Originally posted by: LTC8K6
at a load of 29A, you're stepping down to 8ga wire.
No, I'm not. You can if you wish. It certainly won't hurt anything.
Do whatever the hell you want, but don't tell people they'll be OK installing 10ga wire for a load of 29A. If it's continuous, you can't. If it isn't, it's OK but you're not giving yourself much breathing room (and you give yourself the option of upgrading the circuit down the line if you use 8ga).
It doesn't matter what you think. It matters what 3 bodies think. The NEC, the local building code, and the inspector.
Also, below 120gal water heaters are considered continous loads. For wire ga caluclations, you would have to take that rating and multiply it by 1.25. So (for arguments sake we'll use the 29A figure) 29A at 1.25, you're now looking at 38A.
This is exactly why people tell you to multiply the amperage rating (of the wire/breaker) by 0.8. If it's not continuous, you can load up the line to it's maximum rating. Expect it to trip though. If it is continuous, it's against code if the continuous amperage rating x 1.25 exceeds the total amperage rating of the wire/breaker.
As for the OP, techincally, it's right below that 20A threshold. 3500W/240V = 14.583~ x continuous load factor of 1.25 = 18.22917. It's below the threshold of the wire and the breaker. So safety wise, you'd be fine. However, this goes back to what I said before.
Part of being in compliance with code is adhering to the manufacturer's instructions. If the manufacturer calls for 10ga wire, while technically you may be able to use 12ga wire concerning the amperage, you have to use 10ga wire.
