10 on 40 amp

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lmchenry

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I am looking at a package unit that has a mca of 27 and a mop of 40. The contractor has run a #10 from the disconnect to the unit and supplied a 40 amp breaker at the disconnect. Is this ok because of 440.33?
 
You need a 27 amp conductor. Even at 60 degrees C a #10 is good for 30 amps. The OCPD does not have to correspond to the size of the conductor.
 
In other words, yes. The over-current protection is within the equipment, so the breaker is relieved of this function.

Plus, with the MCA and MOP spec'ed, we are relieved of having to calculate, well, anything. The numbers given are it.
 
So I have a little trouble with the logic. If I have a fault situation then it is possible to have 40 amps on the #10 before the breaker trips, right?
 
lmchenry said:
So I have a little trouble with the logic. If I have a fault situation then it is possible to have 40 amps on the #10 before the breaker trips, right?
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Right. In the case of a ground fault, you might even have much more than that - for a much shorter period of time, of course.
 
During a fault condition, the amount of current that is available to the system based on Ohms Law may be substantially higher in value than the value on the overcurrent device.

(That is part of the reason for the AIC/AIR rating stamped on an overcurrent device)


When installing circuits/effect ground fault current paths, one would like the circuit to have the least amount of impedance, as more current will flow. With the inverse property of most overcurrent devices, this is exactly what we want to occur during a ground fault.


Manufacturers of A/C equipment are required to perform the calculations of their equipment. In following the nameplate of the equipment, 240.4(G) and Table 240.4(G) will lead you to Art 440 which provides info to let you know that following the nameplate will in effect give one some relief in the size relationship of the overcurrent device and conductor sizing.
 
I understand the concept that there can be more on the circuit for a short period of time and I understand the ratings of the a/c equip. However, I wonder why there is an exception for a/c compared to any other circuit. The conductor is what we are trying to protect, so why give exception in this case?
 
lmchenry said:
IThe conductor is what we are trying to protect, so why give exception in this case?
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Because the thermal overloads in the motor will not allow the conductor to be overloaded beyond its ampacity for any great length of time.
 
lmchenry said:
I understand the concept that there can be more on the circuit for a short period of time and I understand the ratings of the a/c equip. However, I wonder why there is an exception for a/c compared to any other circuit. The conductor is what we are trying to protect, so why give exception in this case?
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The motors start under a load and there is a considerable amount of in rush current for a very short time that is required to get the motor started. The OCPD has to be able to function without tripping during start up. For motors that have to start under a load you can (in many cases must) have an OCPD of many times the rating of the conductors feeding the motor.
 
Couldn't there be a short between the wires prior to getting to the motor, or in some other part of the equipment, since we are dealing with a package unit? And what particular code deals with the package unit?
 
lmchenry said:
Couldn't there be a short between the wires prior to getting to the motor, or in some other part of the equipment, since we are dealing with a package unit?
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If there is a short the breaker will trip instantly because the current during a short will be 1000's of amps, well above the handle rating.
 
lmchenry said:
I understand the concept that there can be more on the circuit for a short period of time and I understand the ratings of the a/c equip. However, I wonder why there is an exception for a/c compared to any other circuit. The conductor is what we are trying to protect, so why give exception in this case?
Click to expand...

The breaker is only providing short circuit protection. As Peter said the compressor has it's own overload protection built in. That is how the conductor is protected from overload.
 
lmchenry said:
How about the code reference for a package unit?
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Nothing special about a packaged unit. They follow the same rules as mentioned in this thread.

Size the conductors for the minimum circuit ampacity listed on the unit, and size the OCPD for the max size listed on the unit, and you will be good to go.
 
lmchenry said:
I understand the labeling. I just cant find the code that allows the 10 to be on a 40 for a package unit.
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There is no special code.

Article 440 covers a packaged unit because it contains a compressor.
 
Just because 10 works for 27 amps and the max ocp is 40, doesnt mean that you cant put a 30 for the 10. And 240 sends me to 440 parts III and VI which doesnt seem to fit for this situation. Sorry. Please help me understand.
 
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