Sizing Overcurrent for A/C Condenser

[jumper]

Not going to argue here, you may well be right.

I can not remember seeing all three of these:

• Minimum Circuit Ampacity
• Minimum overcurrent
• Maximum overcurrent

I can only recall seeing these

• Minimum Circuit Ampacity
• Maximum overcurrent

Uncommon IME, but some do list a min OCPD.

View attachment 16956

 

[Mgraw]

The Data sheet on the Trane example above states Max Fuse/Breaker 25 amps Recommended Fuse/Breaker 20 amps. It is interesting in that only the 2 ton in that series has a different Max/Recommended value. My guess would be this is what manufacturers here are doing instead of putting recommended Fuse/Breaker sizes on the label as foreign manufactures sometimes do.

 

[kwired]

If using 60 deg C conductors (NM cable is part of this), you can have a MCA of up to 20.0 amps and use 12 AWG copper conductor, regardless of what the MOCP is listed at. This for art 440 applications, 430 applications are very similar though, but in most instances you must use 125% of motor FLA instead of MCA.

 

[texie]

Uncommon IME, but some do list a min OCPD.

View attachment 16956

I'm not sure what to make of this. The UL Marking and Application Guide for Heating and Cooling Equipment would require the MCA and the MOCP for a condensing unit. AFAIK the Minimum OCPD marking would be optional on the manufacturers part as far as listing goes as I don't think the standard requires this. Either way, it does not have any impact on the MCA and choosing the conductor sizing.
As for the OP, I don't see how the AHJ can require the use of the MaxOCPD. The down side for the OP is that if they have occasional nuisance tripping on start-up he'll be forced to use the max. value.

 

[iwire]

I'm not sure what to make of this. The UL Marking and Application Guide for Heating and Cooling Equipment would require the MCA and the MOCP for a condensing unit. AFAIK the Minimum OCPD marking would be optional on the manufacturers part as far as listing goes as I don't think the standard requires this. Either way, it does not have any impact on the MCA and choosing the conductor sizing.

Thanks for the info.

 

[mtfallsmikey]

A FB group referenced one of Mike's videos on this subject, but they are non-electricians, but HVAC folks. I'm kind of old school, size the wire to the OCPD size, according to 440/430, and the video, you could drop the wire size down?

 

[cowboyjwc]

A FB group referenced one of Mike's videos on this subject, but they are non-electricians, but HVAC folks. I'm kind of old school, size the wire to the OCPD size, according to 440/430, and the video, you could drop the wire size down?

That's the way I did it too.

 

[mtfallsmikey]

These guys on this FB group really tried to draw and quarter me out, I dropped out of the group, being licensed in both HVAC and electrical, I cannot/could not undersize wiring just because Mike did a video (nothing personal, Mike). sounds like NEC/UL need to have a chat about this.

 

[ActionDave]

These guys on this FB group really tried to draw and quarter me out, I dropped out of the group, being licensed in both HVAC and electrical, I cannot/could not undersize wiring just because Mike did a video (nothing personal, Mike). sounds like NEC/UL need to have a chat about this.

The sizing of conductors for motors has been in place as long as I have been alive and it makes good sense. The NEC and UL don't need to change anything.

The conductors are protected from overload by the motors overload protection and you don't need a conductor as fat as your thumb to carry the running amps of the air conditioner. You do need a breaker big enough to handle the inrush for a few seconds while the motor starts. It all works fine and is as safe as can be.

 

[infinity]

These guys on this FB group really tried to draw and quarter me out, I dropped out of the group, being licensed in both HVAC and electrical, I cannot/could not undersize wiring just because Mike did a video (nothing personal, Mike). sounds like NEC/UL need to have a chat about this.

No offense but you're not getting the concept. You are not under sizing anything if the MCA is 30 amps and you use #10 conductors with a 50 amp OCPD when the MaxOCPD is 50 amps.

 

[peter d]

These guys on this FB group really tried to draw and quarter me out, I dropped out of the group, being licensed in both HVAC and electrical, I cannot/could not undersize wiring just because Mike did a video (nothing personal, Mike). sounds like NEC/UL need to have a chat about this.

What exactly is the problem that you see that the NEC and UL need to correct?

 

[texie]

These guys on this FB group really tried to draw and quarter me out, I dropped out of the group, being licensed in both HVAC and electrical, I cannot/could not undersize wiring just because Mike did a video (nothing personal, Mike). sounds like NEC/UL need to have a chat about this.

Let's see if we can help you understand that this not something that Mike Holt just made up. As Dave mentioned, these concepts have been in the code since dirt was invented, is well understood and rooted in sound engineering. Politely, you are just wrong and don't understand the concepts behind Art 430, 440. There is no requirement, and worse, no value or benefit in in making the conductor size match the OCPD in a case such as this or most any motor circuit. In fact in some cases it can be detrimental from an electrical standpoint or not even possible due to using larger conductors than the equipment was designed for. Hopefully we can help you understand the why of these rules/concepts by walking you through it.

Let's start with a question. What would you believe would be the required conductor size for a 100 HP, 480 volt motor that is supplied by a code allowed 350 amp inverse time breaker?

 

[electricalist]

Not to get off topic, but , is the purpose of the MIn OPD to protect the circuit and the the Max OPD to protect the equipment?


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[GoldDigger]

Not to get off topic, but , is the purpose of the MIn OPD to protect the circuit and the the Max OPD to protect the equipment?


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NO.
The purpose of the overload (not even mentioned in this sizing discussion) is to protect the equipment.
The purpose of MCA is to safely provide the needed running current of the device.
The purpose of Max OCPD is to protect the wire against shorts and ground faults.
And if the maker specifies a Min OCPD that is greater than the MCA it is probably in an attempt to reduce customer complaints of nuisance trips while starting. No safety consequences at all.

 

[electricalist]

Ty sir. I go with the max fuse and min wire size unless the drawings say otherwise


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[mtfallsmikey]

Ok fellas, I think I've got it now, thanks for your patience and the explanations.

 

[peter d]

The purpose of Max OCPD is to protect the wire against shorts and ground faults.

I disagree with you on this point, or at least the way you explained it. A short circuit or ground fault will be hundreds, if not thousands of amps, well above the handle rating and obviously in the instantaneous trip range. So let's say the Max OCPD is 60 amps, clearly even a 100 amp breaker will protect the wire against SC and GF.

The purpose of the max OCPD is to be large enough to allow for inrush without being too large for the motor/compressor combination while allowing for moderate overloading.

 

[ActionDave]

I disagree with you on this point, or at least the way you explained it. A short circuit or ground fault will be hundreds, if not thousands of amps, well above the handle rating and obviously in the instantaneous trip range. So let's say the Max OCPD is 60 amps, clearly even a 100 amp breaker will protect the wire against SC and GF.

The purpose of the max OCPD is to be large enough to allow for inrush without being too large for the motor/compressor combination while allowing for moderate overloading.

He's saying the same thing just coming at it from the other end of the rope.

 

[GoldDigger]

I disagree with you on this point, or at least the way you explained it. A short circuit or ground fault will be hundreds, if not thousands of amps, well above the handle rating and obviously in the instantaneous trip range. So let's say the Max OCPD is 60 amps, clearly even a 100 amp breaker will protect the wire against SC and GF.

The purpose of the max OCPD is to be large enough to allow for inrush without being too large for the motor/compressor combination while allowing for moderate overloading.

I think that we more or less agree. The OCPD is protecting the wires against shorts and ground faults, and the equipment maker, for whatever reasons of their own, has not passed tests on their own wiring before the overload with any larger external OCPD.
So it not the MaxOCPD designation itself the protects the circuit wires, it is the presence of any OCPD no less than MCA and no greater than MaxOCPD.
You are right that there is an incentive for the maker to test with as large an OCPD as necessary to avoid nuisance trips, but these days we are also seeing devices with MaxOCPD equal to MCA.