I'm hoping someone can help me understand this. I got a feeling this section is kinda miss leading.
WARNING - Its likda long post.
NEC quote:
Size these branch-circuit conductors no smaller than thespec on the equipment nameplate. If the equipment does not have a nameplatespecifying the branch-circuit conductors, size the conductors per [440.32](Figure 440-5).
One Motor-Compressor. Suppose your equipment is not marked with minimumcircuit ampacity. For each single motor compressor, use a branch-circuit conductorampacity not less than 125 percent of the motor-compressor current.Protect branch-circuit conductors against short circuits and ground faultsbetween 175 percent and 225 percent of the rated-load current. See[440.22(A)].
So if I read this correctly. Here is an example; I have achiller without a name plate. Chillerhas one compressor RLA 52amps. So here we go.
52RLA divided by 1.56to find compressor current (per UL). NowI multiply that by 125% = 42amps (rounded off) conductor required looks like a #8 THHN 90c or even a #8 75c would coverit. Now for branch circuit protection 52amp RLA (rated load current) x 175% = 91amps. I guess a 100amp breakerwould suffice.
NOW does anyone feel mydiscomfort in protecting a #8 conductor with a 100amp breaker???
Here is another NEC quote:
Several Motor-Compressors. Conductors that supply severalmotor-compressors must have an ampacity of not less than 125 percent of the highest-ratedmotor-compressor current of the group plus the sum of the rated-loadcurrents of the other compressors. Protect these conductors against shortcircuits and ground faults per [440.22(B)(1)].
Now lets do the multiple compressor scenario. This chiller has two compressors of the samesize.
1st compressor: 52amps RLA x 125% = 65amps
2nd compressor: 52amps RLA added to above = 117amps
Branch circuit protection would be 117 x 175% = 204amps breaker size? 225amps I guess.
---------------------------------------------------------------------------------------------------------------------------------
Now lets assume same chiller has a tag for the wholechiller. It says MCA 125.6 recommended 150amps MOCP 175
Using the 440 nec code and assuming we go with recommened 150amp (this is not RLA or actualcurrent, it’s recommended and has been adjusted for UL and NEC requirements) Icould use 1/0 for my conductor in either 90c or 75c. and the circuit protection breaker would also be 150amps. After all that is the recommended size protection by themanufacture.
Conclusion; the first two up there IMO really don't make any logical sense.
The last one, of course you need a name plate to go by. But even in this case, why would you multiply the MCA or recommended by 125% if it has already been adjusted by the manufacture? AND reading this "Size these branch-circuit conductors no smaller than the spec on the equipment nameplate." What spec are they reffering to? the recommended amps, the RLA listed for each compressor, the MCA, the MOP???
Sorry for long post.... any light on this would really be appreciated.
WARNING - Its likda long post.
NEC quote:
Size these branch-circuit conductors no smaller than thespec on the equipment nameplate. If the equipment does not have a nameplatespecifying the branch-circuit conductors, size the conductors per [440.32](Figure 440-5).
One Motor-Compressor. Suppose your equipment is not marked with minimumcircuit ampacity. For each single motor compressor, use a branch-circuit conductorampacity not less than 125 percent of the motor-compressor current.Protect branch-circuit conductors against short circuits and ground faultsbetween 175 percent and 225 percent of the rated-load current. See[440.22(A)].
So if I read this correctly. Here is an example; I have achiller without a name plate. Chillerhas one compressor RLA 52amps. So here we go.
52RLA divided by 1.56to find compressor current (per UL). NowI multiply that by 125% = 42amps (rounded off) conductor required looks like a #8 THHN 90c or even a #8 75c would coverit. Now for branch circuit protection 52amp RLA (rated load current) x 175% = 91amps. I guess a 100amp breakerwould suffice.
NOW does anyone feel mydiscomfort in protecting a #8 conductor with a 100amp breaker???
Here is another NEC quote:
Several Motor-Compressors. Conductors that supply severalmotor-compressors must have an ampacity of not less than 125 percent of the highest-ratedmotor-compressor current of the group plus the sum of the rated-loadcurrents of the other compressors. Protect these conductors against shortcircuits and ground faults per [440.22(B)(1)].
Now lets do the multiple compressor scenario. This chiller has two compressors of the samesize.
1st compressor: 52amps RLA x 125% = 65amps
2nd compressor: 52amps RLA added to above = 117amps
Branch circuit protection would be 117 x 175% = 204amps breaker size? 225amps I guess.
---------------------------------------------------------------------------------------------------------------------------------
Now lets assume same chiller has a tag for the wholechiller. It says MCA 125.6 recommended 150amps MOCP 175
Using the 440 nec code and assuming we go with recommened 150amp (this is not RLA or actualcurrent, it’s recommended and has been adjusted for UL and NEC requirements) Icould use 1/0 for my conductor in either 90c or 75c. and the circuit protection breaker would also be 150amps. After all that is the recommended size protection by themanufacture.
Conclusion; the first two up there IMO really don't make any logical sense.
The last one, of course you need a name plate to go by. But even in this case, why would you multiply the MCA or recommended by 125% if it has already been adjusted by the manufacture? AND reading this "Size these branch-circuit conductors no smaller than the spec on the equipment nameplate." What spec are they reffering to? the recommended amps, the RLA listed for each compressor, the MCA, the MOP???
Sorry for long post.... any light on this would really be appreciated.