Motor Feeder, Fuse and Wire sizing

[mkgrady]

I looked at a job today where I am going to wire two 75HP compressors. I'd like to have some eyeballs go over my plan. Here's what I have:

Switchboard with existing Westinghouse 400 amp CB (where the trip unit can be changed out to reach up to 800 amps). It is an LM frame style. I hope to leave the CB alone but I suspect I need to change the trip unit.

The compressors will be fed through a new transfer switch so only one can run at a time. The load side(s) of the transfer will feed two fused disconnects. One for each compressor package.

Compressor packages come with internal starters and other controls. One of the packages comes with a built-in VFD for soft starting, etc. and the instructions say to just supply it through fused disconnect with class J fuses. The max starting amps of the unit is 221. The chart on the instructions say to provide a max fuse of 300 amps for this unit.

The non-VFD compressor package has a standard hard start starter and the FLA is 228 amps. The instructions say to use a time delay fuse. So I multiply by 175% to arrive at 400 amp fuse size.

To determine feeder wire size to the transfer switch I multiplied 228 FLA times 125% (= 285 amps) so I figure I will use 300 MCM copper THHN.

To determine feeder breaker size I figured 228 amps times 250% to arrive at 570 amps. So I figure the 400 amp existing breaker is too small.

Any comments? How much did I get wrong? Any responses would be appreciated.

 

[augie47]

compressor

compressor

assuming 208 volt (I don't believe you stated), your 228 is a lil higher than the number the NEC would have you used (based on 430.250).
If your 400 amp breaker has adjustable (or high enough) trip, my experience would be that it will most likely be sufficient.

1 opinion

 

[mkgrady]

assuming 208 volt (I don't believe you stated), your 228 is a lil higher than the number the NEC would have you used (based on 430.250).
If your 400 amp breaker has adjustable (or high enough) trip, my experience would be that it will most likely be sufficient.

Yup, It's 208V. A sticker on the CB indicates there are replaceable trip units. I think the increments were 25 or 50 amps all the way up to 800. So I'm thinking it is not adjustable for that reason but I'm not sure.

 

[micromind]

I think Augie was referring to the mag trip setting. Alot of big breakers have an adjustable magnetic (inrush) trip. Set it low for feeders, high for motors. It's usually near the load end, frequently there is one per phase.

I think a 400 amp breaker is fine for starting a 75 HP motor on 208, even if it's not adjustable. It'll certainly hold the VFD, and likely be ok across-the-lines as well.

The wire size is ok, but I question the need for the 2 fused disconnects. Is it because they're out of sight of the 400 amp breaker? I don't see the need for fuses on the across-the-lines unit, it's protected by the feeder breaker.

Frequently, packaged units come with their own fused switches built in, if so then a separate fused disconnect wouldn't be needed.

 

[mkgrady]

I think Augie was referring to the mag trip setting. Alot of big breakers have an adjustable magnetic (inrush) trip. Set it low for feeders, high for motors. It's usually near the load end, frequently there is one per phase.

I think a 400 amp breaker is fine for starting a 75 HP motor on 208, even if it's not adjustable. It'll certainly hold the VFD, and likely be ok across-the-lines as well.

The wire size is ok, but I question the need for the 2 fused disconnects. Is it because they're out of sight of the 400 amp breaker? I don't see the need for fuses on the across-the-lines unit, it's protected by the feeder breaker.

Frequently, packaged units come with their own fused switches built in, if so then a separate fused disconnect wouldn't be needed.

I'll look for some adjustment on the existing 400A CB tomorrow.

I agree the fused disconnects seem unnecessary but the manufacturer emphatically states that a disconnect with Class J fuses must be provided for the VFD unit or the warranty is void.

On the accross-the-line unit it calles for time delay fuses but I suspect they would not have a problem with the feeder CB doing the OCPD.

Each unit needs a disconnect because the feeder CB is out of sight. I just started thinking maybe I could use the transfer switch as the disconnect(s) and have one load out with class J fuses and the other load non-fused. Sounds like an expensive switch but may be cheaper than three switches described in OP. Trouble is I need the Class J fused switch ASAP so I may have to keep them seperate.