# Which electrican is right?



## steu4718 (Jan 19, 2009)

Job involves updating circuits for new, larger air handler and heat pump. Currently, 10 AWG with 30 amp breaker goes to existing heat pump. 8 AWG with 50 amp breaker goes to existing air handler. Name plate on new heat pump states minimum circuit ampacity (MCA) is 29 amps, minimum breaker is 35 amps, maximum breaker is 50 amps. Name plate on new air handler (which requires two circuits) states for circuit 1: MCA = 30 amps, max circuit protection = 30 amps; circuit 2: MCA = 50 amps, max circuit protection = 50 amps. Air handler has 2 built-in 60 amp breakers. There is a 60 amp disconnect at heat pump. 

Electrician 1 (me): Wants to use existing 10 AWG wire with 40 amp breaker for heat pump (nothing on circuit other than heat pump). 

Electrician 2: Wants to use 35 amp breaker for heat pump because, he argues, you absolutely can't use a 40 amp breaker with 10 AWG wire.

Both electricians want to run new 2 AWG aluminum circuit (with 100 amp breaker) from main panel to new subpanel at air handler. 

Electrician 1 (me): From new subpanel, wants to run 10 AWG with 30 amp breaker to circuit 1 and 6 AWG with 50 amp breaker to circuit 2 of air handler

Electrician 2: From new subpanel, wants to run 2 6 AWG lines with 60 amp breakers, one to EACH of the two circuits in air handler because, he argues, the 60 amp breakers built into the air handler suggest that 60 amps is needed for the two circuits. I argue that those are just intended to be disconnects, and not to provide overcurrent protection.

So, which of the two electricians is right and, in the interest of education, why?


----------



## electricista (Jan 11, 2009)

steu4718 said:


> Job involves updating circuits for new, larger air handler and heat pump. Currently, 10 AWG with 30 amp breaker goes to existing heat pump. 8 AWG with 50 amp breaker goes to existing air handler. Name plate on new heat pump states minimum circuit ampacity (MCA) is 29 amps, minimum breaker is 35 amps, maximum breaker is 50 amps. Name plate on new air handler (which requires two circuits) states for circuit 1: MCA = 30 amps, max circuit protection = 30 amps; circuit 2: MCA = 50 amps, max circuit protection = 50 amps. Air handler has 2 built-in 60 amp breakers. There is a 60 amp disconnect at heat pump.
> 
> Electrician 1 (me): Wants to use existing 10 AWG wire with 40 amp breaker for heat pump (nothing on circuit other than heat pump).
> 
> Electrician 2: Wants to use 35 amp breaker for heat pump because, he argues, you absolutely can't use a 40 amp breaker with 10 AWG wire.


I give this one to you. You are correct and in fact you can put a 50 amp breaker on it


----------



## electricista (Jan 11, 2009)

steu4718 said:


> Both electricians want to run new 2 AWG aluminum circuit (with 100 amp breaker) from main panel to new subpanel at air handler.


Better check 310.16. Are you running ser or seu-- then check 338.10(4)


----------



## steu4718 (Jan 19, 2009)

electricista said:


> Better check 310.16. Are you running ser or seu-- then check 338.10(4)


XHHW-2 ser. 310.16 indicates should be good for 100 amps, but if you think otherwise, I'd be interested to hear.

Also, I fail to see significance of 338.10. Am I missing something?


----------



## electricista (Jan 11, 2009)

steu4718 said:


> XHHW-2 ser. 310.16 indicates should be good for 100 amps, but if you think otherwise, I'd be interested to hear.
> 
> Also, I fail to see significance of 338.10. Am I missing something?


When ser cable is used-- talking 2008 NEC--- the cable must be used at 60C-- you can never use the 90 c rating except for derating. The 75C column is usually the col. to use however since you are using ser -- 60C


----------



## electricista (Jan 11, 2009)

Also check out art. 440. 22 in terms of the answer in part one. The #10 wire does not need to be protected from overload by the breaker since the units have overload protection in them. The breaker, in this case, just protects the circuit from ground fault and short circuit.


----------



## hardworkingstiff (Jan 22, 2007)

#2 AL @ 75C is rated for 90-amps. What is the reasoning for fusing it at 100-amps?

As pointed out earlier, in the 2008 NEC you must use the 60C column and can only use it @ 75-amps (5-amps short of your MCA).


----------



## Elec Tek (Jan 28, 2008)

electricista said:


> I give this one to you. You are correct and in fact you can put a 50 amp breaker on it


 
440.6 A exception 1


----------

