NEC: wire size to VFD input

[Rob#]

Normally, you size your wires according to the fuse before those wires. Except for drives (VFDs). NEC says that you have to size your wires according to the capacity of the drive times 1.25 (125%). (the drive's electical capacity, not the overload setting in the drive parameters!)

So, my Siemens G120X is specified with an input (LO mode) of 120A at 460V, my motor FLA is 110A. It can pull 110% for up to 60s at a time. So my formula for the wire ampacity is:
120A * 1.10 * 1.25 = 165A minimum.

Now, let's say I can buy the motor and the drive in bulk for a very good price and using the same motor+drive combination for various applications saves me a lot of money on spare parts in my warehouse. But in some of my applications this motor will be oversized and never run on more than 50%. I am allowed to do that.

Well, if my motor runs on 50% only, there is no need for a big fuse before the drive. (Siemens says for that drive the max fuse is 200A) 60A will be enough. So my fuse should be: 60 * 1.10 = 66A, let's do 80A. However, my wires still need to be sized for a 165A minimum (NEC!), because the capacity of the drive is what counts for the wires. But 165A wires after a 80A fuse makes no sense, does it?

Another typical scenario: customer calls: "need drive xy, production is down!" My Siemens supplier says: "xy has a lead time of three weeks, but we have yz on hand, which is one size bigger but will work with your motor". Then I say: "With yz the wires to the drive will be too small and need to be replaced (NEC) even though the motor will never pull more amps only because it has a bigger drive now."

 

[kwired]

I agree as worded it only makes sense if supplying the max allowed motor on the output.

More than once I have supplied a motor with a larger drive and kept original OCPD and supply conductors as it was less than max input current to the drive. Haven't burned anything down yet because of this. Code writers wear blinders at times.

 

[don_resqcapt19]

I agree as worded it only makes sense if supplying the max allowed motor on the output.

More than once I have supplied a motor with a larger drive and kept original OCPD and supply conductors as it was less than max input current to the drive. Haven't burned anything down yet because of this. Code writers wear blinders at times.

Just like the current tables for motors that we are required to use to size motor conductors, the rule for the size of the power conversion equipment input conductors is based on the idea of "worst case".

 

[kwired]

Just like the current tables for motors that we are required to use to size motor conductors, the rule for the size of the power conversion equipment input conductors is based on the idea of "worst case".

I get that, but if you are powering a motor only half the capacity of the drive then worst case goes down.

Not talking about putting a 480 V half HP motor on a 2 HP drive, where you will probably have same OCPD and conductor size for either motor but rather when you have a 100 HP drive but only supplying a 50, 60 or 75 HP motor with it. Ignoring there is the variable drive for a moment, chances are if you ever decide that you need a bigger motor you are likely needing to increase conductors, probably the OCPD, motor overloads unless electronic and still has new value in range, maybe motor contactor, and also maybe driven load isn't rated for higher HP than whatever was driving it anyway.

Nothing wrong IMO with reduced supply conductor if properly protected at it's ampacity and is not expected to exceed current level determined for the application other than maybe for temporary overload situations.

 

[tom baker]

RE: size of conductors for smaller drive
Code rules as Don said, are worst case. Code is Code as Mike Holt once said. You could try for a change to the 2026 NEC...

 

[Rob#]

Just like the current tables for motors that we are required to use to size motor conductors, the rule for the size of the power conversion equipment input conductors is based on the idea of "worst case".

In the example worst case is 80A. The worst case for ALL wires in my panel is based on their fuses/circuit breakers.

 

[texie]

I never could understand why the NEC has this requirement. To me it is like saying if you have a 200 MLO panelboard you must supply it with conductors of at least 200 amps of ampacity just because somebody might increase the load. It is also a widely violated rule and has no harm that I can see.

 

[paulengr]

It’s continuous rating, so you shouldn’t use the overload factor.

You are sizing for the drive front end which includes precharge. So this dictates OCPD. The drive manual has the OCPD required.

Drive current rating is often nothing to do with motor size. On a pump/fan application drive current can be sized closely to the motor. However many applications such as machine tools, conveyors, and especially crushers require significant overload capacity, up to 250% of motor size. So strict name plate interpretation of AWG would be vast overkill. Most people use motor sizing (125% of FLA).

 

[kwired]

Just like the current tables for motors that we are required to use to size motor conductors, the rule for the size of the power conversion equipment input conductors is based on the idea of "worst case".

A motor is a load a VFD is a power conversion device, what it draws worst case is limited by the motor it drives.

 

[Eddie702]

I think 125% of the drive input is all that's required

 

[Rob#]

I think 125% of the drive input is all that's required

The first question is: what's considered the drive input? The drive capacity as specified on the drive's data sheet? Or 110% or 150% of that value because the drive can pull up to 150% for 60s, depending on just a parameter setting on the drive? (paulengr answered that one) Or do I say my max drive input is the FLA of the motor (as paulengr said most people do)? And: what you are saying is the NEC/UL rule for just the wires to the drive. NEC does not tell me the min OCPD ampacity I have to use, so my OCPD can be 50% of the drive capacity, that is fine with NEC. Worst case I cannot even stick the wires in the terminals of the OCPD because the wires must be so ridiculously thick just because of the drive's rating.

 

[nhee2]

430.122(A) 125% of the rated input current of the drive. Take the nameplate current (at the voltage you are using), multiply by 1.25.

 

[bwat]

The first question is: what's considered the drive input?

Usually they have a "rated input current" number.

And: what you are saying is the NEC/UL rule for just the wires to the drive.

430.122 which says not less than 125%

NEC does not tell me the min OCPD ampacity I have to use, so my OCPD can be 50% of the drive capacity, that is fine with NEC.

Yes and no. 430.128: Disconnect (breaker) has to be at least 115% of rated input… so almost treat as the OCPD minimum

 

[Rob#]

430.122(A) 125% of the rated input current of the drive. Take the nameplate current (at the voltage you are using), multiply by 1.25.

Again, 430.122(A) specifies the CONDUCTORS, not the OCPD. NEC (NFPA70) cares only about melting wires. But if your OCPD is rated 50A why should the wires be sized for 120A? The only restriction I have for the OCPD is that it cannot be >200A, that's specified by Siemens.

 

[Rob#]

Usually they have a "rated input current" number.


430.122 which says not less than 125%


Yes and no. 430.128: Disconnect (breaker) has to be at least 115% of rated input… so almost treat as the OCPD minimum

That's because NEC (NFPA70) wants to make sure your disconnect will not melt. The disconnect does not have to be an OCPD!

 

[nhee2]

But if your OCPD is rated 50A why should the wires be sized for 120A?

Because that's what the code says. If you want to deviate from the code, then do something different. or put in a proposal for code change as suggested.

 

[bwat]

The disconnect does not have to be an OCPD!

Of course, but if the OCPD is your disconnect