breaker size for 7.5kw aux heat strips

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gator07

Member
Location
florida
I,ve wired several single package heat pump units, and i have been getting some grief lately. These units get two circuits, im questioning the heating circuit. I always pull #8 thhn on 40 amp breaker because the strips are rated 7500 watts @240v 31 amps, add 125% for continuous load (which i dont think they would be continuous because they are aux heat strips) and im still under 40 amps. Never had any problems 10+ years until lately ac contracter wants them on 50 amp breaker my guess is because of the blower motor. Im just curious how you guys wire them!
 

ceb58

Senior Member
Location
Raeford, NC
I try not to listen to them either but i failed two inspections this week because of them

And what code # was sited? If the unit has two circ. one should be for the comp. and blower. The other should be for aux. heat only. If you wired to name plate speck. for the comp. and calculated for the size heat strip then I dont see the problem.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
You use 125% because of 424.3(b)
And yes, on most units I've seen the blower motor is connected to the heat circuit so the added blower motor load will take you over 40 amps.
Most of the units I see have a manufacturing requirement to post the heater nameplate (sticker) which provides a MCA and MOCP.
(NEC 424.28 also requires a nameplate)
Be cautions as to your wiring method and remember that NM and SE (under '08) have a 60? ampacity.
 

iwire

Moderator
Staff member
Location
Massachusetts
I,ve wired several single package heat pump units, and i have been getting some grief lately. These units get two circuits, im questioning the heating circuit. I always pull #8 thhn on 40 amp breaker because the strips are rated 7500 watts @240v 31 amps, add 125% for continuous load (which i dont think they would be continuous because they are aux heat strips) and im still under 40 amps. Never had any problems 10+ years until lately ac contracter wants them on 50 amp breaker my guess is because of the blower motor. Im just curious how you guys wire them!

For what its worth 424.3(B) requires that we treat these heaters as a continuous load.

Now with that out of the way the calculation comes to 39 amps, which is acceptable on a 40 amp breaker as you have already figured in the extra 25%.

Me, I if I always ran 8 AWG THHN for these circuits I would also install the 50 amp breaker and make sure I used a disconnect that had 75 C terminals.
 
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