You're right, I think my max is around 70A at 120 volt. I guess I was presuming you feed a subpanel with 240V so that it provides a balanced load to both bus bars. So you're saying I could feed the panel with a
60A 240V breaker using 6/3 romex, and then pull more than 60A at 120V (e.g. install 7 20A 120V circuits with a max simultaneous draw around 70A)?That would make this all easier.
cite that, it sure isn't what 310.15(B)(6) says
That's one reason. The other reason is to supply 240V loads.
Absolutely -- as long as you don't pull more than 60A on any one leg at any time. 30 on one leg, 40 on the other, will be just fine.
Yep.
From what you say there 40a with #8/3 would be more than enough. You will balance 80a across the 2 hot legs but going 60a is just a few bucks more for 6/3. The breakers and panel will probably be the same.
I think I started the talk of aluminum, saying that I had used it for the cost savings over copper.
JK
The Table in 310.15(B)(6) says, for a dwelling, which we are assuming this is, for a "Service or Feeder", which this sounds like, the rating in amps for a #4 copper or #2 aluminum shall be 100A. Are you saying that the problem is that it isn't a service but a subpanel? This should fall under the definition of "feeder" and I'm sure that my AHJ and electrician had that in mind when they allowed & ran my feeder, protected by a 100A breaker, through a piece of 2-2-2-4 Aluminum SE cable.
If I'm wrong, tell me how.
JK
see what big jake said.
s
Read the text above the table. It says "feeder conductors that serve as the main power feeder to a dwelling unit". That means if this is the feeder to the main breaker you can use the table. This assumes the diversity of the total house load. You won't be using everything at once. That may not be true in a feeder to a sub panel since we don't know what the sub panel serves. Don't feel bad. A lot of people get confused by this, even inspectors who should know better.
Here's a couple more for you - (I don't do this for a living)
According to another table in 310-16, my SE cable would have been ok for 100 amps if the wire was rated for 90c. and #4 Copper ok for 95 amps, which would can be "rounded up" to the next common breaker size, no?
JK
That really doesn't mean anything since you won't find a termination listed for 90c. The 90c column is really just there for derating purposes. An example is if you had 8 THHN conductors in a pipe you have to derate them down to 70% of their ampacity 310.15(B)(2) but since THHN is a 90c conductor you can start derating from the 90c ampacity. So for a #12 you would start derating from 30a, get 21a but you are still limited by 240.4(D) that makes it 20.
So the termination at the breaker of the main panel and the connection to the buss at the other end aren't rated for 90c?
JK
So does that mean I would only need a 60A double-pole breaker in the main to feed the subpanel? This breaker will feed two hots (240v) to the subpanel bus bars, and then I can pull up to 96A (80% of 120) of
120v from that subpanel?
Almost certainly not -- but they're marked. Check 'em.
Yes.
Yes.
The 80% rule applies to continuous loads -- defined as maximum current for more than three hours.
You only identified about 70A of load anyway...
It means you can pull 60 amps on each leg for a total of 60 amps @ 240 volts or 120 amps @ 120 volts
Should that 60A double-pole breaker be a 120/240v or just a 240v? I would presume the latter, but I see both at the store so thought I'd ask.
Thanks to *everyone* who has been so generous with their time and expertise!
You just need a regular 240v (2 pole) breaker. One labelled 120/240 is probably a GFCI. ($50 or more?)
You want a 240V breaker that has two breakers tied together with a common bar. When you turn the breaker off, you want to turn off BOTH legs of the
240V supply.You DO NOT want the double breakers with two individual switches, as that could still leave one half of the supply powered if you didn't flip both switches.
Just to clarify, if you run 6/3 wire with a 60A breaker, that's 60 amps on EACH side of neutral (60 on the black wire, 60 on the red wire).
If you balance half of your 120V loads on one side, and the other half of your 120V loads on the other, it should support close to 120Amps total. I'm not sure what the derating requirements are, but it should be more than enough to support the 70A loads you are planning.
Anthony
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