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Old 10-14-2007, 01:23 AM   #1
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100a or 125a sub-panel in 40X50 metal building


I have been lurking around here for 3 days, been reading so much my head hurts and Iíve gotten nothing done.

So maybe someone can just tell me what I need?

I want to add a 100a or 125a panel to garage it is used for storing motor home and toys and will be using typical hand tools and probably have an air compressor and maybe a welder later on.

I have a 200a panel at house and the sub panel in building will be 84 feet away from MP.

I was going to run 2224 underground for a 125a panel I have but maybe I can get by w/ a 100a and save some cost on wire?

1. What size panel would you use?

2. What size and type of feeder and ground should I use (I have 2in conduit from MP to SP)

3. I was going to put a 100a breaker in MP to run the feed to SP, this would be the disconnect. Does the 6 breaker rule apply? or do I need disconnect at SB, I thought I read somewhere that a sub panel is a non rated service panel and the rule donít apply?

Any help would be appreciated,

Thanks

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Old 10-14-2007, 07:03 AM   #2
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100a or 125a sub-panel in 40X50 metal building


Your 2,2,2,4 aluminum is good to 100 amps as a feeder to a sub panel. This is probably enough for your needs, as you will not be likely to be using all of the tools at the same time. The air compressor and welder with a radio and a couple lights seems like the max load at any given point in time.

Your distance is not enough to worry about voltge drop.

If you shop is attached to your house you do not need a main disconnect at the shop building. If your shop is detached you need to have a main disconnect of less than six breakers total. (sometimes called the six handle rule) Since you will want several recepticle circuits as well as the welder, air compressor and lights, it is highly likely that you will go way over six circuits fast. I would suggest a 12 circuit panel minimum.

If it were me I would look for a 30 space main breaker panel, either 100 or 125 amp. Remember that the main breaker is being used for disconnect purposes only. Your overload/short circuit protection is at the 100 amp breaker in the main panel.

You will also need to install at least one ground rod at the detached building and bond it to the ground wire from the house.

The grounds and neutrals should be seperated at the sub panel, and the ground should be bonded to the ground rod and the panel enclosure.

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Old 10-14-2007, 10:11 AM   #3
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100a or 125a sub-panel in 40X50 metal building


40 x 50 is a fairly small space. 100 amp is all you need. Many houses have 100 amp services.
Follow (jwhite's) procedure exactly. Just remember at 100 amp you will not need #2 wire.

Also, Since you have to dig trench for the UF cable, why not use PVC conduit instead, and pull individual wires. 3 THWN #3 for 100 amp and 1 #8 green for ground. Easier to terminate on both ends. Nicer looking to.
It will cost a little more money.
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Old 10-14-2007, 10:59 AM   #4
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100a or 125a sub-panel in 40X50 metal building


J.V. I am sorry to differ with you, but table 310.15 (B) (6) says that one needs number 2 aluminum or number 4 copper for a 100 amp feeder in a dwelling service or feeder application.

Also the OP has already said that he/she has a 2 inch conduit from one location to the next.

masco, either use no 2 aluminum with a number 4 ground or number 4 copper with a number 6 ground. In either case use number 6 copper to the ground rod. Do not use aluminum to the ground rod.
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Old 10-14-2007, 12:32 PM   #5
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100a or 125a sub-panel in 40X50 metal building


Quote:
Originally Posted by jwhite View Post
J.V. I am sorry to differ with you, but table 310.15 (B) (6) says that one needs number 2 aluminum or number 4 copper for a 100 amp feeder in a dwelling service or feeder application.

Also the OP has already said that he/she has a 2 inch conduit from one location to the next.

masco, either use no 2 aluminum with a number 4 ground or number 4 copper with a number 6 ground. In either case use number 6 copper to the ground rod. Do not use aluminum to the ground rod.
Thank you for all the replies.

So here is my plan as I understand you.

I have a 200amp MP at house that I will install a 100amp breaker in

I have a 125amp 30 space sub panel w/ a 100amp main breaker installed.

Will run (3) no 4 copper, will mark 1 of them white at each end and run (1) no 6 green copper for ground to sub panel and a no 6 green copper to UFER ground next to sub panel.

My question is this,
Is THHN ok or do I use THWN? Itís going in 2in conduit. (I'm thinking THWN is correct).

I understand keeping the neutral and ground separated at sub panel.

Is my thinking correct now? Is the UFER ok or do I still need ground rod?

Thank You in advance.

Masco
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Old 10-14-2007, 12:52 PM   #6
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100a or 125a sub-panel in 40X50 metal building


If your area requires a UFER ground, you need both a rod and UFER but you will not need a second ground rod. If the building will be served by water you must also have a water pipe ground. If the building is served by TV or phone you must also bond them to the grounding system. If metal gas lines consult the gas company for proper grounding in your area.

You definately need THWN, but it has been years since I have seen wire that is not duel rated THHN/THWN. (and ofter for many other uses as well)
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Old 10-14-2007, 06:35 PM   #7
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100a or 125a sub-panel in 40X50 metal building


Quote:
Originally Posted by jwhite View Post
If your area requires a UFER ground, you need both a rod and UFER but you will not need a second ground rod. If the building will be served by water you must also have a water pipe ground. If the building is served by TV or phone you must also bond them to the grounding system. If metal gas lines consult the gas company for proper grounding in your area.

You definately need THWN, but it has been years since I have seen wire that is not duel rated THHN/THWN. (and ofter for many other uses as well)

If you have the "ufer" you don't need the rod according to the NEC.
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Old 10-14-2007, 08:34 PM   #8
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100a or 125a sub-panel in 40X50 metal building


The table in 310.15(B)6 is not for feeders to outbuildings.
It is only for services and feeders that serve as the main power to the dwelling.

....120/240-Volt, 3-Wire, Single-Phase Dwelling Services
and Feeders. For individual dwelling units of one
family, 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.....

The #2 aluminum does not have the required ampacity for a 100 Amp feeder. It is listed in the 75įC column of Table 310.16 as 90 Amps.

Last edited by DuncanB; 10-14-2007 at 08:41 PM. Reason: Clarification
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Old 10-14-2007, 08:48 PM   #9
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100a or 125a sub-panel in 40X50 metal building


Quote:
Originally Posted by DuncanB View Post
The table in 310.15(B)6 is not for feeders to outbuildings.....
It is only for services and feeders that serve as the main power to the dwelling.

.
The #2 aluminum does not have the required ampacity for a 100 Amp feeder. It is listed in the 75įC column of Table 310.16 as 90 Amps.
The OP should check with their local authority about whether this is allowed or not. Some jurisdictions say it is OK, others say no. Simple solution is to run the 2-2-2-4 as planned and swap the breaker if necessary.
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Old 10-14-2007, 09:17 PM   #10
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100a or 125a sub-panel in 40X50 metal building


He's in SoCA, it's not allowed anywhere in the State
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Old 10-14-2007, 09:58 PM   #11
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100a or 125a sub-panel in 40X50 metal building


Quote:
Originally Posted by DuncanB View Post
He's in SoCA, it's not allowed anywhere in the State
Then he should buy a 90A breaker.
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Old 10-14-2007, 11:27 PM   #12
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100a or 125a sub-panel in 40X50 metal building


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Originally Posted by HouseHelper View Post
The OP should check with their local authority about whether this is allowed or not. Some jurisdictions say it is OK, others say no. Simple solution is to run the 2-2-2-4 as planned and swap the breaker if necessary.

Just because a inspector allows something does not make it right.

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