Ok to feed 100amp panel with less than 100amps?

cj7jeep81 · Mar 29, 2010

I'm getting ready to run electric out to my 16'x16' shed. I'm planning on installing a panel with a 100amp main breaker. I really don't need this much power, but would rather have the room just in case.

My plan is to run aluminum feeder wire from my panel through 1.5" conduit (totaly run will be about 60-75 feet). I will then install 2 ground rounds at the shed.

I have a couple of questions. Due to cost, I plan on running 2 gague aluminum. I've read that 2 gauge wouldn't work for 100 amps (due to 2008 code), which I'm fine with (probably only need 30). Is it ok for me to put a smaller breaker in my panel to feed the 100 amp panel (say 60amp)?

Second, if I'm having seperate grounds at the shed, do I need to run a ground wire from the main panel to the sub? Or woudl I be ok with 2 hots and a neutral? Thanks for any help.

Falcon67 · Mar 29, 2010

I use a 125 amp 16 circuit panel in my shop with a 60A feed. It's not an issue, especially if the feed is sized for the run length and installed overcurrent protection at the main panel. Meaning - you can use a whatever amp panel with a 60 amp feed, if you size the run for 60 and fuse it for 60 that's all you'll be able to use even if you have 45 breakers on the other end. You can't go back and put a 100amp fuse on an 60 amp run later, so it's up to you if you want better wire installed now. I picked 60 because it fits within the design parameters of what's installed on the house and the feed was economical. It's plenty. I used a box with a lot of circuits so I could split things out and have space to run several 220 circuits.

Someone with the right code knowledge will surely comment on the rest.

sberry · Mar 29, 2010

I have installed dozens of 100A panels, 2 alum wire fed with a 60A breaker.

Aceman · Mar 29, 2010

The only thing you'll have to worry about are the lugs. Your subpanels main breaker should be fine, but the 2 pole breaker in your main panel is where you'll probably have to do some adjusting. I'm not sure if a 60 will do it, or if you'll have to bump up to an 80-90 amp breaker to get the lug size you need.

The rest of your plan sounds fine.

Oh I missed your second question. You do need two hots, one neutral and a ground. 4 wires.

cj7jeep81 · Mar 30, 2010

thanks everyone for the quick replies, sounds like i'm on the right track.

aceman, i'll definitely check that out, hadn't thought of it. now i just need it to dry up some so i can get to digging.

cj7jeep81 · Apr 1, 2010

one other question about wire. i was looking at the hardware store today, and they had some 2-2-2-2 (or maybe 2-2-2-4) aluminum service entrance cable, but it was for dry locations only, which wouldn't work since I'm running conduit, correct?

they also had some 2-2-2 that I'm pretty sure was ok for wet conditions, but wouldnt' have the ground wire obviously. would i be ok to buy that a length of either #4 or #6 bare copper to use as the ground?

walrus · Apr 1, 2010

cj7jeep81 said:
would i be ok to buy that a length of either #4 or #6 bare copper to use as the ground?

No, needs to be insulated and rated for a wet location. THWN is what you want if its going into conduit

sberry · Apr 1, 2010

The box stores sell a quad wire just for this application, if there are more than 2 turns 2 inch is a lot easier. To add a ground for number 2 alum conductors 8 copper is sufficient.

Aceman · Apr 1, 2010

walrus said:
No, needs to be insulated and rated for a wet location. THWN is what you want if its going into conduit

Which code article requires insulated THWN copper grounds?

I run bare #4 direct buried between ground rods all the time.

walrus · Apr 1, 2010

Aceman said:
Which code article requires insulated THWN copper grounds?

I run bare #4 direct buried between ground rods all the time.

I thought the question was him pulling a ground into conduit along with triplex or whatever the hardware store had for a subpanel.

Aceman · Apr 1, 2010

walrus said:
I thought the question was him pulling a ground into conduit along with triplex or whatever the hardware store had for a subpanel.

It was. I was trying to say if it's legal to direct bury it, it's definitely legal to pull into conduit. 250.119

It doesn't really matter anyhow because you can't buy a 3 wire cable and pull a seperate ground alongside it. It must be a 4 wire cable. 300.3(B)

walrus · Apr 2, 2010

Aceman said:
It was. I was trying to say if it's legal to direct bury it, it's definitely legal to pull into conduit. 250.119

It doesn't really matter anyhow because you can't buy a 3 wire cable and pull a seperate ground alongside it. It must be a 4 wire cable. 300.3(B)

I've never seen anyone pull bare wire into conduit, either inside or outside. 300.5b says it must be listed for wet locations. Didn't realize bare copper was?

Aceman · Apr 2, 2010

walrus said:
I've never seen anyone pull bare wire into conduit, either inside or outside. 300.5b says it must be listed for wet locations. Didn't realize bare copper was?

The first sentence of that article says "Cables and insulated conductors shall be listed....."

It's not referring to bare copper grounds.

But I'll also say I haven't ever seen anyone pull bare into conduit, but it's perfectly legal from everything I've read.

sberry · Apr 2, 2010

I have seen bare in pipe, about the only place I know its prohibited is for specialty isolated grounded circuits. As I recall in 250 it lists bare as acceptable (as well as pipe) and on some larger conductors it lists stripping the insulation as a means of identifying it as a grounding conductor.

walrus · Apr 2, 2010

Aceman said:
But I'll also say I haven't ever seen anyone pull bare into conduit, but it's perfectly legal from everything I've read.

I emailed Southwire and they say the same thing. I asked specifically if it was listed for the purpose and they say yes.
I've seen alot of insulated copper wire go bad here. Conduits are generally full of water here and a nick in the insulation inevitably turns to a broken wire so I'd never pull a bare wire into a pipe. I have seen some listed direct burial aluminum flexible conduit(looks like liquid tite with conductors in it) with a bare aluminum ground. Used it once would never use it again.

Norcal · Apr 3, 2010

If someone wanted to use a 400 A panel w/ a 60 A feed it would be fine, the only limitation is the wire range on the lugs they may or may not allow smaller wire sizes, but on a 100-125 A panel that is not a issue..

comedyman809 · Apr 7, 2010

#3 copper wire for 100amp.

im doing a panel in my shop at work.

i ran a #3 neutral, the 2 hots will be #3 copper(the rest of my wire is on order) and the ground is a #10.

after one 3 and the 10 wire there iis plenty of room for 2 more 3 wires.

and it is in 1'' emt.

if you arent to far away for the run, i would do the #3 copper for the hots and neutral, and maybe a 10 or 8 for ground.

if im wrong, please advise.

Aceman · Apr 8, 2010

comedyman809 said:
if you arent to far away for the run, i would do the #3 copper for the hots and neutral, and maybe a 10 or 8 for ground.

if im wrong, please advise.

It's been posted on here many, many times that a #8 ground is required for a 100 amp subpanel. There's no reason to guess the ground size required when a search would of yielded the correct answer.

It's moot anyway since it sounds like this guy plans to only have a 60 amp subpanel which only requires a #10 ground.

Ground size is based on the breaker feeding the subpanel, not the main breaker size in the subpanel.

cj7jeep81 · Jun 15, 2011

it finally dried up long enough for me to run the conduit, so i'm getting ready to run the wire. i ended up running 2" conduit to my shed (price wasn't much different than 1.5", so figured i'd upsize to make pulling easier). It has a few bends in it, but shouldn't be too bad. Total length of the run is roughly 65'.

Going to install a 60amp breaker in my panel to feed the 100amp panel i've already installed. Will 6 gauge aluminum be able to handle 60 amps? A local electrical supply place has triplex 6 gauge aluminum wire. My dad has some old coils of 10 gauge stranded copper (marked THW) which I planned on using for a ground if I went this route.

If 6 gauge isn't enough, they also have single conductor 4 gauge aluminum that I can run. If I have to go this route, will the 10 gauge still be big enough for a ground? If not, my dad also has some 6 gauge copper I could use (also marked THW).

I'd just do it all in 6 gauge copper, but he only has about 150' of it, so not enough for all my conductors.

mrb · Jun 15, 2011

if you are supplying #2 with a 60 amp breaker, using a cable with a #4 EGC you need to do the calculation for the EGC to make sure you are complient with 250.122-B

cj7jeep81 · Jun 15, 2011

mrb said:
if you are supplying #2 with a 60 amp breaker, using a cable with a #4 EGC you need to do the calculation for the EGC to make sure you are complient with 250.122-B

sorry for the confusion, but i've changed my mind a bit from the initial post a few months ago. right now i'm planning on feeding the 60amps with either #4 aluminum or #6 aluminum. would like to use #6 since its a bit cheaper, but more importantly easier to work with. with either one, i've already got some copper wire to use as a ground, just not sure which I would need for each gauge. i have available both #10 and #6 copper for ground wire.

mrb · Jun 15, 2011

cj7jeep81 said:
sorry for the confusion, but i've changed my mind a bit from the initial post a few months ago. right now i'm planning on feeding the 60amps with either #4 aluminum or #6 aluminum. would like to use #6 since its a bit cheaper, but more importantly easier to work with. with either one, i've already got some copper wire to use as a ground, just not sure which I would need for each gauge. i have available both #10 and #6 copper for ground wire.

cant use #6 AL for 60 amps, have to use #4. This is individual conductors. If it is SE or NM you can only use #6 AL for 40 amps.

cj7jeep81 · Jun 15, 2011

the #4 is individual conductors, so I'll go with that. would #10 copper be too small for the ground? if so, i can use #6. also, i assume there's no issue with using aluminum for the hots and neutral, and copper for the ground?

mrb · Jun 15, 2011

#4 AL on a 60 amp breaker I believe #10 CU is ok for the ground.

cj7jeep81 · Jun 15, 2011

mrb said:
#4 AL on a 60 amp breaker I believe #10 CU is ok for the ground.

thanks for the info. for some reason, my brain quit working while i was getting the wire, and forgot I had wire to run for the ground until after he had gone back to cut the wire. got 4 #4 aluminum type use-2.

Ok to feed 100amp panel with less than 100amps?

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