Check my plans? 100a sub panel wiring.

[jthelman]

I have a newer house with 200a main service, but the garage wiring *****. I've got 2 outlets, 2 lights, and plugs for the garage door openers, but that's it. Its a 3 car 22 x 30 garage.

I'm going to run a 100a sub panel to fix the problem and allow my to have a few dedicated 20a plugs, juice for a welder, compressor, and possibly a powder coat (kitchen) oven. I'm going to leave the original lights and garage opener circuits alone, so that the sub is dedicated to the tools.

I'm calculating 70' conservatively as the run from the main panel to the sub.

The garage is *attached*, and the cable will go through the basement the entire way.

My plan is stay conservative side of the NEC codes in case the inspector gets a bug up his ***, which he tends to do here in the sticks.

I'm thinking aluminum 2-2-2-3 SER cable (no conduit) would do the trick and I will connect it to a 100a breaker in the main panel. No ground rods for the sub, everything goes back to the main panel.

It looks like 2-2-2-3 is a fairly common size, and almost no one stocks copper anymore? Hopefully Depot or Lowes will have the aluminum, otherwise I'll have to pay a bit more the electric supply house.

Let me know if I'm missing anything?

Thanks,
John

 

[Jim Johnstone]

Check your local codes. I was told that #2 aluminum is not allowed for 100 amp branch circuit wire, only for 100 amp service wire.

Might have to go bigger if you want 100 amps on aluminum, or use the #2 and go with a 90 amp sub.

 

[jthelman]

Yeah, this is what I was afraid of...

I think I'm going to do a cost comparison for the upsized #1 wire. Wire has gotten ridiculously expensive. If I can get the bigger wire for < $100 more, I'll do that. Plan B is that I use the 100a breaker, and wait to see if the local inspector *******. If the 100a gets rejected, I'll just grab a 90a and stuff that in, which takes 5 minutes.

Thank you for the quick response!

 

[Jim Johnstone]

Yeah, this is what I was afraid of...

I think I'm going to do a cost comparison for the upsized #1 wire. Wire has gotten ridiculously expensive. If I can get the bigger wire for < $100 more, I'll do that. Plan B is that I use the 100a breaker, and wait to see if the local inspector *******. If the 100a gets rejected, I'll just grab a 90a and stuff that in, which takes 5 minutes.
Thank you for the quick response!

The highlighted part is EXACTLY what my plan is, since the house I bought already has #2 aluminum buried under the driveway.

 

[sammm]

I recently ran a 100amp sub-panel to my new detached garage. I used #2 alum (4-wires), but used an 80-amp breaker in my main-panel to feed it.

 

[jthelman]

I recently ran a 100amp sub-panel to my new detached garage. I used #2 alum (4-wires), but used an 80-amp breaker in my main-panel to feed it.

Also a good point. That just means that the main breaker will trip if you pull more that 80, but that's pretty unlikely.

I'm only 1 man, so that chances that I could run more than 2 pieces of equip simultaneously pulling more than the 80 (or 90) are slim.

Good call.

 

[Aceman]

It looks like 2-2-2-3 is a fairly common size, and almost no one stocks copper anymore?

Just realize the NEC only considers this 75 amp wire. You're allowed to go to an 80 amp breaker if the calculated load isn't over 75 amps.

If you want the full 100 amp subpanel, you'll need 1/0 AL SER, and that'll have to come from a supply house. I doubt Home Depot sells it.

 

[jthelman]

Just realize the NEC only considers this 75 amp wire. You're allowed to go to an 80 amp breaker if the calculated load isn't over 75 amps.

If you want the full 100 amp subpanel, you'll need 1/0 AL SER, and that'll have to come from a supply house. I doubt Home Depot sells it.

God I love the electric codes. Lots of rules, yet no one can agree on what makes sense. You get **** like "Subsection 6 paragraph 5, line 5 of the 315 NEC code for 2008", unless your local township says otherwise.

75 amps? Every calculator I used said 90-100.

I was OK with going up to #1, but 1/0! Holy ****, the garage wiring will cost more than the house.

Thanks.

 

[Gary S]

I solved that problem by running #2 copper. It is rated higher than aluminum.

 

[jthelman]

I solved that problem by running #2 copper. It is rated higher than aluminum.

Which brings up another point: most of the sub panels I'm looking at can't handle 1/0 wire.....

#2 copper should fit better.

I'm thinking this is gonna cost a grip o' cash tho.

 

[Jim Johnstone]

I solved that problem by running #2 copper. It is rated higher than aluminum.

Priced a bit higher too

 

[Aceman]

75 amps? Every calculator I used said 90-100.

I was OK with going up to #1, but 1/0! Holy ****, the garage wiring will cost more than the house.

Thanks.

Most calculators are a joke. They don't usually take insulation type into account which pretty much makes them all worthless.

Price the 1/0 AL SER before you scare yourself into sticker shock...

A 100 amp panel should have no problem accepting 1/0 wire.

 

[May Pop]

Why not pipe it and run #3 copper and be done with it for good.

 

[AussieDan]

Why not just pick up the 90A breaker and be done with it?

If you really expect to be running at the full 100A then maybe you should be thinking about upgrading the whole shebang to 150A for when you inevitably need to add something down the road. If you're not going to be running at the full 100A then the 90A breaker will serve just as well and save you headaches.

 

[Norcal]

Why not just pick up the 90A breaker and be done with it?

If you really expect to be running at the full 100A then maybe you should be thinking about upgrading the whole shebang to 150A for when you inevitably need to add something down the road. If you're not going to be running at the full 100A then the 90A breaker will serve just as well and save you headaches.

Because SER cable MUST be sized from the 60 degree column of table 310.16, so #2 AL is only rated @ 75A.

See 338.10 (B)(4)(a) which refers to 334.80 in the 2008 NEC

This limits SER to the 60 degree ampacity just like NM cable.

For 100A AL SER, 1/0 is required.#1 AL SER is rated for 85A, if computed load is 85A or less then a 90A overcurrent device can be used.

 

[AussieDan]

Ah that makes sense now, I was thinking 75f for USE but the OP is talking attached.

Sounds like it might be cheaper in the end to run conduit and THHN or RHW-2 (I've seen dual-rated USE-2/RHW-2 2/2/2/4 which should be good for 90A).