35 amp breaker on 10AWG for A/C?

[TSPORT]

Hi all. This is an HVAC electrical question. I'm dual-posting in the HVAC section to solicit input from the HVAC pros as well.
We had our complete HVAC system replaced last year. Twice in the past month the 30 amp breaker for the A/C compressor has tripped.
Never happened before with current system or prior system. The house was built in 1997 and the breakers are original. The breaker is on a dedicated circuit for the outside compressor unit. I called the HVAC company. They came out and checked things over. The tech seemed knowledgeable. He said everything looks OK with the HVAC system but that the compressor is rated for a 35 amps. He said the 30 amp breaker is old and recommended I replace the breaker with a 35 amp breaker and said the wiring is 10 awg and it's fine.
Is it safe to replace the 30 amp breaker w/ a 35 amp breaker in this situation?
If not, is it possible the 30 amp breaker is "wearing out" causing it to trip prematurely and should be replaced?

Thanks!

 

[bigb56]

Yes, replace with a Cutler Hammer CH235. The 30 is too small for the unit because although the total RLA is under 16 amps, the LRA (locked rotor amps) is 92 which means when the unit starts it can draw up to 92 amps for a nanosecond. HACR circuit breakers are designed for this and will hold for that nanosecond of high inrush current but the 30 is too close and the manufacturer knows that which is why they put 35 amps in your maximum fuse and circuit breaker spec. The MCA or minimum circuit ampacity is shown at 19.8 amps which means technically it would be legal to use #12 wire on that unit with a 35 amp breaker so your #10 is fine. This is a common setup for motor and compressor circuits and perfectly safe and legal. (Depending on the distance the wire may need to be upsized from the minimum allowed to prevent voltage drop as excessive voltage drop on a motor will increase current draw and damage motors)

Just make sure your new 35 amp breaker is a Cutler Hammer CH series with the tan handle, no other breaker will fit that panel which BTW is a very good panel. Do not buy one online on Amazon or Ebay as you risk getting a counterfeit.

 

[Sponsor]

 

[bigb56]

In answer to why it didn't trip last year there are multiple possibilities including lower voltage this season caused by an aging neighborhood transformer or more load on the transformer from neighbors, hotter weather, aging fan motor, aging circuit breaker etc. If you were right on the edge any slight change would put you over.

 

[TSPORT]

In answer to why it didn't trip last year there are multiple possibilities including lower voltage this season caused by an aging neighborhood transformer or more load on the transformer from neighbors, hotter weather, aging fan motor, aging circuit breaker etc. If you were right on the edge any slight change would put you over.

Thank you for your replies. We did have an unusual # of hotter than normal days this summer. The HVAC system is less than a year old so it's not the fan's age. Could you expand on why it's OK to replace the 30 amp w/ a 35 amp as also recommended by the HVAC tech that came out. I am seeing that 10 gage wiring is only rated up to 30 amps. Is it because of the short duration of the motor start-up? Something else particular to the application? Thirsting for knowledge Thanks again!

 

[bigb56]

Thank you for your replies. We did have an unusual # of hotter than normal days this summer. The HVAC system is less than a year old so it's not the fan's age. Could you expand on why it's OK to replace the 30 amp w/ a 35 amp as also recommended by the HVAC tech that came out. I am seeing that 10 gage wiring is only rated up to 30 amps. Is it because of the short duration of the motor start-up? Something else particular to the application? Thirsting for knowledge Thanks again!

Besides the fact that it's printed right there on your label, the 35 amp breaker is allowed per NEC Article 440 for motors and compressors. The load protects the wire from overload and the breaker protects it from short circuit. The compressor has built in overload protection which will not allow the wire to be overloaded. See my reply in your other thread.

 

[bigb56]

The part circled in blue gives permission to use #12 wire. The part circled in red gives permission to use a 35 amp circuit breaker regardless of wire size as long as the wire size is at least #12 cu.

 

[TSPORT]

Besides the fact that it's printed right there on your label, the 35 amp breaker is allowed per NEC Article 440 for motors and compressors. The load protects the wire from overload and the breaker protects it from short circuit. The compressor has built in overload protection which will not allow the wire to be overloaded, so the wire will never see more than 30 amps. See my reply in your other thread.

View attachment 105934

Thank you! I just read your reply to the other post and the links. I understand now. Much appreciated!

 

[wwhitney]

Is it because of the short duration of the motor start-up? Something else particular to the application?

Yes, and also because the motor(s) in the HVAC unit have their own thermal protector(s) that guard against overloads. So the breaker supplying the HVAC unit only has to protect against short circuits and ground faults. Which means that the nominal rating can be higher than normal to prevent precisely the short of issue you have having.

Cheers, Wayne