A/C wiring question

[brewchief]

As an HVAC installer for over 20 years I have have become fairly knowledgeable about the electrical codes that determine wire size and type to power an A/C condenser but a recent facebook discussion made me wonder if I understand it 100%.

It was posted that a unit with a minimum circuit ampacity of 18 amps and a max breaker size of 30 amps was wired with #14 thwn and a 30 amp breaker.

I feel that this is code compliant as long as all of the equipment has terminals rated at 75c or better.

Many felt that it would burn the world down and kill a busload of nuns for a variety of reasons.

#14 thwn is good for 20 amps in the 75c column and we are allow to protect at the max breaker size so it would be compliant.

Am I wrong and this does not meet code?

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[Dennis Alwon]

Welcome to the forum

Firstly, what is your wiring method. If it is nm then it is only rated 60C however you have the correct idea. If the wiring method is thhn in conduit then you are correct because the overload protection in the unit will protect the conductors and the ocdp only protects from ground fault and short circuit.

 

[brewchief]

Since this is a semi hypothetical question we will say it is thhn in conduit. If nm-b was used it would need to be #12 as far as I can tell.


Another point was raised about having to derate the wire due to high ambient temp, say if this was in Vegas or Phoenix . This is really where it got a little fuzzy for me. My understanding is that you can use the higher rating of the wire(if marked for 90c) and use the 90c column to make the necessary adjustment( or is it a correction) even though the terminals are only rated for 75c.

#14 thwn has an amp rating of 25 amps in the 90c column so I can use that and multiply it by the adjustment factor and if it's still over the 20 amps it's good for in the 75c column then it can be used for the full 20 amps if I understand correctly.

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[Dennis Alwon]

Since this is a semi hypothetical question we will say it is thhn in conduit. If nm-b was used it would need to be #12 as far as I can tell.


Another point was raised about having to derate the wire due to high ambient temp, say if this was in Vegas or Phoenix . This is really where it got a little fuzzy for me. My understanding is that you can use the higher rating of the wire(if marked for 90c) and use the 90c column to make the necessary adjustment( or is it a correction) even though the terminals are only rated for 75c.

#14 thwn has an amp rating of 25 amps in the 90c column so I can use that and multiply it by the adjustment factor and if it's still over the 20 amps it's good for in the 75c column then it can be used for the full 20 amps if I understand correctly.

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Sounds like you have a good understanding of the de-rating process. Use the 90C rating to de-rate as long as the final ampacity adjustment is not higher than the 75C rating

If you are using nm you still de-rate from the 90C but the final ampacity would be at the 60C rating

 

[Coppersmith]

Since this is a semi hypothetical question we will say it is thhn in conduit. If nm-b was used it would need to be #12 as far as I can tell.

You said this is the condenser and condensers are outside. You cannot use NM-B in any outside conduit or in the whip. The inside of conduit used outside is considered a wet area. NM is not allowed in wet places. You would have to use UF.

It would be OK to use NM if it is coming through a wall directly into the disconnect. Then no conduit is involved.

 

[kwired]

You said this is the condenser and condensers are outside. You cannot use NM-B in any outside conduit or in the whip. The inside of conduit used outside is considered a wet area. NM is not allowed in wet places. You would have to use UF.

It would be OK to use NM if it is coming through a wall directly into the disconnect. Then no conduit is involved.

And UF also needs minimum ampacity selection based on 60 C but can have ampacity adjustments made from 90C if it is UF-B cable.

 

[texie]

Wow, an A/C guy who understands conductor sizing better than many electricians and inspectors.

 

[brewchief]

You said this is the condenser and condensers are outside. You cannot use NM-B in any outside conduit or in the whip. The inside of conduit used outside is considered a wet area. NM is not allowed in wet places. You would have to use UF.

It would be OK to use NM if it is coming through a wall directly into the disconnect. Then no conduit is involved.

Yes I understand that, when replacing old equipment we frequently find whips made from nm-b, apparently many of the electricians in my area didn't get the message that you can't run nm-b outside (unless it was permitted in the 90s when most of the units I find wired kike that were installed)FWIW it would be very rare for me to use less then a #10 thwn whip from the disconnect to the unit, we can buy a premade whip for less then the cost of the parts needed to build one.

How about this as a scenario, old unit minimum circuit ampacity of 16, max breaker 30, unit is fed with #12 nm-b and is on a 30 amp breaker. Replacement unit has similar minimum circuit ampacity but has a max breaker size of 25, can I replace the unfused disconnect with a fused disconnect with 25 amp fuses and meet code or must the breaker be changed to a 25 amp?

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[Dennis Alwon]

How about this as a scenario, old unit minimum circuit ampacity of 16, max breaker 30, unit is fed with #12 nm-b and is on a 30 amp breaker. Replacement unit has similar minimum circuit ampacity but has a max breaker size of 25, can I replace the unfused disconnect with a fused disconnect with 25 amp fuses and meet code or must the breaker be changed to a 25 amp?

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Yes you can replace the unit with a fused disconnect but personally I would replace the breaker. Around here a 25 amp breaker is in stock at the supply companies but not so much in other places.

 

[david luchini]

Yes you can replace the unit with a fused disconnect but personally I would replace the breaker. Around here a 25 amp breaker is in stock at the supply companies but not so much in other places.

Wouldn't changing the disconnect to a fused disconnect make the 30A c/b a feeder c/b? The #12 feeder conductors would then not be properly protected by the feeder c/b.

 

[Dennis Alwon]

Wouldn't changing the disconnect to a fused disconnect make the 30A c/b a feeder c/b? The #12 feeder conductors would then not be properly protected by the feeder c/b.

Technically, yes however I brought this up to the cmp at our yearly meeting in Raleigh and they said that they would look at the entire run as a branch circuit.

Interesting when I tried to write a proposal to change the definition I was turned down as they felt there was no need for clarification... Who knows. Seriously, though.... will the fuses really make a difference in the functioning? No so I wouldn't worry too much about it but you are correct- it could be rejected.

As a safety issue I think it is a no brainer

 

[paulengr]

That's no different than the pull fuse boxes I've seen all over the country.

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[kwired]

That's no different than the pull fuse boxes I've seen all over the country.

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Many of those type probably aren't listed as a branch circuit device either, just a supplementary device.

 

[mtfallsmikey]

Wow, an A/C guy who understands conductor sizing better than many electricians and inspectors.

Not all of us are hacks, even those of us who do electrical and plumbing too!
The good HVAC installers have had to learn about conductor sizing over the years as things have changed.

 

[FionaZuppa]

Sounds like you have a good understanding of the de-rating process. Use the 90C rating to de-rate as long as the final ampacity adjustment is not higher than the 75C rating

If you are using nm you still de-rate from the 90C but the final ampacity would be at the 60C rating

So, as good as the weakest link, regardless, yes? Kinda like using "short" #8 wire on a 15A ocpd.

 

[brewchief]

So another question, was powering an A/C with a disconnect directly off the meter can ever code legal? I know it would fail if done today because of the disconnects not being grouped but did it ever meet code? FWIW it is a service rated fused disconnect.

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[electrofelon]

It could be perfectly fine, however potential issues are:

1. Grouping of service disconnects (as you mentioned)

2. How two sets are landed on the load side if the meter.

3. Now the original set is not carrying the entire load of the dwelling unit, so may be undersized if next size up rule doesn't work out.

 

[MAC702]

Where is the other service disco?

 

[brewchief]

Where is the other service disco?

Other service disconnect would be the main breaker in the panel that's below the meter in the basement.

Here in MI there is(or was in the previous code cycle at least) an exception to the grouping of disconnects rule for separately metered A/Cs but it certainly wouldn't apply here.

I hadn't thought about how it was connected to the load side, I'm sure it's simply both wires under each lug, that's the same way the utility does it when a separate meter is set for the A/C, big difference is that it's on their side of the meter in that case.

While not super common here we do see enough A/C units powered this way that it must have been found acceptable, however having the inspector pass it and have it truly meet code can be two different things.

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[kwired]

So another question, was powering an A/C with a disconnect directly off the meter can ever code legal? I know it would fail if done today because of the disconnects not being grouped but did it ever meet code? FWIW it is a service rated fused disconnect.

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Per NEC, if it was ever legal I think it would have been long enough time ago that equipment in your picture wasn't being made yet.