NEC Motor FLA table for single phase AC motors

[Twophase]

I have been advised to use the NEC FLA tables for sizing OCPDs, wires, etc (as opposed to the motor nameplate). I am sizing the OCPD for a VFD that feeds a 20HP single phase motor. NEC table 430.248 (2017 version) only goes up to 10HP. I can convert 20HP to amperes (HP*746/PF), but wanted to confirm if this is what should be done in this scenario. Or, would I just use the VFD nameplate value?

 

[LarryFine]

Or, would I just use the VFD nameplate value?

That.

 

[Twophase]

Thanks. If I didn't have a VFD, how would I size the OCPD given that the NEC table does not go past 10HP for single phase AC motors?

 

[Eddie702]

Thanks. If I didn't have a VFD, how would I size the OCPD given that the NEC table does not go past 10HP for single phase AC motors?

Good question and I don't know the answer. 20 hp on single phase is pretty rare. But with VFDs you size by the VFD nameplate

 

[texie]

I have been advised to use the NEC FLA tables for sizing OCPDs, wires, etc (as opposed to the motor nameplate). I am sizing the OCPD for a VFD that feeds a 20HP single phase motor. NEC table 430.248 (2017 version) only goes up to 10HP. I can convert 20HP to amperes (HP*746/PF), but wanted to confirm if this is what should be done in this scenario. Or, would I just use the VFD nameplate value?

You have a VFD feeding a single phase motor? This should be a single phase supply feeding a 3 phase motor.

 

[infinity]

I have been advised to use the NEC FLA tables for sizing OCPDs, wires, etc (as opposed to the motor nameplate). I am sizing the OCPD for a VFD that feeds a 20HP single phase motor. NEC table 430.248 (2017 version) only goes up to 10HP. I can convert 20HP to amperes (HP*746/PF), but wanted to confirm if this is what should be done in this scenario. Or, would I just use the VFD nameplate value?

What size OCPD does the VFD manufacturer recommend? You size the VF conductors by 125% of the VFD nameplate input current.

 

[petersonra]

I have been advised to use the NEC FLA tables for sizing OCPDs, wires, etc (as opposed to the motor nameplate). I am sizing the OCPD for a VFD that feeds a 20HP single phase motor. NEC table 430.248 (2017 version) only goes up to 10HP. I can convert 20HP to amperes (HP*746/PF), but wanted to confirm if this is what should be done in this scenario. Or, would I just use the VFD nameplate value?

Part X. Adjustable-Speed Drive Systems
430.120 General. The installation provisions of Part I through
Part IX are applicable unless modified or supplemented by
Part X.
430.122 Conductors — Minimum Size and Ampacity.
(A) Branch/Feeder Circuit Conductors. Circuit conductors
supplying power conversion equipment included as part of an
adjustable-speed drive system shall have an ampacity not less
than 125 percent of the rated input current to the power
conversion equipment.

430.130 Branch-Circuit Short-Circuit and Ground-Fault Protection
for Single Motor Circuits Containing Power Conversion
Equipment.
(A) Circuits Containing Power Conversion Equipment.
Circuits containing power conversion equipment shall be
protected by a branch-circuit short-circuit and ground-fault
protective device in accordance with the following:
(1) The rating and type of protection shall be determined by
430.52(C)(1), (C)(3), (C)(5), or (C)(6), using the fullload
current rating of the motor load as determined by
430.6.
(2) Where maximum branch-circuit short-circuit and groundfault
protective ratings are stipulated for specific device
types in the manufacturer's instructions for the power
conversion equipment or are otherwise marked on the
equipment, they shall not be exceeded even if higher
values are permitted by 430.130(A)(1).
(3) A self-protected combination controller shall only be
permitted where specifically identified in the manufacturer’s
instructions for the power conversion equipment or if
otherwise marked on the equipment.

 

[Twophase]

You have a VFD feeding a single phase motor? This should be a single phase supply feeding a 3 phase motor.

I meant to say single phase power to the VFD, then VFD outputs three-phase to the motor. Not the best configuration, but the manufacturer allows it.

 

[tom baker]

I meant to say single phase power to the VFD, then VFD outputs three-phase to the motor. Not the best configuration, but the manufacturer allows it.

That is a very good method for a 3 ph motor on a 1 ph supply. Much better than the rotary phase converters

 

[Twophase]

That is a very good method for a 3 ph motor on a 1 ph supply. Much better than the rotary phase converters

A quick web search indicates that many people have bought VFDs for this purpose alone.

 

[wolfsburged]

Be aware that some VFDs are rated for single phase operation and others are for three phase input but can operate with de-rating on single phase power input. All of the "single phase input" VFDs I have seen have been for smaller HP (typically under 3HP). I would just make sure that the VFD you specify for your 20HP motor is properly sized/de-rated for that 20HP load on single phase input. It will be pulling a lot of amps.