Sizing wire size for AC Condensors

Status
Not open for further replies.
kwired said:
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.
Click to expand...

Isnt 14 rated 20amps at 75*C? 25 at 90*C? Ill check my code book.

But still, if its ok to overload a conductor here, then why not allow for it in other applications?
 
mbrooke said:
But if its safe to let a 60 amp breaker on 30amp wire clear an overload then why not extend it other application?
Click to expand...

Well they do. What size EGC is contained in a 6/3 cable?


Welders are another example, you could wire a welder that draws maybe 50 amps, use 12 AWG and a 60 amp breaker dependent on the duty cycle of the welder.
 
mbrooke said:
I have seen compressors put 30 amps on #14 for over half an hour with a dirty condenser.
Click to expand...

I find that hard to believe but even if so .... Did the 14AWG start a fire? Burn up a terminal? Discolor?

You have to remember that the NEC ampacity charts are very conservative.
 
iwire said:
Well they do. What size EGC is contained in a 6/3 cable?


Welders are another example, you could wire a welder that draws maybe 50 amps, use 12 AWG and a 60 amp breaker dependent on the duty cycle of the welder.
Click to expand...


#10. But chances are that egc will see a short circuit, not sustained overload.
 
iwire said:
I find that hard to believe but even if so .... Did the 14AWG start a fire? Burn up a terminal? Discolor?

You have to remember that the NEC ampacity charts are very conservative.
Click to expand...

Believe what you want if it helps with your own understanding of the world. Im only telling you what I encountered. I have no reason to make this up.

The 14 gauge wire didn't melt or burn. My honest opinion this isnt a code issue rather a manufacturing issue with a few AC compressors. As I said, I haven't seen it every time, but I have seen it enough to ponder it.
 
Sorry, I was looking at wrong table. One problem I have with the 2014 E-book is those tables have the title at the bottom instead of the top for some reason:( and I thought I was looking at table - 16 instead was looking at 17.
 
kwired said:
Sorry, I was looking at wrong table. One problem I have with the 2014 E-book is those tables have the title at the bottom instead of the top for some reason:( and I thought I was looking at table - 16 instead was looking at 17.
Click to expand...

Don't sweat it. :) Im having computer issues as well :(
 
Are Hermetic Compressors just motors?

Are Hermetic Compressors just motors?

No! ... They are more than motors. They get help from the return suction line vapor, which cools the motor.
 
We must also remember that the trip curve of the overloads in a compressor is sized for a minimum voltage drop, get into a case where the cable limits the current just a little and this can change, I always used a 60' rule of thumb to go to the next size wire above the minimum, I have seen some very long runs and customers having nothing but problems with compressor failures in these cases, but like Bob said, if close to the panel it should not be a problem, we just have to use some common sense.
 
I wonder if voltage drop is even a real problem with modern units. The unit on the side of my house has a minimum voltage of 187 volts (overall range of 187-253) with the standard supply of 240 volts it seems like voltage drop even in the double digit amount shouldn't keep the unit from running normally.
 
Voltage drop may not be an issue with modern AC units with capillary expansion tubes but longer than specified refrigerant line is:compressor suction pressure becomes low thereby causing lower cooling and burn out to the compressormotor.
 
mbrooke said:
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:
Click to expand...
Whether it is the breaker or the over load relay that clears a fault depends on the nature of the fault: a overload relay has no capacity to clear a short circuit whereas a breaker has. So the combination is to be designed to operate accordingly in case of a fault. Hence there is no safety issue if breakers clear overloads but there is one if over load relays were to take over short circuit clearing.
 
Sahib said:
Whether it is the breaker or the over load relay that clears a fault depends on the nature of the fault: a overload relay has no capacity to clear a short circuit whereas a breaker has. So the combination is to be designed to operate accordingly in case of a fault. Hence there is no safety issue if breakers clear overloads but there is one if over load relays were to take over short circuit clearing.
Click to expand...

Ive seen compressor under an overload condition fail to trip while the breaker does it. Don't know why but Ive seen it more than once.
 
mbrooke said:
Ive seen compressor under an overload condition fail to trip while the breaker does it. Don't know why but Ive seen it more than once.
Click to expand...
It is easy to see whether it is an abnormal condition or not: by plotting current-time characteristics of over load relay and circuit breaker obtained from manufacturers. If CB operates instead of OLR for a fault below the cross over point, it may be a maintenance issue.
 
Sahib said:
It is easy to see whether it is an abnormal condition or not: by plotting current-time characteristics of over load relay and circuit breaker obtained from manufacturers. If CB operates instead of OLR for a fault below the cross over point, it may be a maintenance issue.
Click to expand...


Id say abnormal since the breaker that tripped was well over the minimum circuit ampacity on the label.
 
Sahib said:
If the OLR was okay, there might be a mismatch between the OLR and CB. Have you checked?
Click to expand...





Id say the overload was not correct. Its should trip when the unit is drawing over the MCA. In this case it was so high it tripped the breaker which was 2 sizes over the MCA.
 
Status
Not open for further replies.
Top