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My HVAC guy is telling me I need a 25 amp breaker and a 25 amp disconnect for my soon-to-be-installed central air compressor. I've asked him for more details but he's less than forthcoming.

My assumption is that I will install:

    - a double pole 30 amp breaker on my breaker box (30 amp because they don't sell 25 amp breakers at my local big box store)
    - I'll run 10/2 NM from said breaker to a junction box on the interior of my house near the exit point. In that junction box, I'll splice to a weatherproof conduit and run out to the disconnect box.
    - The disconnect box will be a 60 amp disconnect (because, again, that's all they sell at the big box) but I'll get a fused version and put 25 amp fuses in there.

Hopefully the HVAC guys will take it from there. Am I way off? Is a 30 amp breaker too much for the 25 amp requirement that my HVAC guy spec'd?

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Get and follow the manufacturers specs. 25 Amp breaker may stop a small over heat were 30 Amp will hurt your motor/unit before the breaker pops. The disconnect fuses should be enough. *Did not check NEC for 10/2 ratings –  Jason Aug 20 '13 at 14:31
    
Are you saying the disconnect (25 amp) fuses will (probably) protect the motor/unit even with an actual 30 amp breaker at my main breaker box? –  milkboneUnderwear Aug 20 '13 at 14:37
    
Yes, just like your individual breakers works in comparison to your main/service breaker. –  Jason Aug 20 '13 at 14:45
    
In my limited research on this, I think I found that the 125 or 115% doesn't apply to central air condensers: The manufacturers of the condensers already build in the extra allowance on their spec's. I don't have the unit to look at the nameplate, but the HVAC company is telling me that 25 amps are required. Turns out I can't put a 25 amp fuse on a 60 amp disconnect because the 25 amp fuses are too small. So it looks like I'll be ordering a 25 amp breaker and going with a non-fused disconnect. I'll verify that 10/2 NM wire is sufficient for 25 amps. –  milkboneUnderwear Aug 20 '13 at 17:55
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1 Answer

up vote 2 down vote accepted

In the US, it's required that manufacturers include the following items on condensing unit's nameplate (NEC 440.4(B)):

  • Maker's name
  • Rating in volts
  • Frequency
  • Number of phases
  • Minimum supply circuit conductor ampacity
  • Maximum rating of the branch-circuit short-circuit and ground-fault protective device

You'll simply use the minimum supply circuit conductor ampacity value from the name plate, and NEC 2011 Table 310.15(B)(16) through Table 310.15(B)(19) to determine the proper conductor size.

As for the branch-circuit short-circuit ground-fault protection, you'll have to select one less than or equal to the maximum rating listed on the nameplate. However, you'll have to choose a device with a high enough rating to handle the inrush current, so you'll want to choose a device close to this value.

Example Nameplate

tl;dr

To answer your questions directly...

  • A 30 ampere breaker is the wrong one, if the nameplate lists 25 as the maximum rating of the branch-circuit short-circuit and ground-fault protective device.

  • For 25 amperes, you're likely correct to use 10 AWG cable.

  • Usually the disconnect is not fused, and simply functions as a safety disconnect. You'll just have to make sure the disconnect is rated for greater than 25 amperes, and the appropriate voltage.

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