# Service sizing?



## Cletis (Aug 20, 2010)

Joshua86 said:


> So this is not something that I do at all.Normally the power company handles it. I hate to even ask but part of my brain is arguing with the other part of my brain and then my business partner is agreeing with both parts and lord knows the code book doesn't spell anything out for you. The code book has a lot of factors that allow you to downsize wire and so on. Just want to make sure I am correct. I have a 24 unit apartment building with 2 1200 amp 240 vt disconnects feeding the 125 amp services. To run the secondary to the 1200 amp disconnect to the transformer I need to run large enough wire to pull the full 1200 amps correct? The demand factors and wire downsizing were applied while coming up with the disconnect size and no further downsizing can occur?
> 
> So 6 pulls of 500mcm and 3 pulls of 350 mcm for the neutral. Or am I completely wrong?
> 
> Also the longest pull is about 35 foot. Thanks.


backwards. think your completely wrong


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## RePhase277 (Feb 5, 2008)

The code books tells you everything you need to know about this. The conductors only need be large enough to serve the calculated load. Why isn't the POCO providing the lateral? Is it 3-phase?


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## McClary’s Electrical (Feb 21, 2009)

I didn't crunch the numbers because I'm driving but you are allowed to size those conductors by the calculated load, not just by the 1200 amps.


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## Dennis Alwon (May 9, 2009)

I agree that the wire can be sized for the load but I would size it to the 1200 amps. The neutral may be downsized if a load calculation is done to justify it.

3 runs of 500 mcm copper is only going to give you 1140 amps. If the calculated load is less than that then you are okay.


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## Joshua86 (Nov 29, 2012)

mcclary's electrical said:


> I didn't crunch the numbers because I'm driving but you are allowed to size those conductors by the calculated load, not just by the 1200 amps.



Thanks not looking for a handout here on the number crunching as it wasn't long ago I could do this with no issues. Thanks for all the replies.


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## Joshua86 (Nov 29, 2012)

I actually have 4 services total. 

2 1000amp disconnects and 2 1200 amp disconnects. After doing the math and checking everything. The 1000amp disconnects have a total load of 846 amps
and the 1200 amp have a total calculated load of 1079amps. 

If I just have to supply for the calculated load I can use three parallel runs of 500mcm but if I have to supply the full 1200amps I will need to run three parallel runs of 600mcm. Still trying to find in the code book where it states which one is correct.


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## McClary’s Electrical (Feb 21, 2009)

Joshua86 said:


> I actually have 4 services total.
> 
> 2 1000amp disconnects and 2 1200 amp disconnects. After doing the math and checking everything. The 1000amp disconnects have a total load of 846 amps
> and the 1200 amp have a total calculated load of 1079amps.
> ...


 
Hereya go dude.

*230.31 Size and Rating.
(A) General.​*​​​​Service-lateral conductors shall have sufficient
ampacity to carry the current for the load as calculated
in accordance with Article 220 and shall have adequate​
mechanical strength.


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## Joshua86 (Nov 29, 2012)

mcclary's electrical said:


> Hereya go dude.
> 
> *230.31 Size and Rating.
> (A) General.​*​​​​Service-lateral conductors shall have sufficient
> ...


Thanks!

and thank's for all the replies. I honestly didn't expect this place to be so active. Will visit more often. I could sure stand to learn a few more things.


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## 123electric (Jun 3, 2012)

75c column. 500 McM is only good for 380 amps per set. We usually do 1200 amp services with (4) sets of 350 as its easier to work with. Utility company's here don't like derated neutrals for commercial, always full size.


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## CopperSlave (Feb 9, 2012)

Dennis Alwon said:


> I agree that the wire can be sized for the load but I would size it to the 1200 amps. The neutral may be downsized if a load calculation is done to justify it.
> 
> 3 runs of 500 mcm copper is only going to give you 1140 amps. If the calculated load is less than that then you are okay.


If the service is single-phase, wouldn't 310.15(B)(7) allow for the use of 400MCM Cu in 3 sets?


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## 123electric (Jun 3, 2012)

Table 310-15 (B)(6) only goes up to 400 amps period with no modification allowance for larger services. If the total calculated load is 1200 amps and you can't do it with (3) sets of 500 Mcm cu 380x3=1140 amps. You feel you can create a sub code and multiple the allowable residential allowance 400 Mcm (400 as per table), (3) sets to give you 1200 amps? As long as your AJH isn't that sharp and you feel you can get away with it. Would be rather difficult to add another set down the road!


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## 123electric (Jun 3, 2012)

I don't know of to many single phase 1200 amp services out there in my neck of the woods. Usually 1200 amp 3-phase is always open bottom switch gear.


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## McClary’s Electrical (Feb 21, 2009)

123electric said:


> Table 310-15 (B)(6) only goes up to 400 amps period with no modification allowance for larger services. If the total calculated load is 1200 amps and you can't do it with (3) sets of 500 Mcm cu 380x3=1140 amps. You feel you can create a sub code and multiple the allowable residential allowance 400 Mcm (400 as per table), (3) sets to give you 1200 amps? As long as your AJH isn't that sharp and you feel you can get away with it. Would be rather difficult to add another set down the road!


 
He never mentione 310.15 b 6. Everybody here knos thats for single and two family dwellings.

And second, he's not "getting away" with anything. The install is code compliant.


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## 123electric (Jun 3, 2012)

He's not getting away with anything if the total connected load is 1200 amps and he is using (3) sets of 400 McM. Please educate me?


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## 123electric (Jun 3, 2012)

Bare in mind I was commenting on "copper slaves" post


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## 123electric (Jun 3, 2012)

Maybe we were referring to different posts? 13k posts that you have, I'm going with you thought I was referring to the original post. I apologize if I threw you off, my bad.


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## McClary’s Electrical (Feb 21, 2009)

123electric said:


> Maybe we were referring to different posts? 13k posts that you have, I'm going with you thought I was referring to the original post. I apologize if I threw you off, my bad.


 
just read the whole thread. The calculated load is not 1200 amps.


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## 123electric (Jun 3, 2012)

Your the man, I have read quite a few of your posts! You too know stuff. I wouldn't even consider putting a fellow electrical contractor down here. I wasted allot of time getting my 4 licenses, most of which we never use as you know but needed it for license preparation.


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## 123electric (Jun 3, 2012)

Lol, wow just broke 100 posts. Nothing compare to you! Holly crap 13k!


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