VFD wire gauge: manual vs NFPA 79

I wired up a few VFDs and I'm confused by the required wire gauge. I used a pair of Nidec (Control Techniques) Unidrive M300's, running on single-phase 110VAC. One is 1/3HP, the other 1.5HP. Per the manual, both of these required a 15A breaker.

With the wire ampacity tables from NFPA 79, I need 14ga from the breaker to the drive, and, because there's no circuit protection after the drive, I also have 14ga from the drive to the motor. If this were a direct-on-line motor, I would put a small breaker on the 1/3HP motor and be able to use much smaller wire.

If I go look at the manual, however, Nidec recommends 16ga on the input to the 1/3HP and 10ga on the input to the 1.5HP. They recommend 16ga on the output for both of them.

This problem would appear to be far worse if I'd used a different brand of VFD. The 1.5HP Delta drive wants a 50A breaker but has a maximum wire size of 14ga.

I'm trying to understand the correct way to wire these. In reality, the answer is that the gauges from the manual should be fine, but I would like to understand how to do an NFPA 79 compliant installation.

The standard pretty much does not recognize the existence of anything under 14 ga for power wiring.

There is no particular issue with using larger wire than required, aside from economic issues (where applicable).

When you put those two facts together, you come up with just using 14 ga copper (12 ga aluminum) as the smallest power wiring.

As for the 10 ga, that will depend on the length of run, although there can be some advantage to larger wire when used with rectifier circuits. Rectifier circuits for VFDs use average and RMS current that is usually much lower than the peak, because conduction tends to be in short peaks, nothing like a sine wave. So the larger (lower resistance) wire can lead to a better (higher) voltage on the VFD internal bus. With the 15A breaker you would otherwise be perfectly justified in using 14 ga copper wire.

I might mention that it is unlikely that you can get 1.5 HP through a 15A breaker, unless that VFD has a power factor correction circuit (PFC). And, if it did, then you would not need to up-size the wire going to the VFD, since a PFC eliminates the high peak currents.

JST said:

There is no particular issue with using larger wire than required, aside from economic issues (where applicable).

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The economic issues are negligible. Any difference in wire costs is rounding error. The big problems for me are physical. First, getting the wire into the VFD. I like to ferrule everything, and then 14ga won't fit, so I had to skip the ferrules. Second, it drives up the size of the cable going to the motor, which makes the cable bigger and less flexible. Now I need a bigger, larger radius cabletrack, etc.

15A breaker cannot be right for the 1-1/2HP if the input is 115V. Remember, the OUTPUT current is not the same as the input current on a VFD, especially one that is converting single phase to 3 phase, then again one that is doubling the voltage. A 1-1/2HP 230V 3 phase motor is 6A, so the single phase 230V input would be 6 x 1.732 = 10.4A at 230V, so at 115V it will be double that, roughly 21A, which is why you need the 10ga wire on the input side (input conductors must be sized at 125% of the VFD input current, 12ga is only good for 20A). So you are likely reading something incorrectly.

Only the OUTPUT conductors are to be based on the motor FLA, and 14ga is the smallest you can use on motor circuits anyway. NFPA 79 is the standard for building a machine control panel, the wires that leave the panel going to the motor fall under the NEC, which is where the 14ga minimum comes into play.

I am assuming that either there is confusion, OR the 15A breaker is required by UL for the VFD.

Agree that 1.5HP is not coming through that w/o a PFC. And it does not seem as though there is a PFC in the unit.

If the OP has a problem with total wire size including the insulation he might look into the wire that has the insulation rating required for the voltage used and the various types that are correctly rated but are very thin such a THHN and others. Might be able to fit them where thicker insulated will not fit. Though fill limits exist.

Thank you all for the replies, sorry for the delay in replying. The customer was here for testing, and things got busy.

Jraef said:

15A breaker cannot be right for the 1-1/2HP if the input is 115V. Remember, the OUTPUT current is not the same as the input current on a VFD, especially one that is converting single phase to 3 phase, then again one that is doubling the voltage. A 1-1/2HP 230V 3 phase motor is 6A, so the single phase 230V input would be 6 x 1.732 = 10.4A at 230V, so at 115V it will be double that, roughly 21A, which is why you need the 10ga wire on the input side (input conductors must be sized at 125% of the VFD input current, 12ga is only good for 20A). So you are likely reading something incorrectly.

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screenshot of the manual attached. It says 15A unless I'm completely misreading it.

Jraef said:

Only the OUTPUT conductors are to be based on the motor FLA, and 14ga is the smallest you can use on motor circuits anyway. NFPA 79 is the standard for building a machine control panel, the wires that leave the panel going to the motor fall under the NEC, which is where the 14ga minimum comes into play.

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My understanding is the NFPA 79 covers the entire machine. UL508A is the standard that only covers panels.

I would have to say that something got lost in translation for that table..... The "MCB" rating listed is obviously wrong in the last two rows.

Nobody would consider a 15A breaker for 18 or 24A "continuous draw". The breaker might pass the current, for a considerable time, because breakers are not very precise, and they have response time curves vs current.... But any "molded case circuit breaker" has to be rated at 125% of continuous draw per the standard. The definition of "continuous draw" is one issue here. last I saw, that was continuing at the given current level for 3 hours or more, which is arguably unlikely in the case of a lathe or mill in that HP category...... but the manual for the VFD would cover an all day operation of a fan, or a pump as well, which would easily meet the 3 hour time.

Bottom line is that the table appears to be unreliable... wrong. Going purely on the basis of continuous pump load, the 24A draw ought to show a 30A breaker, as that is 125% of 24A. A 25A breaker would be suitable for the 18.8A draw, if that rating is available. Otherwise that also would be 30A.

Note that I have NO IDEA whether UL tested the unit with those breaker sizes, so I am not giving a recommendation.

Also, the table lists 100V as the input voltage, so the currents are presumably higher than would be appropriate for 120V.

Is there a similar table for 120V input? Does it agree with the table you posted?

Yes, UL508 covers control panels. The NEC does not get into those details, simply using the description "listed for the purpose" and leaving the listing agency open. UL, ETL, and so forth, there are at least a half dozen of so that I have seen at one time or another. They all would, however, test to UL 508.