apply?

310.12(A) Services - for a service rated 100a through 400a, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family OR MULTIFAMILY DWELLING, shall be permitted to have an ampacity not less than 83 perceent of the service rating. If not adjustment or correction factors are reuquired, Table 310.12(A) shall be permitted to be applied.

who said it wouldn't apply?
 
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