Sizing wire size for AC Condensors

[vino82]

For years now I run 10/2 to my AC condensers on a 240v 30a breaker. I have noticed lately that some condensers have max overcurrent protection size of 25a. Which I install accordingly instead of the 2pole30. Still running the 10/2. I have read on a few different websites that electricians are sizing their wire according to the nameplate rating "minimum circuit ampacity" 2 - 2 1/2 tons 14amps, 18amps... Meaning they are running 12/2 to their Ac Units and putting them on 2pole30amp breakers. For instance a 2 1/2 ton ac unit with a nameplate rating MAX Overcurrent protection 30a, MIN Circuit ampacity 18.7a

Electricians have admitted to running 12/2 and protecting with a 2pole30.

Is that legit? I am unsure

 

[texie]

For years now I run 10/2 to my AC condensers on a 240v 30a breaker. I have noticed lately that some condensers have max overcurrent protection size of 25a. Which I install accordingly instead of the 2pole30. Still running the 10/2. I have read on a few different websites that electricians are sizing their wire according to the nameplate rating "minimum circuit ampacity" 2 - 2 1/2 tons 14amps, 18amps... Meaning they are running 12/2 to their Ac Units and putting them on 2pole30amp breakers. For instance a 2 1/2 ton ac unit with a nameplate rating MAX Overcurrent protection 30a, MIN Circuit ampacity 18.7a

Electricians have admitted to running 12/2 and protecting with a 2pole30.

Is that legit? I am unsure

I think this thread will help you: http://forums.mikeholt.com/showthread.php?t=162987

 

[StarCat]

HVACR Recc's

HVACR Recc's

Its unfortunately not a well known or possibly well respected job practice with many Electricians running service for these types of units to install service to the MAX rating on the nameplate, but in the simplest terms its always a better way to go. If you do it this way you can never be blamed or requested to return and run the MAX service to the unit.
I am speaking as an HVACR Tech with a Mechanical License and near 30 years experience.
There may be many installations that seemingly can get away with minimum service sizing but there will always be some that definitely will not and the only solution at that point will be to run new service and most often on single phase jobs add factory specified hard start components. The ones that don't work out will be tripping the breaker on start under different and varying conditions. Maybe not all the time but they will trip.
The matter being is that compressors are hard to start especially at 208V-1ph.
Instantaneous starting current on a 5 ton unit in high heat conditions can easily be over 175A.

 

[kwired]

For years now I run 10/2 to my AC condensers on a 240v 30a breaker. I have noticed lately that some condensers have max overcurrent protection size of 25a. Which I install accordingly instead of the 2pole30. Still running the 10/2. I have read on a few different websites that electricians are sizing their wire according to the nameplate rating "minimum circuit ampacity" 2 - 2 1/2 tons 14amps, 18amps... Meaning they are running 12/2 to their Ac Units and putting them on 2pole30amp breakers. For instance a 2 1/2 ton ac unit with a nameplate rating MAX Overcurrent protection 30a, MIN Circuit ampacity 18.7a

Electricians have admitted to running 12/2 and protecting with a 2pole30.

Is that legit? I am unsure

You probably have gotten away with your approach of 10AWG and 30 amp breaker because most units you have connected are probably in the 2.5-3 ton range and are 240 volt single phase. Chances are most of them were for single family dwelling units, with approximately same cooling capacity needs as well.

Get into different voltages, number of phases or even just different capacity units and you really need to know how to size conductors and overcurrent protection.

Over time units have gotten more efficient as well and the amp ratings have dropped some because of increased efficiency. We also have different design types for compressors that may mean different starting characteristics which may effect the setting of overcurrent protection as it may need to hold a different level of starting current then some older unit needed.

 

[iwire]

Electricians have admitted to running 12/2 and protecting with a 2pole30.

Is that legit? I am unsure

Yes it can be legit.

You can (and in my opinion should) install the largest overcurrent device on the label. Even when you do that you may use conductor that is only rated to cover the the minimum circuit rating. It is safe, it is legit and I can point you to 240.4(G) as the code section that allows this.

The reason it is allowed is because the HVAC unit has built-in overload units that will shut the unit down if it is drawing to much current. The breaker or fuse will still protect the conductors from short circuits or ground faults.

A note on wiring sizing, while I am fully behind taking advantage of the code rules you must consider the length of the run. If it is a short run the smallest allowed conductor will likely work fine. If the run is long you may have hard starting issues due to voltage drop.

 

[infinity]

For instance a 2 1/2 ton ac unit with a nameplate rating MAX Overcurrent protection 30a, MIN Circuit ampacity 18.7a

Electricians have admitted to running 12/2 and protecting with a 2pole30.

Is that legit? I am unsure

With a 18.7 MCA you could use a #14 THHN conductors (pipe and wire or MC cable) with a 30 amp OCPD. NM cable would require #12 AWG conductors.

 

[iwire]

With a 18.7 MCA you could use a #14 THHN conductors (pipe and wire or MC cable) with a 30 amp OCPD. NM cable would require #12 AWG conductors.

I agree with Robs point above and will only add that the reason NM would have to be 12 AWG is because the NEC forces us to use the 60 C ampacity of NM while we can often use the 75 C ampacity of pipe and wire or MC cable.

(Although using cheap pull out type disconnects may limit you to 60 C for any wire type)

 

[vino82]

I appreciate the input. I have learned quite a bit, really quick. I think I will approach wiring size in the future appropriate to the nameplate rating while considering length of the run. Thanks again.

 

[tom baker]

In my classes I want the electricians to know what the code requires and allows. But then in the field they may elect to use a larger size conductor. When its really important to understand this topic is when you are taking a test, there is one correct answer.

 

[augie47]

Be prepared to "teach" your inspector. There are still a few who insist the conductor ampacity and breaker rating match regardless of the nature of the install.

 

[kwired]

Be prepared to "teach" your inspector. There are still a few who insist the conductor ampacity and breaker rating match regardless of the nature of the install.

And hope he is one that is willing to learn when he makes mistakes or you will get nowhere in a hurry.

 

[augie47]

Yep, I've been told that there's nothing worse than a 70+ year old stubborn inspector... no idea where that attitude originates, but I've been told

 

[mbrooke]

Yes it can be legit.

You can (and in my opinion should) install the largest overcurrent device on the label. Even when you do that you may use conductor that is only rated to cover the the minimum circuit rating. It is safe, it is legit and I can point you to 240.4(G) as the code section that allows this.

The reason it is allowed is because the HVAC unit has built-in overload units that will shut the unit down if it is drawing to much current. The breaker or fuse will still protect the conductors from short circuits or ground faults.

A note on wiring sizing, while I am fully behind taking advantage of the code rules you must consider the length of the run. If it is a short run the smallest allowed conductor will likely work fine. If the run is long you may have hard starting issues due to voltage drop.

You are 100% correct. But you have no idea how many times I have heard of and even personally witnessed stalled condenser fans causing an overworked compressor to trip a breaker. Yes, I know, the intention is to trip the compressor overload, but real world someone is dropping the balls on these overloads.

 

[kwired]

You are 100% correct. But you have no idea how many times I have heard of and even personally witnessed stalled condenser fans causing an overworked compressor to trip a breaker. Yes, I know, the intention is to trip the compressor overload, but real world someone is dropping the balls on these overloads.

Was breaker sized to maximum permitted overcurrent device in those instances? Did compressor stall in those cases?

Trip curve of the compressor overload will not necessarily be the same as that of the breaker. The breaker is designed to protect conductors not motors. A hermatic compressor will have different heat distribution characteristics then a NEMA general purpose motor as well, and may have completely different trip curve designed into overloads. Heat from compressor motor is likely taken away from windings by the refrigerant, and because of this can possibly take higher current for longer time then other motor designs.

If a condenser fan fails, refrigerant pressure is going to rise, it may get to the point of compressor stalling and sudden locked rotor current - the breaker trip curve maybe catches it first when that happens, and if not the max permitted breaker - even sooner.

 

[mbrooke]

Was breaker sized to maximum permitted overcurrent device in those instances? Did compressor stall in those cases?

Trip curve of the compressor overload will not necessarily be the same as that of the breaker. The breaker is designed to protect conductors not motors. A hermatic compressor will have different heat distribution characteristics then a NEMA general purpose motor as well, and may have completely different trip curve designed into overloads. Heat from compressor motor is likely taken away from windings by the refrigerant, and because of this can possibly take higher current for longer time then other motor designs.

If a condenser fan fails, refrigerant pressure is going to rise, it may get to the point of compressor stalling and sudden locked rotor current - the breaker trip curve maybe catches it first when that happens, and if not the max permitted breaker - even sooner.

Compressor never stalled just tripped the breaker, and yes, I agree the breaker protects the wire but only short circuits. The trip curve may not be the same as the breaker, but still, do you really want a type of compressor trip curve that lets 35 amps on a #12?

 

[iwire]

Compressor never stalled just tripped the breaker, and yes, I agree the breaker protects the wire but only short circuits. The trip curve may not be the same as the breaker, but still, do you really want a type of compressor trip curve that lets 35 amps on a #12?

Yes, absolutely safe. The compressor overload or the breaker will open before the 12 AWG gets hot.

If this was not safe the rules for this and other motor loads would have been changed long ago. The only reason this seems wrong is because it different.

 

[mbrooke]

Yes, absolutely safe. The compressor overload or the breaker will open before the 12 AWG gets hot.

If this was not safe the rules for this and other motor loads would have been changed long ago. The only reason this seems wrong is because it different.

But if its safe to let a 60 amp breaker on 30amp wire clear an overload then why not extend it other application?



I know what the theory implies, the overload trips before the breaker. But real world that's not what I always see. If it truly was the case the overload would always trip unless it was a short circuit. I have seen breakers clear overloads:blink:

 

[kwired]

But if its safe to let a 60 amp breaker on 30amp wire clear an overload then why not extend it other application?



I know what the theory implies, the overload trips before the breaker. But real world that's not what I always see. If it truly was the case the overload would always trip unless it was a short circuit. I have seen breakers clear overloads:blink:

Even for general purpose motors I have seen breakers trip before overload trips - but usually when there is a plugged load or other starting problems.

15 and 20 amp 120 volt loads - I don't look at too hard, especially some power tools that are trying to get all they can out of a 15/20 amp circuit, that is going to happen there. But other loads I seldom if ever have seen breaker trip first other then when there are stalling/starting type issues.

There is a difference between a motor steadily overloaded to say 135% vs one having starting/stalling issues that is maybe easily drawing 300-400% at times. The latter will be a race to see which protective device reaches it's designed trip point first.

 

[mbrooke]

Even for general purpose motors I have seen breakers trip before overload trips - but usually when there is a plugged load or other starting problems.

15 and 20 amp 120 volt loads - I don't look at too hard, especially some power tools that are trying to get all they can out of a 15/20 amp circuit, that is going to happen there. But other loads I seldom if ever have seen breaker trip first other then when there are stalling/starting type issues.

There is a difference between a motor steadily overloaded to say 135% vs one having starting/stalling issues that is maybe easily drawing 300-400% at times. The latter will be a race to see which protective device reaches it's designed trip point first.

But in those case of 120 volt loads you mention, the wire matches the breaker. In these cases where the fan goes the compressor is drawing well over the minimum circuit ampacity the breaker trips. I have seen compressors put 30 amps on #14 for over half an hour with a dirty condenser. A correct time current curve would trip it at most within 5 minutes. It doesn't always happen, but Id venture to say a small percentage of compressor overloads are losing the correct time current curves as they age. Perhaps not intended but it seems to be that way.

 

[kwired]

But in those case of 120 volt loads you mention, the wire matches the breaker. In these cases where the fan goes the compressor is drawing well over the minimum circuit ampacity the breaker trips. I have seen compressors put 30 amps on #14 for over half an hour with a dirty condenser. A correct time current curve would trip it at most within 5 minutes. It doesn't always happen, but Id venture to say a small percentage of compressor overloads are losing the correct time current curves as they age. Perhaps not intended but it seems to be that way.

Table 310.15(B)(16) says a 14 AWG copper is good for 30 amps @ 75C and the insulation is good for 35 amps @ 90C - what is the problem? The primary function of that overload is to protect the compressor, and that inherently protects the conductor.

I am not disagreeing with some of what you have seen, but I have more often seen overloads cutting out when there is a hard starting situation then I have seen breakers trip - mostly on single phase units and usually when trying to start too soon after a stop - before refrigerant head pressure has been reduced.