HVAC circuit wire and breaker sizing

Status
Not open for further replies.
V

vernon_15ss

Member
Location
Goldston, NC
Occupation
Electrical Contractor
If I have an air handler that has a minimum current requirement of 31.6A, with a maximum breaker size allowed of 45A, can I run a 10/2 wg NM-B and install a 45A breaker. Or do I need to run an 8/2 wg NM-B?
 
#10 AWG NM cable can only be used at its 60° C ampacity which is 30 amps so that will be too small. #10 AWG MC cable would work with an 75° C ampacity of 35 amps.
 
infinity said:
That distance is not an issue even with #10 conductors.
Click to expand...


0.050X, 1.2R per 1000 feet

#10cu is 0.00005X, 0.0012R per foot

At 80 feet thats 0.096 ohms

At 120 volts that would give 1,250 amps

45x10 = 450 amps minimum instantaneous pickup, 45 x 20=900 amps max pickup


The breaker will trip in about a cycle preventing I2R let through energy from heating copper mass approaching let alone exceeding 90*C.
 
mbrooke said:
0.050X, 1.2R per 1000 feet

#10cu is 0.00005X, 0.0012R per foot

At 80 feet thats 0.096 ohms

At 120 volts that would give 1,250 amps

45x10 = 450 amps minimum instantaneous pickup, 45 x 20=900 amps max pickup


The breaker will trip in about a cycle preventing I2R let through energy from heating copper mass approaching let alone exceeding 90*C.
Click to expand...
Sorry M but I have no idea what any of that means. #10 conductors are permitted to serve this load (just no NM cable), at 40' the conductor length will have no affect on the circuit performing properly.
 
infinity said:
Sorry M but I have no idea what any of that means. #10 conductors are permitted to serve this load (just no NM cable), at 40' the conductor length will have no affect on the circuit performing properly.
Click to expand...


Right, 40 feet will not present any hazard when #10 THHN/THWN-2 is placed on a 45 amp breaker.
 
Thank you all very much...you have been very helpful. It was what I thought. I was told that the 3rd floor Air Handler would be a 30A circuit. So we ran 10/2 wg NM-B. When we came back for trim out, low and behold...it wasn't what they had said! Now another circuit has to be ran and there's no access to panel on 2nd floor.
 
vernon_15ss said:
Thank you all very much...you have been very helpful. It was what I thought. I was told that the 3rd floor Air Handler would be a 30A circuit. So we ran 10/2 wg NM-B. When we came back for trim out, low and behold...it wasn't what they had said! Now another circuit has to be ran and there's no access to panel on 2nd floor.
Click to expand...
I suggest telling them they have to supply a unit that will run on the 30a circuit.
 
Status
Not open for further replies.
Top