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Hey guys and guyettes!

Getting ready to install a 9 kw steam generator in a new construction house.

Need to know what size circuit breaker and size wire to use.

thanx

This sounds like an electric boiler. 9KW is the power consumed to generate maximum steam production.

Volts times Amps equals Watts.

Watts divided by volts equals amps.

You want amps for the electrical supply size.

Divide 9,000 Watts by 230 volts.

This comes out to just under 40 amps.

Since this load could run for long periods you need to go one size larger to a 50 Amp breaker and wiring. 8 gauge THHN wire is good, 6 gauge THHN wire is better.

Well the "one size larger" rule of thumb is pretty old school. The real way to figure it is 125% of the load for certain things like electric heat and water heaters and continuous loads.
We use 240v in figuring this. The actual amperage is 37.50, x 125% = 46.88, so yes a 50A circuit and breaker are required.

Remember, #8NM cable is NOT rated for 50A. #8THHN in conduit is. VERY few places will use conduit in a residential setting, so if you will be using NM cable then #6 it is.
Typically this would be 6/2 cable, since pretty much every steam genny I have seen or wired has been straight a 240v unit.

Thanx for the great help. That is valuable info.

Speedy Petey
I'm seeing romex cable with THHN insulated wire in it. Does this affect the temperature rating or ampacity? I know that the flexibiliity is better for pulling and installing.

NEC 334.80 tells us that me must use the 60 deg C column of T310.16 when figuring circuit ampacity for NM cable, even thought the conductors and cable are rated for 90 deg C.

If you have to figure adjustment factors in (derating) THEN you are allowed to use the 90 deg C column.

For THHN in conduit, again, for adjustments you can use the 90 deg C column, but you must use the 75 deg C column for circuit ampacity because there is very little that is 90 deg C rated except for the wire. Breakers are typically 75 deg C, and so are most terminations. We must use the lowest rated item when figuring circuit ampacity.

Good details. Thanks