Sizing OCPDs and Wire Sizes for Mutiple VFDS on one Branch Circuit

[jtgrace87]

I am in the middle of installing a feeder circuit from an MCC to a panel that contains three separate VFDs. Each VFD will be controlling a 25HP motor (per drive). I am wanting to verify that my calculations are correct in sizing my main feeder breaker, wire size from MCC to panel, individual breakers in front of VFDs and wires going out to motor. Could someone please confirm that I have used the correct NEC rules?

1. Sizing Feeder Protection Device in the MCC:

Per NEC 430.52- For 25HP Motor- FLA= 34 Amps

34A*1.5= 51 Amps

Per NEC 430.62- 51A + 34A + 34A= 118 Amps

** So my protection device in the MCC would be 110 Amp Molded Case Breaker

2. Sizing Feeder Conductors from MCC to drive panel

Per NEC 430.24- (34A*1.25)+34A+34A= 110.5 A
Using THHN I would use #3 AWG since it is good for 115A

3. Sizing Circuit Breaker on Line Side of VFDs

Per NEC 430.52- For 25HP Motor FLA= 34 Amps

34A*1.5- 51A

So I would use a 50 Amp Circuit Breaker on the line side of each VFD.

4. Sizing Conductors from 50A Circuit Breaker to VFD and from VFD to Motor

Per NEC 430.122-

34A*1.25= 42.5A

So therefore #8 AWG conductors shall be used.

 

[petersonra]

I am in the middle of installing a feeder circuit from an MCC to a panel that contains three separate VFDs. Each VFD will be controlling a 25HP motor (per drive). I am wanting to verify that my calculations are correct in sizing my main feeder breaker, wire size from MCC to panel, individual breakers in front of VFDs and wires going out to motor. Could someone please confirm that I have used the correct NEC rules?

1. Sizing Feeder Protection Device in the MCC:

Per NEC 430.52- For 25HP Motor- FLA= 34 Amps

OK

34A*1.5= 51 Amps

where does this come from?

Per NEC 430.62- 51A + 34A + 34A= 118 Amps

It is not a violation, but this is not what 430.62 requires.

** So my protection device in the MCC would be 110 Amp Molded Case Breaker

2. Sizing Feeder Conductors from MCC to drive panel

Per NEC 430.24- (34A*1.25)+34A+34A= 110.5 A
Using THHN I would use #3 AWG since it is good for 115A

agreed

3. Sizing Circuit Breaker on Line Side of VFDs

Per NEC 430.52- For 25HP Motor FLA= 34 Amps

34A*1.5- 51A

where is this 1.5 factor coming from?

So I would use a 50 Amp Circuit Breaker on the line side of each VFD.

4. Sizing Conductors from 50A Circuit Breaker to VFD and from VFD to Motor

Per NEC 430.122-

34A*1.25= 42.5A

So therefore #8 AWG conductors shall be used.

that is not what 430.122 actually says.

I would go back and look at what these sections actually say.

 

[iwire]

You must supply the VFDs per their rated input current not the motor HP.

 

[jtgrace87]

I am in the middle of installing a feeder circuit from an MCC to a panel that contains three separate VFDs. Each VFD will be controlling a 25HP motor (per drive). I am wanting to verify that my calculations are correct in sizing my main feeder breaker, wire size from MCC to panel, individual breakers in front of VFDs and wires going out to motor. Could someone please confirm that I have used the correct NEC rules?

1. Sizing Feeder Protection Device in the MCC:

Per NEC 430.52- For 25HP Motor- FLA= 34 Amps

OK

34A*1.5= 51 Amps

where does this come from?

Reply: I was under the impression that you could use an overcurrent protection device that did not exceed 250% of the Motors FLC (table 430.52). I used a 1.5 as my factor. Should I use 2.5 instead?

Per NEC 430.62- 51A + 34A + 34A= 118 Amps

It is not a violation, but this is not what 430.62 requires.

Reply: Can you clarify this rule for me?

** So my protection device in the MCC would be 110 Amp Molded Case Breaker

2. Sizing Feeder Conductors from MCC to drive panel

Per NEC 430.24- (34A*1.25)+34A+34A= 110.5 A
Using THHN I would use #3 AWG since it is good for 115A

agreed

3. Sizing Circuit Breaker on Line Side of VFDs

Per NEC 430.52- For 25HP Motor FLA= 34 Amps

34A*1.5- 51A

where is this 1.5 factor coming from?

Reply: I was under the impression that you could use an overcurrent protection device that did not exceed 250% of the Motors FLC (table 430.52). I used a 1.5 as my factor. Should I use 2.5 instead?

So I would use a 50 Amp Circuit Breaker on the line side of each VFD.

4. Sizing Conductors from 50A Circuit Breaker to VFD and from VFD to Motor

Per NEC 430.122-

34A*1.25= 42.5A

So therefore #8 AWG conductors shall be used.

that is not what 430.122 actually says.

Reply: Can you clarify this rule for me. Should I use the rated input current of the drive * 1.25 to get conductor size?

 

[Jraef]

Going the other direction:

You must actually read the VFD installation requirements FIRST. Many VFDs are NOT approved to be protected by circuit breakers, they must have fuses and often on the ones that require fuses, they will have a narrowly specific size based on their UL listing. Then if they do have UL listing behind CBs, the VFD mfr may have a maximum size limitation. Start there by reading the installation manual for the VFDs, paying close attention to the section on fuses and/or circuit breakers and SCCR requirements.

I would use the 125% rule for the Feeder size calcs, not 150%. The 250% allowance on the CB is for one motor to allow for the possibility of starting current surges causing nuisance tripping. There will be no starting current surge with a VFD. So (34 x 1.25) + (2x34) = 110.5A total, I'd use a 125A Feeder, unless there is something else in that control panel beside the VFDs.

430.122 only relates to sizing the conductors for the INPUT of the VFD based on the VFD max input amp rating, making those conductor sizes no longer tied to the motor size. 430.122 does not say anything about the conductor size going to the motor from the OUTPUT of the VFD. Nothing special there, 125% of the NEC 430.52 FLA chart for the HP of motor.

Just pay attention to what TYPE of cable you run based on what type of raceway you use. No THHN on the output of the VFD... <brace for flame war>: use RHH/RHW on the output side if you are using steel conduit, you want the XLPE insulation. If anything other than steel conduit, use shielded VFD cable; ground the shield drain wire at BOTH ends, ground the enclosure, ground the motor, bond the conduits etc. etc.. When in doubt, ground it out...

 

[petersonra]

There is nothing in the code that limits the feeder OCPD rating.

There is a restriction on the largest OCPD rating that can be used with the minimum ampacity allowed for the feeder conductors. However, it has no 1.5 or 1.25 factor in it. One might argue it has a 250% factor in it though.

If you want a feeder OCPD rating larger than this, you just have to increase the size of the feeder conductors.

 

[augie47]

I don't deal with drive installations much so this question sent me to the books and I have confused myself.
'08 leaves a whole lot to be desired when it comes to drive system feeders in general and definitely when it comes to multiple drives. '11 & '14 make do better at addressing feeders.
It seems to me that the drive input is used to size branch circuit conductors and the protection of those is most often base on manufacturer requirements.
Beyond the branch circuit supplying the individual drives all other calculations including conductors to the motor and feeders seem to be based on the Table values for the motor itself.
Is that a correct read ?