HVAC min. circuit ampacity v. older wire

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  #1  
Old 06-09-12, 08:00 PM
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HVAC min. circuit ampacity v. older wire

Hello NEC readers,

A contractor recently replaced my HVAC. Whenever the condenser starts up, the 30A 2-pole breaker buzzes momentarily. Twice in many weeks the breaker has tripped.

The name plate on the condenser lists "Min. Circuit Ampacity" as 29.3 and "Maximum Fuse or Circuit Breaker" as 50. There is a good article from 2000 on HVAC overcurrent protection on the web at...

[SIZE=2][SIZE=2]www.iaei.org/magazine/2000/07/overcurrent-protection-for-air-conditioning-and-refrigeration-equipment/[/SIZE][/SIZE]

About five pages into the article, the section "Branch Circuit Requirements" addresses my concerns. I have been going back and forth in the local NEC to figure out the capacity of my #10 AWG in this situation.

My house wiring is about 32 years old. The #10 wires (and all the wires in my panel) have no markings on them. I figure at best they are type TW. So I am using the 60 degC column in Table 310.16 to determine that the maximum amps for my #10 wire is 30A. When I de-rate this using the correction factors at the bottom of the table for 87 - 104 degF, I calculate the maximum safe current is 26.4A - 28.2A.

Bottom line, I do not think the HVAC wiring is code-compliant as it is installed.

To meet code I believe that the conductors need to be type THHW or THHN #10 or preferably #8. I also believe that I cannot go higher than the 75 degC column because the breaker is only rated for 75 degC., and that the breaker should be raised to 40A.

Is my analysis correct, or am I splitting hairs here?

Does the scope of a mechanical permit pulled by an HVAC contractor require that the electrical hook-up meet code requirements?

Comments appreciated.
Thanks,
Lynnx
 
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  #2  
Old 06-09-12, 09:03 PM
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You probably have NM wiring with 60 degree insulation. NM-B started around 1984 and has 90 degree insulation.

Why do you feel you need to derate for temperature?

You can change the breaker to a 2 pole 50.
 
  #3  
Old 06-09-12, 09:29 PM
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I would recommend changing the breaker to the correct size. Starting amps are much higher than run amps and will cause the 30A breaker to trip. The replacement breaker should be HACR type.
 
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Old 06-09-12, 09:33 PM
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I do not think the HVAC wiring is code-compliant as it is installed.
You said that the branch circuit for your condensing unit has 10AWG conductors protected at 30A. That's code compliant.

The condensing unit nameplate lists 29.3A as the minimum circuit ampacity and 50A as the maximum overcurrent protection. The branch circuit meets the manufacturer's specifications.

Given that
Whenever the condenser starts up, the 30A 2-pole breaker buzzes momentarily. Twice in many weeks the breaker has tripped,
you can use an ammeter to determine the actual load at startup. You can pay to have the conductors changed to #8 and the breaker changed to a 40A. Or you can try re-torquing the terminal screws on the existing 2-pole breaker to 10 or 15 ft/lbs (read the label on the inside of the panel door for the recommended value). If this hasn't been done in 32 years, it's probably time.

Question: Is the service disconnect for the condensing unit fused or unfused? If it is fused, what is the rating of the fuses (or the breaker)?
 
  #5  
Old 06-09-12, 09:36 PM
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All circuit wiring is individual conductors pulled through EMT. That's why I guessed TW rather than NM. Either way it's the same 60 degC temp rating.

The current-carrying capacity of a conductor is reduced at higher temperatures. When calculating the maximum ampacity of a conductor, Table 310.16 in Article 310.15 normally requires correcting for ambient temperature.

However, I do not know all the considerations that go into the manufacturer's specification of "Minimum Circuit Ampacity". Maybe I do not need to correct for temperature in this situation. I don't know. I am hoping someone in this forum will know for sure.

Isn't a 50A breaker on a #10 circuit pretty high?

Lynnx
 
  #6  
Old 06-09-12, 09:45 PM
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The disconnect for the condensing unit is fused at 40A.

Good idea to re-torque!

I will look for an ammeter, presumably one that holds the highest current.

Thanks,
Lynnx
 
  #7  
Old 06-09-12, 09:45 PM
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Isn't a 50A breaker on a #10 circuit pretty high?
Yes. See my earlier post.

The current-carrying capacity of a conductor is reduced at higher temperatures. When calculating the maximum ampacity of a conductor, Table 310.16 in Article 310.15 normally requires correcting for ambient temperature.
Generally speaking, using the standard of 30 C (86 F) for the ambient temperature works for any residential situation when consulting Table 310.16. IOW, no, you do not need to correct for temperature in this situation.
 
  #8  
Old 06-10-12, 07:47 AM
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Isn't a 50A breaker on a #10 circuit pretty high?
It is acceptable, but since the 30 amp breaker trips only occasionally, I'd change the breaker to a 40 amp 2 pole. The wire is fine.
 
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