AC Conductor sizing

Please help me understand this. AC Condenser has a name plate rating as follows. (Min Circuit amp.) 24.4 (Max Fuse or Circuit BKR.) 40 amps

Branch Circuit is #10 (60 degree C 30 amps.) How is the the correct size branch circuit?

The name plate lists the MAX circuit BKR not the Required circuit brk.

Would this not require a #8 branch circuit or a 30 amp brk?

Thank you in advance for your input on this.
 

Dennis Alwon

Moderator
Staff member
You could actually use #12 however the reason that you can use 40 amp overcurrent protective device on a #10 is because the overcurrent protective device is only giving protection for short circuit and ground fault. The overloads in the compressor are protecting the wire from overloading-- or from overcurrent. Without the overloads there would be an issue
 

Dennis Alwon

Moderator
Staff member
BTW, look at 240.4(D) which takes you to 240.4(G). This is assume #12 is not at 60C -- if nm cable is used then #10 would be needed
 

ActionDave

Moderator
Staff member
Please help me understand this. AC Condenser has a name plate rating as follows. (Min Circuit amp.) 24.4 (Max Fuse or Circuit BKR.) 40 amps

Branch Circuit is #10 (60 degree C 30 amps.) How is the the correct size branch circuit?

The name plate lists the MAX circuit BKR not the Required circuit brk.

Would this not require a #8 branch circuit or a 30 amp brk?

Thank you in advance for your input on this.
Kinda freaks you out huh? It's a legit and legal install.
 
You could actually use #12 however the reason that you can use 40 amp overcurrent protective device on a #10 is because the overcurrent protective device is only giving protection for short circuit and ground fault. The overloads in the compressor are protecting the wire from overloading-- or from overcurrent. Without the overloads there would be an issue
Should there not be some type of information on the device indicating this protection?
 

ActionDave

Moderator
Staff member
Should there not be some type of information on the device indicating this protection?
There is. That is what the MCA - Minimum Circuit Ampacity and MOCP - Maximum Over Current Protection printed on the nameplate of the unit is all about.
 

Carultch

Senior Member
Please help me understand this. AC Condenser has a name plate rating as follows. (Min Circuit amp.) 24.4 (Max Fuse or Circuit BKR.) 40 amps

Branch Circuit is #10 (60 degree C 30 amps.) How is the the correct size branch circuit?

The name plate lists the MAX circuit BKR not the Required circuit brk.

Would this not require a #8 branch circuit or a 30 amp brk?

Thank you in advance for your input on this.
Motors change a lot of what you though you knew about how load current relates to OCPD, and thus relates to wire size. So expect a lot of sizing results that wouldn't match what you'd do for any other load on a 40A breaker. And for A/C systems, the dominant load is the motor that drives the compressor, usually part of the outdoor unit. For feeders and branch circuits in general, it is the case that you need as much wire size as is necessary to "round up" to the OCPD you use. But for A/C units, this isn't the case. The reason the NEC treats motor loads and HVAC units this way, is that motors tend to spike the load current upon startup, but settle during the steady state on a much lower full load current. A resistive heating unit in an HVAC system will likely have the same sizing strategy that you would use for loads in general, but a motor load HVAC unit will not.

The way to work with AC units is to observe the MCA (minimum circuit amps) and maximum overcurrent device (MOC) values. Minimum circuit amps selects the wire at (usually) 75C, and maximum overcurrent device tells you what branch circuit breaker to use. If applicable, you would also follow (usually) the 90C column for your temperature and bundling derate calculations, to verify this is also above the MCA. You select as high of amps on your maximum circuit breaker as you can, to avoid the outcome of nuisance tripping. There is internal protection in the unit that will protect against overload current, and that internal protection will be selected with a slow time-current curve to avoid nuisance tripping. The branch circuit breaker is mostly there to trip when protecting against short circuits.
 
Motors change a lot of what you though you knew about how load current relates to OCPD, and thus relates to wire size. So expect a lot of sizing results that wouldn't match what you'd do for any other load on a 40A breaker. And for A/C systems, the dominant load is the motor that drives the compressor, usually part of the outdoor unit. For feeders and branch circuits in general, it is the case that you need as much wire size as is necessary to "round up" to the OCPD you use. But for A/C units, this isn't the case. The reason the NEC treats motor loads and HVAC units this way, is that motors tend to spike the load current upon startup, but settle during the steady state on a much lower full load current. A resistive heating unit in an HVAC system will likely have the same sizing strategy that you would use for loads in general, but a motor load HVAC unit will not.

The way to work with AC units is to observe the MCA (minimum circuit amps) and maximum overcurrent device (MOC) values. Minimum circuit amps selects the wire at (usually) 75C, and maximum overcurrent device tells you what branch circuit breaker to use. If applicable, you would also follow (usually) the 90C column for your temperature and bundling derate calculations, to verify this is also above the MCA. You select as high of amps on your maximum circuit breaker as you can, to avoid the outcome of nuisance tripping. There is internal protection in the unit that will protect against overload current, and that internal protection will be selected with a slow time-current curve to avoid nuisance tripping. The branch circuit breaker is mostly there to trip when protecting against short circuits.
Thank you for this information.
 

infinity

Moderator
Staff member
Just keep in mind that the OCPD can be sized according to the minimum circuit ampacity and is not required to be the MaxOCPD listed on the unit.
 

kwired

Electron manager
Motors change a lot of what you though you knew about how load current relates to OCPD, and thus relates to wire size. So expect a lot of sizing results that wouldn't match what you'd do for any other load on a 40A breaker. And for A/C systems, the dominant load is the motor that drives the compressor, usually part of the outdoor unit. For feeders and branch circuits in general, it is the case that you need as much wire size as is necessary to "round up" to the OCPD you use. But for A/C units, this isn't the case. The reason the NEC treats motor loads and HVAC units this way, is that motors tend to spike the load current upon startup, but settle during the steady state on a much lower full load current. A resistive heating unit in an HVAC system will likely have the same sizing strategy that you would use for loads in general, but a motor load HVAC unit will not.

The way to work with AC units is to observe the MCA (minimum circuit amps) and maximum overcurrent device (MOC) values. Minimum circuit amps selects the wire at (usually) 75C, and maximum overcurrent device tells you what branch circuit breaker to use. If applicable, you would also follow (usually) the 90C column for your temperature and bundling derate calculations, to verify this is also above the MCA. You select as high of amps on your maximum circuit breaker as you can, to avoid the outcome of nuisance tripping. There is internal protection in the unit that will protect against overload current, and that internal protection will be selected with a slow time-current curve to avoid nuisance tripping. The branch circuit breaker is mostly there to trip when protecting against short circuits.
One more thing to add to this- The higher device setting is needed to prevent tripping during the short startup period, that setting does protect from short circuits and ground faults, conductor overload protection is still necessary but along with such loads is required motor overload protection - which inherently will never allow the conductors to be overloaded as long as they are sized according to minimum conductor requirements of 430 or 440.
 
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