100 amp sub off 100 amp main?

kmacht · Sep 16, 2010

I'm ready to start wiring my garage this weekend. As a background, it is a detached garage about 5' from the house but will require about a 60' run to get from the inside house panel to where I want to mount a sub panel. I went to HD today and bought the sub panel. It is a 100 amp main breaker panel. It came with the 100 amp breaker and 5 20 amp breakers plus room to add 9 more (14 slots). I plan on using 2-2-2-4 aluminum service cable to make the run between panels. It was the least expensive option I could find at $1.49 a foot.

So here is my question. The main panel in my house is 100 amps with a 100 amp breaker. I know that the wire I am using is rated around 100 amps and the sub panel is rated at 100 amps. Can I or should I put a 100 amp breaker in the main panel to feed the sub panel or should I go smaller Realisticly I don't ever see me using 100 amps at a time in the garage and even if I did I would have to shut everything off in the house. The only reason I see to go that route is because the price difference between breakers. A 60 amp breaker is about $10, a 70 amp breaker is around $35 and a 100 amp breaker is $45. Thoughts?

Keith

Charles (in GA) · Sep 16, 2010

kmacht said:
I plan on using 2-2-2-4 aluminum service cable to make the run between panels........

So here is my question. The main panel in my house is 100 amps with a 100 amp breaker. I know that the wire I am using is rated around 100 amps and the sub panel is rated at 100 amps. Can I or should I put a 100 amp breaker in the main panel to feed the sub panel or should I go smaller ?

The 2-2-2-4 "mobile home feeder" quadplex wire is only good for 90 amps per NEC 310.16 (310.15(B)(6) does not apply as this is not a dwelling service)...... so........ you need a 90 amp breaker in your main panel for the feeder to the garage. The 100 amp main is OK in your garage panel, as it is only being used as a disconnect in this installation.

Charles

sdowney717 · Sep 16, 2010

https://keithspecialty.com/elec.cable_wire.htm

if only good for 90, then why is it sold as 100 amp service cable?

2-2-2-4 100amp aluminum SER cable, above-ground use

Is there a difference when buried versus in the air? (air runs cable cooler so a higher rating?)

Stuart in MN · Sep 16, 2010

sdowney717 said:
https://keithspecialty.com/elec.cable_wire.htm

if only good for 90, then why is it sold as 100 amp service cable?

Is there a difference when buried versus in the air? (air runs cable cooler so a higher rating?)

It's rated at 100 amps for the main service coming in from the electric utility to the main panel, but not when it's used from the main panel to a sub panel. It's just one of those things in the NEC codebook.

Also, since you mentioned overhead and underground, make sure the stuff you buy is rated for where you're going to install it.

Charles (in GA) · Sep 16, 2010

As noted, for a RESIDENTIAL service, table 310.15(B)(6) allows 100 amps for this, but since its not a residential service, but a feeder to a subpanel in a non-residential structure, then table 310.16 applies, and that only allows 90 amps.

Charles

mrb · Sep 16, 2010

does your mobile home feeder cable also have RHW or XHHW marking? if it just has USE or XLPE it cant enter a structure. Also, has to be in conduit once it comes out of the ground, cant just run this type of cable in the building (we've been through that before here...)

kmacht · Sep 17, 2010

Your last post really confuses me. What do you mean it cant enter a structure. This is the same wire that runs down the side of my house and then enters it and runs over to my main panel. I also assumed that it had to be run in conduit underground (buried 18" deep). Am I missing something here?

Keith

mrb · Sep 17, 2010

kmacht said:
Your last post really confuses me. What do you mean it cant enter a structure. This is the same wire that runs down the side of my house and then enters it and runs over to my main panel. I also assumed that it had to be run in conduit underground (buried 18" deep). Am I missing something here?

Keith

it depends on the marking on the wire. if it says RHW or XHHW youre ok because those are NEC listed wire types. there is some mobile home feeder out there that is not 'dual listed'. if the wire only says USE or XLPE on it the wire cannot be run in a structure.

sberry · Sep 17, 2010

I usually use a 60 breaker, its cheap and have never had one call because of a tripped main, my bud runs auto repair shop from the same set up, 30A comp, fair amount of lights and even a 20A air cond on occasion.

bochnak · Sep 17, 2010

I have done exactly what you are proposing.

I ended up using a 60amp breaker in the main panel to feed the 100amp sub out in the garage.

A 100amp main panel is WAY cheaper than a 60amp panel. I had a 60amp breaker laying around, that is why I chose it. At the moment, I only have 2 circuits going in the garage, a 15 and 20 amp. I am a long way from maxing out the 60amps.

I also chose this route if I ever upgrade the home panel to 200amp. I'll just pop in a 100 breaker to sub to take full advantage.

Amp ratings:

http://www.westernextralite.com/resources.asp?key=23

Charles (in GA) · Sep 18, 2010

mrb said:
does your mobile home feeder cable also have RHW or XHHW marking? if it just has USE or XLPE it cant enter a structure. Also, has to be in conduit once it comes out of the ground, cant just run this type of cable in the building (we've been through that before here...)

Here is the "dual" marked mobile home feeder quadplex, 2-2-2-4. I have a 150 ft roll of it I bought very cheap one day at HD out of the mis-cut and end of spool basket. Its marked "TYPE USE-2 RHH RHW-2" (the last -2 got chopped off in the pic). I had to lighten and change the contrast to make it show in the pic.

Charles

unclejunk · Sep 20, 2010

What do you mean by 'USE/XLPE cannot be run in a structure' exactly?

It must be in conduit when it's run inside a structure, or it can't enter a structure at all..??

mrb · Sep 20, 2010

unclejunk said:
What do you mean by 'USE/XLPE cannot be run in a structure' exactly?

It must be in conduit when it's run inside a structure, or it can't enter a structure at all..??

cant enter structure at all unless it has dual rating with another listed wire type typically RHW or XHHW. most of the cable out there is ok, but there is some that is not.

this type of cable has to be run in conduit when its not in the ground since its just 4 wires twisted together and does not have an outer jacket.

100 amp sub off 100 amp main?

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