#2 or #3 Copper for 100A Subpanel

DB2 · Sep 2, 2009

I understand that #3 copper is sufficient for the feed cable to a 100A subpanel.

Does going to #2 copper buy you anything, i.e. what is the max current rating for #2?

I'm just trying to think of all possible future upgrades I might want to do. Currently the house has a 200A main panel.

sberry · Sep 2, 2009

How far is the run? If its short it wont add much to the cost, if its long you may need it for V drop if you anticipate heavy sustained loads, most garages really use very little power though, most demands are way over estimated. Most load is intermittent, biggest electric usage is usually lights. 310.16 I believe, I will let someone else look it up, is this cable or piped wire?

DB2 · Sep 2, 2009

Run would be approximately 75 ft, with individual cables (1/2 of run is underground).

For the moment, highest sustained load would be a 3 hp table saw + 1.5 hp dust collector + lights (26 x 24 shop) + electric dryer (laundry is in garage) + 24k btu mini-split AC, though all of my woodworking equipment and the mini-split are wired for 220V.

Falcon67 · Sep 2, 2009

I used #2 for service entrance on the house, since it's a 100A panel. IIRC it's good for 150 in that use. Of the two, I'd use the #2 because you'll probably have to special order #3 and #2 you can get anywhere plus it's likely cheaper. It was about $2.10 a foot here. Cost bites when you add it up.

Gary S · Sep 3, 2009

I ran #2 for much the same reason as above. It was readily available, and #3 would have been special order at the cheap places, so it probably would have cost me more than the #2. My run was fairly short, so it didn't cost much.

I think the larger wire was a good idea. I have no flickering on the lights in either my garage, or in the house where the garage panel is subfed from even when my big 220v compressor or my welder cycles. With smaller wire, I might have seen some of that.

hetkind · Sep 3, 2009

Let me see....

Welder 50 amps
Air compressor 30 amps
lights: 20 amps

So there is 100 amps load just for a simple air gouge operation...

I put 400 amp service in mine...

Now I am shopping for single phase machine tools.

Howard

sberry · Sep 3, 2009

Not many home shops will do arc gouging though, it would be a rare day for a home brew deal to see more than 60A.

Gary S · Sep 3, 2009

hetkind said:
Let me see....

Welder 50 amps
Air compressor 30 amps
lights: 20 amps

So there is 100 amps load just for a simple air gouge operation...

I put 400 amp service in mine...

Now I am shopping for single phase machine tools.

Howard

If that is your usage level, then 100 amp service isn't enough. My usage is much less. My lighting is all florescent so I'm at 3-4 amps 110v. My welder is a 110v wirefeed that uses less than 20 amps. Since both of them are 110, they draw off only one leg, and that means they count for only 1/2 of what they draw when you figure off the 110v main.
My compressor needs 22 or 24 amps. I'd have to look at the motor to be sure.
I also have two 220v electric heaters that combined need 40 amps.

But, I've never used the welder and the air compressor at the same time. My welding jobs don't require any compressed air.

My heaviest load is in the winter when my heaters run, and even then, I'm not hitting 50 amps total, so I'm content with 100 amp service.

StumpFJ40 · Nov 18, 2009

hetkind said:
Let me see....

Welder 50 amps
Air compressor 30 amps
lights: 20 amps

So there is 100 amps load just for a simple air gouge operation...

I put 400 amp service in mine...

Now I am shopping for single phase machine tools.

Howard

I don't think that is how you caclulate your demand... If we applied that calc to a house even we'd all be somewhere in the 800amp range: dryer, washing machine, baseboard heat/ furnace, oven, dish washer, microwave, lights+ way over my 200amp service.
My compressor is rated at 20amp at startup but doesn't run continuously (nor should it) or start and stop every minute.

hidollartoys · Nov 21, 2009

Per 2005 NEC Table310.15(b)(6) PG70-141

#2 COPPER IS GOOD FOR 125 AMPS AND #3 COPPER IS GOOD FOR 110 AMPS. THIS IS BASED ON 100 FOOT RUN AND ALL APPLICABLE REQUIREMENTS OF THE CODE.

mrb · Nov 21, 2009

hidollartoys said:
Per 2005 NEC Table310.15(b)(6) PG70-141

#2 COPPER IS GOOD FOR 125 AMPS AND #3 COPPER IS GOOD FOR 110 AMPS. THIS IS BASED ON 100 FOOT RUN AND ALL APPLICABLE REQUIREMENTS OF THE CODE.

310.15(B)(6) is for services and feeders supplying entire dwelling units. Since this is a subpanel that table wouldnt apply, but for #2cu and #3cu its only a few amps difference; #3 is good for 100a.

bimmer1980 · Nov 21, 2009

i'm in a similar situation.... I already have my #2 run in conduit...

Am I limited to 100 amps? If I put in a 125 amp breaker, I'm assuming that will be out of code?

1Garageman · Nov 23, 2009

StumpFJ40 said:
I don't think that is how you caclulate your demand... If we applied that calc to a house even we'd all be somewhere in the 800amp range: dryer, washing machine, baseboard heat/ furnace, oven, dish washer, microwave, lights+ way over my 200amp service.
My compressor is rated at 20amp at startup but doesn't run continuously (nor should it) or start and stop every minute.

I agree also. Heck if that's the case I'd need 3 400amp boxes at my house for my tools, air compressor, computer systems, tv's, fridge etc. That's crazy!!!
:lol_hitti

jvitez · Dec 1, 2009

Calculating loads confused the heck out of me until I learned the right way: add up the wattage, not the amps. A 100 amp subpanel at 240V single phase is 100A x 240V= 24,000 W. Add up the total wattage of all loads and see if you're over 24 KW. That's total wattage of all continuous running loads, not intermittent.

Aceman · Dec 2, 2009

Platt, Graybar, North Coast Electric, Stoneway Electrical Supply are some supply houses around here.

#2 or #3 Copper for 100A Subpanel

DB2

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sberry

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DB2

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Falcon67

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Gary S

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hetkind

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sberry

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