Motor Feeder Question...

[chevyx92]

I have been giving the following info for a new install. New RTU will have an MCA of 98A 3Phase 208V with Max Fuse Size listed at 125A. There will also be an indoor blower motor rated at 1.5HP 208V single phase. 430.248 shows that motor to be 11 amps.

Scoped job out and found a nearby 3 phase 208V feeder not being used. It has #2 CU coming into a Non Fused 200A disconnect which is fed from a 400A Fused disconnect with 250A fuses installed.

My questions are:

1) With an MCA of 98 amps do you think I can get by fusing at 100A instead of 125A? I ask only because there's an existing 100A Fused disconnect there not being used I could possibly use. If I have to fuse at 125A then I will have to install
a 200A fused disconnect with 125A fuses.

2) Is the #2 CU code compliant to feed both these motors? I think so. MCA of 98A plus the 11A of blower motor = 109 A #2 CU is good for 115 A

3) Feeder OCPD. According to 430.62 if I take the largest motor rating which is the RTU of 98A plus 125% = 122.5A plus 100% of all other motor loads, blower is 11A that would equal = 133.5A Correct?????

4) If 133.5A is correct then we fuse down right? Which means the feeder OCPD would be set at 125A? Right?

 

[kwired]

I have been giving the following info for a new install. New RTU will have an MCA of 98A 3Phase 208V with Max Fuse Size listed at 125A. There will also be an indoor blower motor rated at 1.5HP 208V single phase. 430.248 shows that motor to be 11 amps.

Scoped job out and found a nearby 3 phase 208V feeder not being used. It has #2 CU coming into a Non Fused 200A disconnect which is fed from a 400A Fused disconnect with 250A fuses installed.

My questions are:

1) With an MCA of 98 amps do you think I can get by fusing at 100A instead of 125A? I ask only because there's an existing 100A Fused disconnect there not being used I could possibly use. If I have to fuse at 125A then I will have to install
a 200A fused disconnect with 125A fuses.

You mentioned it called for max fuse size of 125, last I knew 100 was less then 125, so you are ok on this.

2) Is the #2 CU code compliant to feed both these motors? I think so. MCA of 98A plus the 11A of blower motor = 109 A #2 CU is good for 115 A

That is true.

3) Feeder OCPD. According to 430.62 if I take the largest motor rating which is the RTU of 98A plus 125% = 122.5A plus 100% of all other motor loads, blower is 11A that would equal = 133.5A Correct?????

MCA already has 125% of largest motor/compressor factored into it, you do not need to add another 25%. 430.62 is giving you maximum permitted protective device, you can still go with something less as long as it permits starting of your loads.

4) If 133.5A is correct then we fuse down right? Which means the feeder OCPD would be set at 125A? Right?

The mentioned 109 amps in #2 would be the correct value after considering my reply to #3. This would then make the max OCPD for #3 be 110 amps. I myself would use the existing 100 amp fused disconnect - it will likely hold the starting current, and is plenty high enough setting once started.

 

[augie47]

I have been giving the following info for a new install. New RTU will have an MCA of 98A 3Phase 208V with Max Fuse Size listed at 125A. There will also be an indoor blower motor rated at 1.5HP 208V single phase. 430.248 shows that motor to be 11 amps.

Scoped job out and found a nearby 3 phase 208V feeder not being used. It has #2 CU coming into a Non Fused 200A disconnect which is fed from a 400A Fused disconnect with 250A fuses installed.

My questions are:

1) With an MCA of 98 amps do you think I can get by fusing at 100A instead of 125A? I ask only because there's an existing 100A Fused disconnect there not being used I could possibly use. If I have to fuse at 125A then I will have to install
a 200A fused disconnect with 125A fuses.

IMO, you can most likely be ok with the 100 amp.

2) Is the #2 CU code compliant to feed both these motors? I think so. MCA of 98A plus the 11A of blower motor = 109 A #2 CU is good for 115 A

#2 at 115 amps would be compliant as the feeder

3) Feeder OCPD. According to 430.62 if I take the largest motor rating which is the RTU of 98A plus 125% = 122.5A plus 100% of all other motor loads, blower is 11A that would equal = 133.5A Correct?????

4) If 133.5A is correct then we fuse down right? Which means the feeder OCPD would be set at 125A? Right?

Your feeder OCP, per 430.62, would be based on the max OCP for the largest motor plus the sum of the FLA for others.
Your OCP for your large motor is 125 plus the 11 = 136 amps.
I would say keep the existing 125 amp protection in the 400 amp disconnect as is.

 

[kwired]

IMO, you can most likely be ok with the 100 amp.

#2 at 115 amps would be compliant as the feeder


Your feeder OCP, per 430.62, would be based on the max OCP for the largest motor plus the sum of the FLA for others.
Your OCP for your large motor is 125 plus the 11 = 136 amps.
I would say keep the existing 125 amp protection in the 400 amp disconnect as is.

His RTU nameplate should already have 125% of largest motor factored into the MCA, as well as OCPD needed for largest motor plus all other loads. All he is doing is adding one more motor that is most likely not the largest motor of the entire group, so all that is needed is to add FLA of that motor to the MCA.

He did say he had existing 100 amp fused disconnect he was hoping to use. It is less then max OCPD permitted - but will likely hold during starting, worst case one may need delay relays to keep all motors from starting simultaneously.