Another subpanel wire gauge question

[K13]

Hey guys.

I have done a search and seem to find a different answer in every different thread I read on this so I thought I would start a new one. I apologize if this is rehashed info.

I am going to run a 100 Amp service to my detached garage and need to know what gauge wire I should be using. The run is approx 50 feet. From what I have been able to find on here I think 2-2-2-6 copper is what I want but I just want to make sure before I get it. Also if using Aluminum do I step up to 1-1-1-4?

I am I on the right track with this? Can I go smaller?

Thanks for any input.

 

[Falcon67]

I just looked this up for mine and at 100' it was more like #4 copper would do the trick.

 

[K13]

I just looked this up for mine and at 100' it was more like #4 copper would do the trick.

HAHA that is the problem everything I read seems to be different I have seen everything from #4 to #2

 

[Falcon67]

You can always run a larger conductor - more money, but you're for sure that it'll carry the load. There is a huge jump in pricing between 2 and 4. Also, keep in mind that the ratings vary by type of wire and how they are run. In my case, part will be in free air with a small run in conduit. I want to use 100A capable equipment from "feed to need" but will be breaking at 70A. I figure if I pop a 70, then I need to really look at what loads were running at that time. Plus, if I scale up some equipment I'd like to be able to run at higher load without replacing a bunch of feed equipment.

http://www.codebookcity.com/codearticles/nec/necarticle310.htm

Tables:
http://www.houwire.com/products/technical/article310.html

 

[Norcal]

I just looked this up for mine and at 100' it was more like #4 copper would do the trick.

#4 will not do the "trick" for 100A.

 

[Falcon67]

#4 will not do the "trick" for 100A.

According to 310.17 if I run 90C wire in free air it will, even if I correct for 120F air temp. Unless I'm reading the wrong table.

 

[K13]

This seems to be what happens on every thread I have looked at. Someone say's something won't work but don't explain what will or why it is no good.

I have a friend that can get me the wire in exchange for an extra garage heater I have so I am not worried about cost but I also don't want him pulling a favor for me for more than I need. Mine will all be underground either in conduit or direct burial depending on what he can get me.

 

[mrb]

cant use the 90deg column in these applications, its for derating only. Have to use the 75deg column.
Copper
#4 - 85 amps
#3 - 100 amps
#2 - 115 amps

Aluminum
#4 - 65 amps
#3 - 75 amps
#2 - 90 amps
#1 - 100 amps

 

[mrb]

I have a friend that can get me the wire in exchange for an extra garage heater I have so I am not worried about cost but I also don't want him pulling a favor for me for more than I need. Mine will all be underground either in conduit or direct burial depending on what he can get me.

that is like $500 worth of wire. Where is it coming from? Hopefully not an employer or company that doesnt know theyre giving away $500 in wire.....

 

[K13]

cant use the 90deg column in these applications, its for derating only. Have to use the 75deg column.
Copper
#4 - 85 amps
#3 - 100 amps
#2 - 115 amps

Aluminum
#4 - 65 amps
#3 - 75 amps
#2 - 90 amps
#1 - 100 amps

Thanks I appreciate it. So am I right in assuming I need a number 6 ground if I go with the #2 copper? What gound for the #3 copper, #8?

that is like $500 worth of wire. Where is it coming from? Hopefully not an employer or company that doesnt know theyre giving away $500 in wire.....

No he knows the owner of a supply company that owes him a favour so it is all above board. Free/bartering is good... stolen is not.

 

[Falcon67]

I'm sure this doesn't help - NEC 310.15(b)(6) seems to allow #4 for 100amp if it's a feeder and not coming from past the main disconnect to a sub panel. So you may need to clarify where your power starts.

In my case, it will be a feeder and not a sub. If someone would flesh this out, I'm sure we'd all appreciate it.

 

[Aceman]

According to 310.17 if I run 90C wire in free air it will, even if I correct for 120F air temp. Unless I'm reading the wrong table.

Use Table 310.16 and the 75C column, then you'll be on the right track.

I'd run 1/0 al or #3 cu.

 

[Charles (in GA)]

I'm sure this doesn't help - NEC 310.15(b)(6) seems to allow #4 for 100amp if it's a feeder and not coming from past the main disconnect to a sub panel. So you may need to clarify where your power starts.

In my case, it will be a feeder and not a sub. If someone would flesh this out, I'm sure we'd all appreciate it.

The OP's garage is detached. 310.15(B)(6) is for residential only, so it cannot be used on feeder to a non-residential structure.

Charles

 

[MrMark]

No 3 copper is designed for your application. If romex you must use #2. You have received the correct answers here.

Does anyone know why there is a #3 copper? Historic reasons?

 

[mrb]

No 3 copper is designed for your application. If romex you must use #2. You have received the correct answers here.

Does anyone know why there is a #3 copper? Historic reasons?

as i remember there is a considerable amount of difference in the amount of copper between 4 and 2, so with 3 you can save a ton of money on larger projects

 

[MrMark]

as i remember there is a considerable amount of difference in the amount of copper between 4 and 2, so with 3 you can save a ton of money on larger projects

Probably and I was thinking that because 100 amp is such a popular subpanel size that #3 was created to fit that bill perfectly.

I always just thought it odd that all awg sizes were even until you get down to #3 but I think it must be because of the 100 amp panel.

 

[mrb]

Probably and I was thinking that because 100 amp is such a popular subpanel size that #3 was created to fit that bill perfectly.

I always just thought it odd that all awg sizes were even until you get down to #3 but I think it must be because of the 100 amp panel.

well really under #4 it stops skipping odd numbers, 4, 3 ,2 ,1 ,1/0, 2/0, 3/0

 

[Falcon67]

The OP's garage is detached. 310.15(B)(6) is for residential only, so it cannot be used on feeder to a non-residential structure.

Charles

I'll bet us folks with 1100 sq/ft shops with heat/air/potty could give the NEC a rash about the distinction LOL. Still, it would be interesting to understand the logic that allows a #4/100A feeder for a building that is occupied a lot of the time and a similar building of similar square footage that was used intermittently.

I'll ask our master at work if he has a source for #3 copper.

 

[Norcal]

310.15(B)(6) is for residential services carrying the entire load only it cannot be used for subfeeds or any panel that does not carry the entire load of a residence.


(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services
and Feeders. For individual dwelling units of onefamily,
two-family, and multifamily 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 serve as the main power feeder to each dwelling unit
and are installed in raceway or cable with or without an
equipment grounding conductor. For application of this section,
the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by
branch circuits or by feeders, or both, all loads that are part
or associated with the dwelling unit. The feeder conductors
to a dwelling unit shall not be required to have an allowable
ampacity rating greater than their service-entrance conductors.
The grounded conductor shall be permitted to be
smaller than the ungrounded conductors, provided the requirements
of 215.2, 220.61, and 230.42 are met.

 

[Falcon67]

310.15(B)(6) is for residential services carrying the entire load only it cannot be used for subfeeds or any panel that does not carry the entire load of a residence.

I agree completely with that it says. My "purely argumentative as a mental exercise" item is:

- #4 OK for 100A main power feeder to panel board for 1100 sq/ft residence with heat/air/sink/potty.

- #4 Not OK for 100A main power feeder to panel board for 1100 sq/ft non-residence with heat/air/sink/potty.

There must be an engineering reason that a residence is "safe" with that size feeder and a non-residence is "unsafe" with the same feeder. I imagine it has something to do with the load calcs.

Think "farm" with a main to the meter, then overheads to 2-3 separate buildings.

 

[Grumpy365]

that is like $500 worth of wire. Where is it coming from? Hopefully not an employer or company that doesnt know theyre giving away $500 in wire.....

Why would you even ask

You will find life is a lot easier when you don't worry about what angle everyone else is working.