100 amps to shop. What wire size?

xbanone · Aug 30, 2012

I just had an electrician install a 200a service upgrade to my house. I would like to run a 100 amp feed to the new shop. I have a 100a panel in the shop and will have to go about 100-120 feet distance underground. What cable do I need to ask for at the electrical supply house? I will have 3 conductors and a ground. #3 THHN? What should I expect to pay for copper?

930dreamer · Aug 30, 2012

Here's some good info. Post # 7 & 9.

http://www.garagejournal.com/forum/showthread.php?t=162740

xbanone · Aug 30, 2012

930dreamer said:
Here's some good info. Post # 7 & 9.

http://www.garagejournal.com/forum/showthread.php?t=162740

Thanks,

according to the chart, #4 copper or 2AL for 100 amps. Is voltage drop something I should consider at this distance?

930dreamer · Aug 30, 2012

Distance is a factor.

pattenp · Aug 30, 2012

For a feeder the #2 Al is good to 90A not 100A. #2 Al used as service entrance carrying the whole load to a dwelling can be used at 100A. That chart is for 3 wire service entrance. You are doing a 4 wire branch/feeder circuit, which is a different chart/table. It's a NEC thing. If you can be happy with 90A use Mobile Home Feeder 2-2-2-4 or 2-2-4-6.

http://www.southwire.com/products/MobileHmFeeder.htm

The #2 is fine for the 120', it will be less than 3% at 90A @ 75C.

sberry · Aug 30, 2012

And 95 and a half plus of the time it will never see much load, I like the 90A with alum also.

CoyoteLL · Aug 30, 2012

Just ran 50' to a 100A subpanel in my garage. Used #4 Copper. Pulled 4 legs through 1 1/2" PVC conduit.

pattenp · Aug 30, 2012

Hope you are feeding it from a max of 90A breaker, because #4 Cu is rated at 85A.

CoyoteLL said:
Just ran 50' to a 100A subpanel in my garage. Used #4 Copper. Pulled 4 legs through 1 1/2" PVC conduit.

Gary S · Aug 30, 2012

I ran #2 copper to my garage. Menards didn't have #3 in stock and the other vendors in town wanted more for #3 than I paid for #2 at Menards. It handles the 100a breaker with room to spare.

mrjaw14 · Aug 31, 2012

I did a similar project earlier this year and was wanting #4 Cu thhn/thwn, but the cost of #2 Al XHHN was literally about half of what it was going to cost for Cu. I did the entire project for the cost of what #4 Cu would have been.

90A breaker > 4 cond AL to a subpanel in detached garage (on 100A "main disconnect")

I had a single 15A circuit prior to this but anything with a motor would heat up really quickly, voltage would drop to around 100 VAC, it was horrible. Things are much happier now that I have an actual sub panel setup. V-drop is next to nothing under the same loads I was putting on it before.

Dave Maxwell · Aug 31, 2012

Just got a price from local electric supply. #2 copper is 1.37 per foot. Menards is 1.89.

fnsane · Aug 31, 2012

Most economical is 1/0 al SER.

'04 Cummins · Aug 31, 2012

Dave Maxwell said:
Just got a price from local electric supply. #2 copper is 1.37 per foot. Menards is 1.89.

Wire other than romex is where the supply houses have the box stores beat by a mile. Panels and breakers usually seem to be cheaper at HD, Lowes, etc. YMMV

Falcon67 · Aug 31, 2012

Dave Maxwell said:
Just got a price from local electric supply. #2 copper is 1.37 per foot. Menards is 1.89.

#2 AL mobile feeder is running about 1.80/ft. That's a 4 wire cable - not a single wire. It's what I used for mine - it was 1.47/ft when I bought it. I fused my 100A panel at 75A in the panel at the meter.

Error - 70A is what's in the can.

Norcal · Sep 1, 2012

Falcon67 said:
#2 AL mobile feeder is running about 1.80/ft. That's a 4 wire cable - not a single wire. It's what I used for mine - it was 1.47/ft when I bought it. I fused my 100A panel at 75A in the panel at the meter.

Where are you finding 75A fuses or circuit breakers? 75 is not a standard size, 70A is.

pattenp · Sep 1, 2012

That would be okay, except for the fact that SER can't be run under ground.

fnsane said:
Most economical is 1/0 al SER.

fnsane · Sep 1, 2012

Pipe it.

Steevo · Sep 1, 2012

Mine is about 90 ft overall from main panel to sub in shop.
We used #2 copper to feed the 100A sub panel.

Charles (in GA) · Sep 1, 2012

fnsane said:
Most economical is 1/0 al SER.

pattenp said:
That would be okay, except for the fact that SER can't be run under ground.

fnsane said:
Pipe it.

338.12 Uses Not Permitted.
(A) Service-Entrance Cable. Service-entrance cable (SE)
shall not be used under the following conditions or in the
following locations:
(1) Where subject to physical damage unless protected in
accordance with 230.50(B)
(2) Underground with or without a raceway
(3) For exterior branch circuits and feeder wiring unless
the installation complies with the provisions of Part I of
Article 225 and is supported in accordance with 334.30
or is used as messenger-supported wiring as permitted
in Part II of Article 396

Falcon67 · Sep 2, 2012

Error - its a 70. 220,221 - whatever it takes

It was in stock and plenty big enough IMHO. I worked out here all day yesterday, cut some stuff on the lathe, built a mobile stand for the handsaw- bet I didn't pull 10~15A peak all day.

fnsane · Sep 3, 2012

Ok, we have an inspector monitoring this thread. So, pipe it with 1 1/2" pvc, pull 3 1/0 al XHHW and 1 #6 XHHW and you are compliant.

Norcal · Sep 3, 2012

fnsane said:
Ok, we have an inspector monitoring this thread. So, pipe it with 1 1/2" pvc, pull 3 1/0 al XHHW and 1 #6 XHHW and you are compliant.

No your not, when you upsize ungrounded conductors you run into the requirements of 250.122(B).

(B) Increased in Size. Where ungrounded conductors are
increased in size, equipment grounding conductors, where
installed, shall be increased in size proportionately according
to the circular mil area of the ungrounded conductors.

Copied from the 2011 NEC.

That section overrides the normal grounding conductor sizing table.

Dave Maxwell · Sep 4, 2012

Supplier says #2 copper. My electrician says too small. Should be 1/0 atleast. Run 2/0 and be done. Om getting lost. 100 amp house to garage. Underground in conduit. 250 foot

pattenp · Sep 4, 2012

The 250' is whats killing you. To get the full 100A at 3% or less voltage drop you do need at least 1/0 Cu or 3/0 Al.

Dave Maxwell said:
Supplier says #2 copper. My electrician says too small. Should be 1/0 atleast. Run 2/0 and be done. Om getting lost. 100 amp house to garage. Underground in conduit. 250 foot

Dave Maxwell · Sep 4, 2012

Dave, calculating the wire size is pretty basic, the challenge is fitting it into the existing lugs of the breaker in your house panel and the lugs of the breaker in the additional panel in your shop. We stock a 2/0- 3 conductor direct bury underground aluminum for this application. Cost is $2.55 per foot.
that is from another supplier.

pattenp · Sep 4, 2012

Dave, I'd say drop your load amps to 90 and go with the 2/0 Al and use a 90A breaker in your main panel. Using 2/0 Al at 90A for 250' you should be right at a 3% volt drop. Also make sure you get 4 conductor (2 hots, 1 Neutral, 1 Ground). Such as 2/0-2/0-2/0-1 or 2/0-2/0-1-4. Otherwise you'll need to use 1/0 copper ($$$) to get it to fit the breaker at 100A. I believe a 100A breaker will take up to a 2/0 wire.

Dave Maxwell · Sep 4, 2012

I was told I could use a grounding rod at building and not have to have the fourth wire..

Wastedwagesracing · Sep 4, 2012

Dave Maxwell said:
I was told I could use a grounding rod at building and not have to have the fourth wire..

I had to have both, ground wire from the main service and two ground rods 8' apart at the building.

pattenp · Sep 4, 2012

So who told you that? You need a grounding electrode system (rods) and a EGC back to the main panel.

Dave Maxwell said:
I was told I could use a grounding rod at building and not have to have the fourth wire..

Dave Maxwell · Sep 4, 2012

Retired electrician and local electric supply house

pattenp · Sep 4, 2012

They gave you out of date info.

Dave Maxwell · Sep 5, 2012

Figures. Supply house has the 2/0 aluminum in a 3 wire. Then they have a #4 copper for ground. Sound good for my 100 amp run

pattenp · Sep 5, 2012

That will work. The 2/0 Al at 100A for 250' will give you a 3.33 % voltage drop which is not too bad. The supply house doesn't have any aluminum wire you could run for the ground? I'm surprised they don't have any 2/0 Quadruplex or 4 wire URD. They can't order it for you?

Edit: If you add your own ground to the three wire cable you will need to mark it as a ground by wrapping the ends where terminated with green tape.

Dave Maxwell said:
Figures. Supply house has the 2/0 aluminum in a 3 wire. Then they have a #4 copper for ground. Sound good for my 100 amp run

forced induction · Sep 6, 2012

I am running 3-#2 and 1-#8 THHN in 1-1/2" PVC conduit running 50' to a 100A sub panel in my detached garage. I didn't think you could use #4 and get away with 100A.
Hope to complete this project soon. Great thread guys and great site!!

Falcon67 · Sep 6, 2012

pattenp said:
They gave you out of date info.

X2 - changed in 2006 or 2008 code IIRC. Did mine that way with 4 conductor and a UFER using 20' of #4 in the slab. Inspection went AOK.

pattenp · Sep 6, 2012

If you're commenting to my post, the 4 wire URD is the number of conductors not the size. The size was 2/0. I was not very clear with the wording.

Also, are the #2's and #8 you used for 100A, copper? I also so assume the THHN you used also carries the THWN rating.

forced induction said:
I am running 3-#2 and 1-#8 THHN in 1-1/2" PVC conduit running 50' to a 100A sub panel in my detached garage. I didn't think you could use #4 and get away with 100A.
Hope to complete this project soon. Great thread guys and great site!!

forced induction · Sep 7, 2012

Yes all my conductors are copper. And yea accd to nec THHN and THWN have same rating. I don't have the cable yet but should arrive today. My #8 is actually gonna be XHHW. I wanted to try that wire type. I bought all my #2's color coded black, red, white. But only black and green was available in XHHW in 2ga size so I stuck with the THHN. At the 8 ga size they have all colors.

That's for explaining your post sorry on that I went back and reread it also.

pattenp · Sep 7, 2012

THHN and THWN are not the same ratings. THHN is for dry locations and THWN is for wet locations. Most if not all wiring today that is THHN also carries the THWN rating too. I asked just to make sure the wire you have was not just THHN only. There's no question about the XHHW being suitable for wet locations.

forced induction said:
Yes all my conductors are copper. And yea accd to nec THHN and THWN have same rating. I don't have the cable yet but should arrive today. My #8 is actually gonna be XHHW. I wanted to try that wire type. I bought all my #2's color coded black, red, white. But only black and green was available in XHHW in 2ga size so I stuck with the THHN. At the 8 ga size they have all colors.

That's for explaining your post sorry on that I went back and reread it also.

forced induction · Sep 7, 2012

The amapcity rating is the same (as far as I can tell). The application of each is different I agree completely. I just got my wire in today and it is THHN and THWN-2 approved. I also really like the jacket on the XHHW. Nice wire. I might start the install tomorrow but don't know if I will get to it. Thx again for all the help.

xbanone · Oct 1, 2012

pattenp said:
For a feeder the #2 Al is good to 90A not 100A. #2 Al used as service entrance carrying the whole load to a dwelling can be used at 100A. That chart is for 3 wire service entrance. You are doing a 4 wire branch/feeder circuit, which is a different chart/table. It's a NEC thing. If you can be happy with 90A use Mobile Home Feeder 2-2-2-4 or 2-2-4-6.

http://www.southwire.com/products/MobileHmFeeder.htm

The #2 is fine for the 120', it will be less than 3% at 90A @ 75C.

So, mobile home feeder @ 2-2-2-4 or 6 with a 90A breaker should work fine for me. I would like to run it through conduit, what size would you suggest?

100 amps to shop. What wire size?

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