ECN Forum Forums General Topics of the Trade General Discussion Area Gutter box for NM installs.

Don't those window A/C units only draw in the range of 4A-8A, depending on size?

Computers, lights, tv's (for some people), a/c's (as mentioned), refrigerators, stereos... I'm sure the list can be expanded, but all of these things are often on for more than 3hrs. And a lot of homes have central air, those units can get a little big. But, the only real continuous load on an a/c is the fan. The compressor only runs when the a/c cycles which can vary in frequency depending on the conditions.

So, as DS has asked, in residential (or any application for that matter) how do you determine "continuous loads"? Or do you just forget about it and do it anyway regardless of the code?

So, even if the total load isn't close to the rating of the circuit breaker, or percent thereof, it must be derated anyways?

derating is for the conductors, not the breaker. you're giving the same current a larger wire to run on, but the current is the same for the breaker. the issue is heat dissapation.

Say you're running a 20A circuit on #12 wire, but due to derating you have to drop down to #10, you're still running a 20A circuit with a 20A breaker. And as far as I can tell, the derating is only applied where you have the wires running together, so you can have #12 in free air, but have #10 in conduit for the same circuit. Am I wrong here?

The "adjustment factor" (derating) changes the maximum permitted ampacity of the conductor. The size of the overcurrent protective device cannot not exceed the maximum permitted ampacity of the conductor.
Don

In reality, there are almost no continuous loads in a dwelling unit.

Think of it this way: How many circuits are loaded up to their full ampacity for 3 hours or more? I would say none.

Peter

I see. Would derating apply to conduit as well?

well, once you've derated, you're now dealing with larger wires, which will require larger conduit.

Derating does apply to conduits.

Also derating is done when you have more than three current carring conductors, regardless if it is a continuous load or not.

Keep in mind that you can use the temp rating of the wire for derating. So for THHN you can use the 90 degree column of 310.16 for you derating. If you notice that column lists the ampacitiy of # 12 as 30 amps.

Chris

Another question, let's say you've caluclated the derating of the circuits and come up that you'd need to use #10 wire. Does this mean that the entire install needs to be in #10 wire, or does it mean that only the homeruns that'll end up in the gutter box need to be #10 wire?