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# Three phase electric Heat gain

For single phase electric appliances the heat gain is 3.41 x Watts (Volts x Amps).

For three phase Watts = Volts x the average Amps of the three legs, can someone please tell me for this equation is the Voltage the average of the three legs (on a 460v around 270v) or the average of two legs (460v)??????

• True Watts requires a power factor. Volt-Amps are more frequently used (and for the case of resistance heating the power factor is essentially one anyway.)

The magic number you need is the square root of three (1.732) which is multiplied by the line to line voltage and then by the current to get VA. For a 460V three phase system drawing 40 Amps:

460V x 40A x 1.732 = 31,869 VA = ~31.87 kW of heat

If the currents or voltages on the legs differ by much, you need a lot more math. I use a calculator for that now.
• Thanks all,

I just wanted to make sure that the volts is measured from leg to leg, and not leg to neutral.
• If you use leg to neutral, You can use that voltage x amps x 3 and you wind up in the same place....
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• With the heater running at full wattage would there be any current flow in the neutral? The voltage measurement would be taken on the phases that had current flowing thru them.

Some heaters could have a need for a neutral for controls blower motors, etc. to operate at a lower voltage.

Also some smaller (20 to 30 KW) would have a single phase 240VAC blower motor which would only use 2 of the 3 phases. This would skew your amp clamp measurements for true heater calc. (most of the electric motor energy use turns to heat anyway)

• For 3 phase resistance heat a neutral is not needed. You are correct that many units also have a single phase component that needs a neutral. This also throws the balance between the phases off a bit.
I think the OP was not trying to measure the actual wattage of an operating unit, which would involve measuring both the volts and amps.
I think he was just making sure he understood how to the calculation works.
Carl
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Carl is spot on here.

Most of the units I've seen use a 3Ø fan motor as well. If they have any onboard controls, there is typically a small transformer wired L-L that with a 24 VAC secondary.