The furnace nameplate should have the MCA (minimum circuit amps) and MOCP (maximum breaker)
Using only the information you supplied at 15KW (assuming this is the heat strips plus fan motor) served by a single branch circuit would be calculated at continuous load. 15000/240 = 62.5 amps ... 1.25 x 62.5 = 78 amps. So you would need #4 THHN with 75C ampacity of 85 amps if using conduit or #3 cable like SER 60C ampacity of 85 amps. Your breaker is 1.25 times the continuous load of 62.5 so you need an 80 amp double pole breaker. Your ground should be #8 copper. Do not use aluminum wire.
Is there any supplemental overcurrent protection at the heating unit? Because you need to be sure this isn't a two circuit Furnace.
It is typical for 15 kw to have 2 overcurrent devices at the unit itself. The supply wiring is usually 60 amps and the other one is 30 from your panel..
" One nice thing about the NEC articles ... you have lots of choices"
Last edited by Stubbie; 01-06-2010 at 11:46 AM.