Condensing Unit Overcurrent Protection

[Dustin Foelber]

We have an condo rehab project where they are replacing the condensing units for HVAC. To these units we have single phase 240 10awg. Units min circuit ampacity is 22 but the maximum overcurrent protection is 35. The inspector is requesting us to change the feeder to #8 and install 35Amp breakers. My question is does the max OCP need to be followed or can your drop it down. Also is it NEC compliant to have 10 AWG on 2pole 30s feeding the unit yet 35 amp fuses protecting? I appreciate any input.

 

[Cavie]

#10 awg and 30 amp protection is all that is manufacturer required per the NEC. I would stand with that, HOWEVER some inspectors think the MAX stated is required. Over sized breakers and fuses are allowed on the MIN ampacity wire for A/C compressor motors. They are thermol protected. If the unit says CB/Fuses, CB is fine. If it says Fuses you must use fuses.

 

[texie]

We have an condo rehab project where they are replacing the condensing units for HVAC. To these units we have single phase 240 10awg. Units min circuit ampacity is 22 but the maximum overcurrent protection is 35. The inspector is requesting us to change the feeder to #8 and install 35Amp breakers. My question is does the max OCP need to be followed or can your drop it down. Also is it NEC compliant to have 10 AWG on 2pole 30s feeding the unit yet 35 amp fuses protecting? I appreciate any input.

The inspector is wrong. If the MCA is 22 amp, you need wire with an ampacity of at least 22 amp. If you are doung this with conduit and THHN it could even be a #12. If the max breaker is shown to be 35 amp, that is fine regardless of the wire size. This complies with the rules of Art 240 and 440. Bottom line:the breaker and wire size do not have to match-this is urban legend.

 

[Dennis Alwon]

There is no need to change the wire size as you can use the max 35 amp overcurrent protective device. The unit has overload protection built in so the overcurrent protective device is only for ground fault and short circuit protection

 

[infinity]

With an MCA of 22 amps there is nothing that would prohibit you from using a 25 amp OCPD either.

 

[GoldDigger]

With an MCA of 22 amps there is nothing that would prohibit you from using a 25 amp OCPD either.

That also came up in the thread discussing Mike Holt's AC disconnect video.
The main thing that would encourage using the maximum OCPD size given by the manufacturer would be to avoid nuisance trips. Rather than evaluate the nuisance trip potential on a case-by-case basis or have to come back to replace the breaker later, going straight to the maximum size has some merit.
Once again, a specific aspect of the topic on which different reasonable people will come to different conclusions.
You are absolutely right that the code itself does not keep you from using a smaller breaker.

 

[infinity]

The inspector is wrong. If the MCA is 22 amp, you need wire with an ampacity of at least 22 amp. If you are doung this with conduit and THHN it could even be a #12. If the max breaker is shown to be 35 amp, that is fine regardless of the wire size. This complies with the rules of Art 240 and 440. Bottom line:the breaker and wire size do not have to match-this is urban legend.

I agree, you could use #12 AWG MC cable also with a 35 amp OCPD. The inspector missed the boat on this one.

 

[kwired]

The inspector is wrong. If the MCA is 22 amp, you need wire with an ampacity of at least 22 amp. If you are doung this with conduit and THHN it could even be a #12. If the max breaker is shown to be 35 amp, that is fine regardless of the wire size. This complies with the rules of Art 240 and 440. Bottom line:the breaker and wire size do not have to match-this is urban legend.

Pretty much my thoughts on this topic also. Many balk at seeing 12 AWG on a 35 amp breaker. Another example where they may balk even more is a 5hp 3ph 208 volt motor that could be connected with 12 AWG and can have a 45 amp breaker protecting it, without having to see if the breaker will hold during starting - otherwise the breaker setting possibly could be even higher if it will not start.

 

[templdl]

Pretty much my thoughts on this topic also. Many balk at seeing 12 AWG on a 35 amp breaker. Another example where they may balk even more is a 5hp 3ph 208 volt motor that could be connected with 12 AWG and can have a 45 amp breaker protecting it, without having to see if the breaker will hold during starting - otherwise the breaker setting possibly could be even higher if it will not start.

This is how I look at it:
Basically the breaker provides short circuit protection for the wire since its rating is much greater than the rating of the wire. And as such the motor itself shold have overload protection which essentially protects the wire from overload. It's almost like a tap rule when you think about it and the wire is terminated of a singe device.
The current of the wire is determined buy what the wire is terminated on. As such is there any way that you could overload the wire?
It's similar to the 250% rule for the primary protection of a transformer when secondary protection is provided.

 

[texie]

With an MCA of 22 amps there is nothing that would prohibit you from using a 25 amp OCPD either.

Agreed. The issue I have with the OP's situation, as harsh as it sounds, this is so basic that I question the expertise of the inspector. We have enough competence issues in the trade without having an inspector making it worse.

 

[jap]

I always have mixed feelings about what everyone considers "Overload" protection when it comes to your standard outdoor
condensing unit. Although I agree with eveybody else that installing a smaller guage wire and a larger short circuit protector is generally
allowed on a motor load with true overload protection such as Overload Heaters installed in conjunction with an actual motor starter.

I've always felt that a simple 2p contactor along with the thermal overload switch on the side of a compressor found on most standard outdoor condensing units is not near as accurate an overload device to be able to compare it to a Starter with Oveload Heaters set
at a specific amperage at which they trip.

To me the thermal overload switch on the side of the compressors is more of a last line of defense just prior to a compressor
burning up rather than a dependable means of taking the compressor out of the circuit do to your general overload situation.

 

[kwired]

I always have mixed feelings about what everyone considers "Overload" protection when it comes to your standard outdoor
condensing unit. Although I agree with eveybody else that installing a smaller guage wire and a larger short circuit protector is generally
allowed on a motor load with true overload protection such as Overload Heaters installed in conjunction with an actual motor starter.

I've always felt that a simple 2p contactor along with the thermal overload switch on the side of a compressor found on most standard outdoor condensing units is not near as accurate an overload device to be able to compare it to a Starter with Oveload Heaters set
at a specific amperage at which they trip.

To me the thermal overload switch on the side of the compressors is more of a last line of defense just prior to a compressor
burning up rather than a dependable means of taking the compressor out of the circuit do to your general overload situation.

Why is this any different than any motor with integrated overload protection? If anything I trust them more than field adjustable overload devices, as nobody is going to tamper with settings.

The reason the compressor burns up is because these devices automatically reset once the overload has cooled. The compressor motor continues to get hotter and hotter if it fails to start and keeps cycling due to overload resetting. This auto reset is an advantage to high pressure in the lines when starting, but if a capacitor should fail they will just keep recycling and motor will get hot. If motor was failing anyway then it doesn't matter if it recycles, the motor was shot anyway.

 

[jumper]

Why is this any different than any motor with integrated overload protection? If anything I trust them more than field adjustable overload devices, as nobody is going to tamper with settings.

The reason the compressor burns up is because these devices automatically reset once the overload has cooled. The compressor motor continues to get hotter and hotter if it fails to start and keeps cycling due to overload resetting. This auto reset is an advantage to high pressure in the lines when starting, but if a capacitor should fail they will just keep recycling and motor will get hot. If motor was failing anyway then it doesn't matter if it recycles, the motor was shot anyway.

Yep. Agreed.

 

[ActionDave]

Yep. Agreed.

Yeppers. Agreed again.

 

[jap]

Why is this any different than any motor with integrated overload protection? If anything I trust them more than field adjustable overload devices, as nobody is going to tamper with settings.

The reason the compressor burns up is because these devices automatically reset once the overload has cooled. The compressor motor continues to get hotter and hotter if it fails to start and keeps cycling due to overload resetting. This auto reset is an advantage to high pressure in the lines when starting, but if a capacitor should fail they will just keep recycling and motor will get hot. If motor was failing anyway then it doesn't matter if it recycles, the motor was shot anyway.

I'd put a field adjustable overload up against an integral overload for motor load protection any day of the wek.

 

[jap]

At least the Field Adjustable Overload can sense the current getting too high and shut the starter down before damage occurs.
An integrated Overload generally trips due to the sheer heat of the motor after the amperage has been too high for too long.

 

[kwired]

I'd put a field adjustable overload up against an integral overload for motor load protection any day of the wek.

I would too if guaranteed to be properly selected and adjusted. Other than phase loss protection feature I like thermal element overloads better because you don't have operators tampering with settings, unless you have a setting where there is a supply room and additional elements readily available, did have a case one time where they did just that, because there was easy access to a supply of elements.

 

[iwire]

I would take an internal over external any day of the week.

Either way the motor is heating up during an overload condition.

 

[jap]

I would take an internal over external any day of the week.

Either way the motor is heating up during an overload condition.

That's right, but it's where the Thermal Overload is sensing its heat from, is what makes the difference.

I feel the actual current going through a set of field overloads is more accurate than an internal thermal
overload since the Internal Overload is usually relying on heat applied to the outside of it to open, instead
of the current going through it like on a external overload.

But to each his own.

 

[jap]

I would too if guaranteed to be properly selected and adjusted. Other than phase loss protection feature I like thermal element overloads better because you don't have operators tampering with settings, unless you have a setting where there is a supply room and additional elements readily available, did have a case one time where they did just that, because there was easy access to a supply of elements.

I didnt figure in the maintenance guys coming in and cranking the overload settings all the way up so
thier motor will run to distruction, course It's not my job to do that. I can't control ignorance.

If you put that into the equation then I'd be favoring the side of the integrated overload also.