100 amp sub panel, 180' from main, what size wire?

[DuramaxPowered]

I 'm putting a 100 amp sub panel in the pole barn. The total length from the main panel will be about 180'. What size wire is needed? Local supply warehouse guy says #2AL, but that doesn't seem to be correct from everything I've seen.

 

[sberry]

How much service do you want, number 2 will deliver about 60A at that distance without too much V drop. You will need to step up a size for 100A.

 

[Aceman]

1/0 al.....

 

[nate379]

#2 is fine, but just barely.

It is rated to 100 amp per 310.15 B-6

The thing though is at 180ft you are looking at just under a 5% voltage drop. If you don't plan on drawing near 100 amps you could be ok, but I would go with a #1 or 1/0 which would be ~4% drop and 3% respectively.


I am not an electrician so don't take my info as gospel... Just info from research I did a while back when my house had an undersized feed line.

 

[walrus]

#2 is fine, but just barely.

It is rated to 100 amp per 310.15 B-6

The thing though is at 180ft you are looking at just under a 5% voltage drop. If you don't plan on drawing near 100 amps you could be ok, but I would go with a #1 or 1/0 which would be ~4% drop and 3% respectively.


I am not an electrician so don't take my info as gospel... Just info from research I did a while back when my house had an undersized feed line.

#2 is fine for a service, this isn't a service its a feeder, #1 aluminum is minimum. I have a sub that far away from my main and I used #1. Why?, I'll never load it at 100 amps so why worry about voltage drop at 100 amps. If I did 60 it'd be amazing. You need to do calculations to get a real answer to your question

 

[nate379]

Right, am I wrong that 310.15 B.6 is not for feeders as well?

310.16 shows #2 AL as 130 amps.

From NEC on 310.15...

"For individual dwelling units of one-family, two-family and multi-family dwellings, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240 volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that are installed in a raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and branch circuit panelboard(s)."

Oh, I am asking not to argue, just to clear up confusion.

 

[Norcal]

Right, am I wrong that 310.15 B.6 is not for feeders as well?

310.16 shows #2 AL as 130 amps.

From NEC on 310.15...

"For individual dwelling units of one-family, two-family and multi-family dwellings, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240 volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that are installed in a raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and branch circuit panelboard(s)."

Oh, I am asking not to argue, just to clear up confusion.

The OP said it was a feed out to a pole barn, which means it ain't gonna meet the criteria....

 

[nate379]

This....

Table 310.15(B)(6), shall be permitted as 120/240 volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that are installed in a raceway or cable with or without an equipment grounding conductor.

What gets used if it's not 310.15? It's not 310.16 as it says 130amp for #2 wire.

 

[AussieDan]

My understanding is that a garage or pole barn does not meet the criteria for 310.15(B)(6), because they are not:

"individual dwelling units of onefamily, two-family, and multifamily dwellings".

You have to use 310.16, which is 130A for #2 copper at 90*C, whereas you must use the rating for Al @ 75*C, which is 90A.

To clarify, you have to use the 75*C column because 110.14(C) states:

"The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device."

So even though USE-2 is rated for 90*C, you're stuck with 75*C because most circuit breakers are rated at 75*C.

For that reason you're going to be best off with 1/0 especially with the length of your run.