240volts X 42amps = 10kw so you should be good to go.
If your wire is labeled 55amps,55 is the max it should be for but #4 al is normally 65 amp.

So are you saying the 10 kw only needs 42 amps if 240 volts-- or am I not understanding? And if wire is 55 amps it's ok?

I also responded to your message in the electrical forum, I'm not sure where you will check back. If the air handler is served by a single circuit, the blower motor and other controls add amps. My air handler manual says that for 10KW heating, you need a 60 amp breaker and #6 copper wire. I have a total of 20KW and one circuit requires a 60 amp breaker and the other circuit only requires a 50 amp breaker with #6 copper.

What size breaker do you have now? It is possible that a 55 amp breaker will be adequate if that is what you have, depending on the actual draw of the blower motor. A 1/3 hp motor would add 3 1/2 amps, a 1/2 hp motor 5 amps plus the draw of the rest of the air handler.

The best thing to do is find your air handler manual online. There is always a table showing the draw, wire sizes, and breaker sizes required for various size heaters. Also check to see if aluminum is recommended. My air handler manual specifically says it does not recommend aluminum.

**I found a Trane Air Handler manual online:

http://www.trane.com/content/dam/Trane/Commercial/global/products-systems/equipment/unitary/split-systems/split-systems-1-1-2-to-5-tons/22-1847-7B_02012014.pdf

For the 9.6 KW, single circuit, it requires a minimum ampacity of 54 or 55 amps depending on your model (see pages 13 through 16) , so the Trane dealer is correct and your wiring is adequate.

Here is a link that might be useful: TAM7 Manual

This post was edited by Dovetonsils on Fri, Dec 5, 14 at 23:17