using #12 on a 25A breaker for motors - OK, except when VFD is present?

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malachi constant

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A few years ago I asked (and received!) an explanation for why it is allowed to feed a motor from a 25A breaker using a #12. Links to the original posts are noted below.

The summary is: The code allows you to size the motor feeder per 430, and size the OCPD per table 310.16. For a 208V/3P 5HP motor, for instance, you can have a #12 feeder on a 25A circuit. We are used to the "small conductor" clause in 240.4(D), but a careful reading of 240.4(D) and (G) clearly shows that the small conductor requirement does not apply to motors.

Well. Yesterday I had an inspector note that the small conductor requirement does pertain to VFDs. 240.4(G) allows an exception from 240.4(D) for Article 430, Parts II-VII. But VFDs are covered in Article 430 Part X. And since 430 Part X is not listed in 240.4(G), then 240.4(D) does apply. Therefore, since I cannot downsize the OCPD to 20A, I need a #10 to feed my motor. This is dizzying, yes?

It appears to apply only to a few very specific motor sizes, where it actually makes a difference if you have or don't have a VFD:
* 208V/1P, 3HP -> 18.7 FLA
* 208V/3P, 5HP -> 16.7 FLA

Unfortunately I have about a dozen 5HP motors, all with VFDs, all with #12s already pulled. I don't see a way around this. Assuming all the above is correct I will tweak my master motor schedule accordingly such that it assigns #10s for all 208/1/3HP and 208/3/5HP motors, in case a VFD is present.

Original post: http://forums.mikeholt.com/showthread.php?t=126987
Follow-up post that clarified grounding: http://forums.mikeholt.com/showthread.php?t=135768
 
I'd say the inspector is incorrect.

240.4(G) references Art 430 Part IV for Motor Branch Circuit Short Circuit and Ground Fault Protection.

Art 430 Part X applies to Adjustable Speed Drive Systems, and says in 430.120...

The installation provisions of Part I through Part IX are applicable unless modified or supplemented by Part X.
Click to expand...

I see nothing in Part X that modifies or supplements the requirements of Part IV for Motor Branch Circuit Short Circuit and Ground Fault Protection. Part IV should apply to the vfd installation.
 
infinity said:
What's the input current on the VFD's?
Click to expand...

key point ! :D

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.
 
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I'd still say op should be per 430 Part IV and that exempts it from the requirements of 240.4 (D).
 
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david luchini said:
The OP notes 18.7A on a 3 HP single phase unit and 16.7A on a 5 HP three phase unit. Both permit #12 AWG based on 125% of the input current.

But that doesn't address the overcurrent protection requirements.
Click to expand...

I only have a draft of '14, but it appears that 430.130 directly addresses the issue. FWIW
david luchini said:
I'd still say op should be per 430 Part IV and that exempts it from the requirements of 240.4 (D).
Click to expand...

I agree. 430 is exempt not just a portion of 430

EDIT: I THINK I AM MISTAKEN See Post 11
 
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infinity said:
What's the input current on the VFD's?
Click to expand...

augie47 said:
key point ! :D

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.
Click to expand...
I still say the motor needs conductors sized per 430 part II. The supply to the drive needs sized according to drive input rating. Remember the power factor on the drive input will be high, the power factor on the drive output will be low. The drive input could be single phase, the drive output (for most general purpose drives anyway) will be three phase. The drive is power conversion equipment not just a simple contact controller.

add: the motor conductors on a drive output are still protected from overloading by the motor overload protection built into the drive.
 
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I'm trying to get the taste of crow from my mouth.
You would think I would learn to read 1st.
ON SECOND THOUGHT, I agree with the inspector
240.4(G) specifically states Art 430 Part III,IV,V,VIO and VII.

Since drives are 430 X, they do not fall under the exception and 240.4(D) would apply, IMO.
 
augie47 said:
I'm trying to get the taste of crow from my mouth.
You would think I would learn to read 1st.
ON SECOND THOUGHT, I agree with the inspector
240.4(G) specifically states Art 430 Part III,IV,V,VIO and VII.

Since drives are 430 X, they do not fall under the exception and 240.4(D) would apply, IMO.
Click to expand...
BUT... 240.4 is regarding OCPD... not conductor ampacity.

As David said earlier, 430.122 states Parts I through IX appply unless modified by Part X. OCPD rating is not modified under Part X [2011]. Even 430.130 [2014] sends you to 430.52(C)(1), (C)(3), (C)(5), or (C)(6)... where branch OCPD is not specified by mfgr... which is in Part IV.
 
I got lost in all that, so I'm at a loss for a reply except to say I have no problem with the #12 conductor for the load..
My only comment is that since we are using #12 for what ever reason,
we would need to follow 240.4(D)(5) since 430 Part X is not excluded.
If the OP can't reduce the breaker to a 20, then he would need to increase the wire size.
 
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OK so is a regular water heater allowed to be wired with #12?

Is it ok on a 30 amp breaker?
 
Sierrasparky said:
OK so is a regular water heater allowed to be wired with #12?

Is it ok on a 30 amp breaker?
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240.4(D) tells you a #12 must be protected by an OCP with a max rating of 20 amps unless as covered in 240.4(E)or(G).
Don't see where a water heater is covered by either.. so... no 30 amp on a #12 feeding a water heater........ although we have all see such.
 
augie47 said:
I got lost in all that, so I'm at a loss for a reply except to say I have no problem with the #12 conductor for the load..
My only comment is that since we are using #12 for what ever reason,
we would need to follow 240.4(D)(5) since 430 Part X is not excluded.
If the OP can't reduce the breaker to a 20, then he would need to increase the wire size.
Click to expand...

The overcurrent protection for the motor circuit supplied via the VFD is specified in Art 430 Part IV. Art 430 Part IV is excluded from the requirements of 240.4 (D) via 240.4 (G).

The overcurrent protection requirements are same for a motor that is on a vfd as for one that is not.
 
augie47 said:
I got lost in all that, so I'm at a loss for a reply except to say I have no problem with the #12 conductor for the load..
My only comment is that since we are using #12 for what ever reason,
we would need to follow 240.4(D)(5) since 430 Part X is not excluded.
If the OP can't reduce the breaker to a 20, then he would need to increase the wire size.
Click to expand...
BUT, AGAIN...

240.4(D)(5) states 20 amperes is the maximum overcurrent protection rating for a 12AWG copper conductor circuit unless specifically permitted to be greater through 240.4(G) in this case. I'm certain we can agree on this part.

Next, forget that the wire size is #12 for the moment and consider here that the VFD is not the load... the motor it controls is the load.

The rating for this circuit's SC/GF protection is determined under Article 430, Part IV... which is included in Table 240.4(G)... and that is how the breaker is specifically permitted to be greater than the 20 amperes maximum stated in 240.4(D)(5).
 
I see the point.. not sure I concede
IF your view the correct one, then if one were under the '14 Code where 430.130 (Part X) specifies GFSC device for VFDs the install would be non-compliant, correct ?
 
augie47 said:
240.4(D) tells you a #12 must be protected by an OCP with a max rating of 20 amps unless as covered in 240.4(E)or(G).
Don't see where a water heater is covered by either.. so... no 30 amp on a #12 feeding a water heater........ although we have all see such.
Click to expand...

Thanks
 
augie47 said:
I see the point.. not sure I concede
IF your view the correct one, then if one were under the '14 Code where 430.130 (Part X) specifies GFSC device for VFDs the install would be non-compliant, correct ?
Click to expand...

I don't think the new language in '14 changes anything.

430.130 says to provide OCP of the size and type as specified in 430.53(C)...which is in Part IV...which still exempted from the 240.4(D) small conductor requirements.
 
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