I have a motor that is rated for 2HP, 480V, 3PH. The feeder originates from a 20A circuit breaker at a panel and terminates to a 30A fused disconnect with 15A fuses. Is the upstream 20A breaker in violation of NEC 430.62 as it is sized greater than the maximum allowed 250% inverse time breaker in table 430.52?
480V, 3PH, 2HP motor feeder
- Thread starter pete25
- Start date
- Location
- New Jersey
- Occupation
- Journeyman Electrician
What size is the wire?
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
The 15 Amp fuses are the branch circuit protection for the motor. The feeder protection does not matter in this case.
IMO, if the feeder coming from the CB to the disconnect meets the tap rules it can be #14. Otherwise it would need to be #12. However, you might check the rules for sizing motor feeders to make sure I remembered this correctly.
#12
There are no motor taps here, its a single feeder serving a fused disconnect that terminates into a VFD.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
The tap rule would be for the tap from the feeder cb to the fuse.
Eddie702
Licensed Electrician
- Location
- Western Massachusetts
- Occupation
- Electrician
If he is running #12 and is on a 20A breaker how is it a tap?
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
It's not. There is no real question about wire sizing if it is a #12.
The question is that based on NEC 430.62(A) if you have a fused disconnect 15A fuses providing branch circuit OCP, your feeder OCP cannot be greater than the branch circuit OCP. My question is if I have a 20A CB at the panel terminating #12 awg to a fused(15A fuses) disconnect at the VFD, am I violating 430.62?
- Location
- Connecticut
- Occupation
- Engineer
See 430.62(B)
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
The tabular FLC is 3.4A. So that would 6A for a time delay fuse (175% of FLC, round up to 6A standard size)? Or 15A for a non-time delay fuse (300% of FLC, round up to 15A standard size?
Cheers, Wayne
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The size of the motor is basically immaterial.
The 20A breaker is a feeder protecting #12 which meets code.
The 15A fuses feed a VFD per the additional info provided in post #6.
The VFD needs to be programmed for the 2HP motor.
What size fuses are required by the VFD manufacturer? What is the full load rating of the VFD input?
The 20A breaker is a feeder protecting #12 which meets code.
The 15A fuses feed a VFD per the additional info provided in post #6.
The VFD needs to be programmed for the 2HP motor.
What size fuses are required by the VFD manufacturer? What is the full load rating of the VFD input?
My error, I( missed the VFD in Post #6.)
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Confused . . to paraphrase (2020 NEC) 430.130(A) on SCGF protection when there is a VFD, it says for sizing the SCGF protection:
(1) use the normal procedure based on the motor FLC, but per Exception 1, if the VFD is listed and marked "Suitable for Output Motor Conductor Protection", you may use the normal procedure based on the VFD input current
(2) but if the VFD documentation has a lower limit, that lower limit applies
So it sounds like the the normal procedure based on motor FLC may be the controlling procedure. Are you saying that in practice in most every VFD circuit either (1) Exception 1 or (2) will in fact apply?
Cheers, Wayne
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Only for the past 30 years or so.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
For Listed devices and those from major manufacturers.
Make-them-yourself or unlisted imports are probably problematic