champion
Member
- Location
- St. Louis Missouri, US
I have a question about the proper size conductor and inverse time brkr that would be required for a Gas Furnace with A/C.
A friend of mine is installing a new furnace in his house and he asked me a question about the proper size wire and brkr he would need to supply the furnace. He has permission from the county he resides in to do his own AC work and the inspector will inspect when he is complete.
The only current data information on the furnace is max amp=12.0. (120v 60hz 1ph)
The controller and electrical components withing the furnace itself that operate dampers, relays etc. have a total max rating of 300 mili amps.
According to the NEC for branch ckt conductors to a single motor, I should be using the FLC of the motor x 1.25 for determining my conductor size and table 430.152 for determining the br ckt and grd fault prot device.
-Using 12.0 amps rating shown on the furnace, the wire size would calculate 15 amps for a no. 14 awg.
-Using table 430.152 for inverse time brkr at 250% of FLC, the brkr size should be 30amps.
-I will tell him to use a Gen Purpose Snap Switch for the disconnect at the furnace.
My dilemma is whether I should be using the max amps of 12.0 listed on the furnace or take the furnace apart and look directly on the motor for the HP of the motor itself. As far as the furnace goes, no other information is written in the manuals obtained with the equipment.
I'am ambarassed to tell my friend I am not sure but when it comes to electricity, I would rather be damn sure and certainly not be responsible for giving the wrong advice.
Can you help?
About the A/C compresser motor-----
I think I have it down but would like some input if possible.
Rating on unit:
----Compresser----
RLA-Rated load current=21.2
LRA-Locked rotor current=96
Min ckt ampacity=28.2
Max fs or ckt brkr=45
220v 1PH
----Compressor Fan Motor-----
FLC-Full load current=1.7
HP=1/4
Branch ckt---
Assuming the max ckt ampacity on the compressor name plate is (br ckt selection current) 440.32 states not less than 125% which would equate to 35.25 amps for a no. 8 gauge cable. (28.2 x 1.25 = 35.25).
Branch ckt short ckt and grd fault device---
Per 440.22, a protection rating not exceeding 175% of br ckt selection current 28.2 x 1.75 = 49.35 amp but I will use a 45amp due to max rating on compressor.
Disconnect----
Per 440-12, 115% of br ckt sel current = 32.43amp.
I was going to tell him to use a 60amp disconnect at the compressor location with 45amp dual element fuse.
Is the "Min Ckt Amps" listed on the compressor equipment nameplate really the branck ckt selection current and in your opinion am I on the right track with the advice that I need to give this friend?
Is the "Max Amps" listed on the furnace designed for ckt calculation or should I be using this max amp as the actual amps applied for br ckt conductor size and overcurrent protection?
Thank you in advance for reading and replying on this dilemma.
A friend of mine is installing a new furnace in his house and he asked me a question about the proper size wire and brkr he would need to supply the furnace. He has permission from the county he resides in to do his own AC work and the inspector will inspect when he is complete.
The only current data information on the furnace is max amp=12.0. (120v 60hz 1ph)
The controller and electrical components withing the furnace itself that operate dampers, relays etc. have a total max rating of 300 mili amps.
According to the NEC for branch ckt conductors to a single motor, I should be using the FLC of the motor x 1.25 for determining my conductor size and table 430.152 for determining the br ckt and grd fault prot device.
-Using 12.0 amps rating shown on the furnace, the wire size would calculate 15 amps for a no. 14 awg.
-Using table 430.152 for inverse time brkr at 250% of FLC, the brkr size should be 30amps.
-I will tell him to use a Gen Purpose Snap Switch for the disconnect at the furnace.
My dilemma is whether I should be using the max amps of 12.0 listed on the furnace or take the furnace apart and look directly on the motor for the HP of the motor itself. As far as the furnace goes, no other information is written in the manuals obtained with the equipment.
I'am ambarassed to tell my friend I am not sure but when it comes to electricity, I would rather be damn sure and certainly not be responsible for giving the wrong advice.
Can you help?
About the A/C compresser motor-----
I think I have it down but would like some input if possible.
Rating on unit:
----Compresser----
RLA-Rated load current=21.2
LRA-Locked rotor current=96
Min ckt ampacity=28.2
Max fs or ckt brkr=45
220v 1PH
----Compressor Fan Motor-----
FLC-Full load current=1.7
HP=1/4
Branch ckt---
Assuming the max ckt ampacity on the compressor name plate is (br ckt selection current) 440.32 states not less than 125% which would equate to 35.25 amps for a no. 8 gauge cable. (28.2 x 1.25 = 35.25).
Branch ckt short ckt and grd fault device---
Per 440.22, a protection rating not exceeding 175% of br ckt selection current 28.2 x 1.75 = 49.35 amp but I will use a 45amp due to max rating on compressor.
Disconnect----
Per 440-12, 115% of br ckt sel current = 32.43amp.
I was going to tell him to use a 60amp disconnect at the compressor location with 45amp dual element fuse.
Is the "Min Ckt Amps" listed on the compressor equipment nameplate really the branck ckt selection current and in your opinion am I on the right track with the advice that I need to give this friend?
Is the "Max Amps" listed on the furnace designed for ckt calculation or should I be using this max amp as the actual amps applied for br ckt conductor size and overcurrent protection?
Thank you in advance for reading and replying on this dilemma.