# Wire size for 400 Amp 3 phase service



## John M. (Oct 29, 2016)

I am building a 400 amp (class 320) service. Is it permissible to use 500 MCM XHHW aluminum (350 amps rated) since the load is not constant on a service?


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## CoolWill (Jan 5, 2019)

John M. said:


> I am building a 400 amp (class 320) service. Is it permissible to use 500 MCM XHHW aluminum (350 amps rated) since the load is not constant on a service?


As long as the calculated load is less than 350 amps, yes.


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## Dennis Alwon (May 9, 2009)

John M. said:


> I am building a 400 amp (class 320) service. Is it permissible to use 500 MCM XHHW aluminum (350 amps rated) since the load is not constant on a service?


Is this a residence?


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## Dennis Alwon (May 9, 2009)

John M. said:


> I am building a 400 amp (class 320) service. Is it permissible to use 500 MCM XHHW aluminum (350 amps rated) since the load is not constant on a service?


Is this a residence?

Never mind I see it is 3 phase- probably not resi


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## tmessner (Apr 1, 2013)

What is your over current device rating?


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## telsa (May 22, 2015)

John M. said:


> I am building a 400 amp (class 320) service. Is it permissible to use 500 MCM XHHW aluminum (350 amps rated) since the load is not constant on a service?


You don't get to use the 90 degree value of 350 Amps... so the rating is 310 Amps.

My Poco dictates wire sizes for Services. 

Down stream feeders are sized to the NEC.


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## John M. (Oct 29, 2016)

tmessner said:


> What is your over current device rating?


400 amp fuses


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## John M. (Oct 29, 2016)

Just talked to my inspector. He says I have to size it to the fuses,


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## tmessner (Apr 1, 2013)

Parallel 250 kcmil al. a lot easier to work with.


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## John M. (Oct 29, 2016)

tmessner said:


> Parallel 250 kcmil al. a lot easier to work with.


That is my plan, but this POCO makes us pull service to the pole and dont want paralleled conductors


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## Forge Boyz (Nov 7, 2014)

John M. said:


> That is my plan, but this POCO makes us pull service to the pole and dont want paralleled conductors


Like as in, you can't do it? Or they just don't like it? Sometimes they will splice a short piece of larger copper on to take parallels down to a single wire.
If it's a rule that's actually kind of dumb, because some services you aren't able to do with a single conductor.

Sent from my Moto Z (2) using Tapatalk


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## Cow (Jan 16, 2008)

Over here, I'd use 750AL XHHW and downsize the neutral to 250AL if there wasn't much neutral load.

If you have multiple service disconnects, your wire size only has to carry the load regardless of the size of the service disconnects.

But, if you only have one service disconnect, as is your case, you have to size to the rating of the OCPD or use the "round up rule" assuming your calculated load doesn't exceed the conductor rating.

Which is why I suggested 750AL, which is rated 385 amps, if I remember right, and it is allowed to feed a 400 OCPD assuming the load is under 385 amps.


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## tmessner (Apr 1, 2013)

Forge Boyz said:


> Like as in, you can't do it? Or they just don't like it? Sometimes they will splice a short piece of larger copper on to take parallels down to a single wire.
> If it's a rule that's actually kind of dumb, because some services you aren't able to do with a single conductor.
> 
> Sent from my Moto Z (2) using Tapatalk


Yeah. Watch out for your lug size and barrels.


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## Southeast Power (Jan 18, 2009)

400 amps gets 600AL unless they upgrade to 500 Copper.


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## brian john (Mar 11, 2007)

Forge Boyz said:


> Like as in, you can't do it? Or they just don't like it? Sometimes they will splice a short piece of larger copper on to take parallels down to a single wire.
> If it's a rule that's actually kind of dumb, because some services you aren't able to do with a single conductor.
> 
> Sent from my Moto Z (2) using Tapatalk


Here they would take parallel 250 kcmilsAL and splice a single 4/0 AL to the transformer


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## Cow (Jan 16, 2008)

Southeast Power said:


> 400 amps gets 600AL unless they upgrade to 500 Copper.


600AL is 340 amp conductor.

Using 600AL on a service with single main 400 amp OCPD is noncompliant even using the "round up" rule as 350 amp OCPD's are available.

A person needs to select a conductor size with an ampacity past 350 amps to be able to select the next size up 400 amp OCPD assuming the calculated load is less than the conductor ampacity.

FYI, I'm referencing 310.15(B)16, 240.4(B), and 240.6(A).


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## sbrn33 (Mar 15, 2007)

Cow said:


> 600AL is 340 amp conductor.
> 
> Using 600AL on a service with single main 400 amp OCPD is noncompliant even using the "round up" rule as 350 amp OCPD's are available.
> 
> ...


I don't like you anymore...


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## VoltElectric (Sep 30, 2020)

John M. said:


> I am building a 400 amp (class 320) service. Is it permissible to use 500 MCM XHHW aluminum (350 amps rated) since the load is not constant on a service?


Parallel the conductors


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## Kevin (Feb 14, 2017)

VoltElectric said:


> Parallel the conductors


Pretty darn sure OP has finished this service by now...


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