jackdawson1049 t1_iw5gjnk wrote on November 13, 2022 at 2:38 AM

To figure current from wattage divide the number of watts by the voltage. 3600 watts / 120 volts = 30 amps

Hattix t1_iw6maji wrote on November 13, 2022 at 10:22 AM

Use peak voltage, not RMS. The peak voltage of a US AC supply is 170V (peak of the cycle), so if OP specifies for 30A, your example of a a 3600 watt nominal load is 4 ohm reactance or resistance.

Solving for current through that 4 ohms at 170V gets 42.5 amps. If specified for 30A + 5A slack and so running through a 35A breaker, that breaker would trip.

You can also just multiply current by root(2) (1.41) to get the peak in the AC cycle, but I like to show how the figure is derived.

jackdawson1049 t1_iw71259 wrote on November 13, 2022 at 1:24 PM

Funny, I was taught to use RMS.

Hattix t1_iw8a6ji wrote on November 13, 2022 at 6:51 PM

A breaker will trip on a peak current. It's not much of a 30A breaker if it lets 42.5A past now, is it?

However, don't just believe some random yahoo on the Internet with electrical safety, test it out in a controlled and safe setup.

JonJackjon t1_iw8p437 wrote on November 13, 2022 at 8:25 PM

> A breaker will trip on a peak current. It's not much of a 30A breaker if it lets 42.5A past now, is it?

A thermal magnetic breaker trips at different rates.

Magnetic circuit trips the breaker when a short or near short is experienced. They are rated to trip at 2 to 5 times the rated current. Whether 2, 3, 4 or 5 times depends on the type of breaker.

The thermal circuit will trip the breaker when enough heat is generated due to the current flow. Since is it heat related the current is sensed as RMS.

This is a safety and a local code issue. Please do not suggest solutions where you don't know the complete situation nor the local code requirements.

jackdawson1049 t1_iw8pufx wrote on November 13, 2022 at 8:30 PM

I was trying to give OP a quick and dirty example to get them in the ballpark. We haven't even considered voltage drop in the wire or the start up current draw. If I was installing this circuit based on the 3600 watt example I would be designing, based on experience, for a minimum 50 amp breaker.

Now I admit, I'm not an electrical engineer or even an electrician. I'm just a lowly PMEL tech who calibrates and repairs all of the test equipment.