Feeder Protection UL508A vs NEC using VFDs

[MhsControls]

When calculating feeder size and protection should we use the VFD input current as in UL508A 30.2? Looking at NEC it seems to always refer to motor FLA or FLC for calculations. Is there a section of the NEC that references calculations for motor controls using VFDs?

 

[petersonra]

When calculating feeder size and protection should we use the VFD input current as in UL508A 30.2? Looking at NEC it seems to always refer to motor FLA or FLC for calculations. Is there a section of the NEC that references calculations for motor controls using VFDs?

take a look at 430 section X that starts at 430.120.

 

[MhsControls]

take a look at 430 section X that starts at 430.120.

Bob,

Thanks, that make sense regarding the conductors. What about the main breaker for the panel? Would we take the largest protective device of the VFD * 1.25 and add the remaining input currents of the VFDs or the FLC of the motors? We have been using the remaining input currents of the VFDs which makes for a larger main breaker to our panels.

Anthony

 

[petersonra]

Bob,

Thanks, that make sense regarding the conductors. What about the main breaker for the panel? Would we take the largest protective device of the VFD * 1.25 and add the remaining input currents of the VFDs or the FLC of the motors? We have been using the remaining input currents of the VFDs which makes for a larger main breaker to our panels.

Anthony

In most cases the motor FLA is more than the input amps of a VFD. Not by a lot, but that is usually the way it works out.

 

[GoldDigger]

Possibly a slight confusion here.
The 125% of FLA is there in part to allow for starting surge which can be close to Locked Rotor Amps (LRA). But it may also be tied in part to continuous operation.
The VFD input current for a motor operating at full load will have to be close to the FLA input of the motor to supply the same power, but does not necessarily need to cover the reactive current of the motor
So it does not need the higher KVA input to supply the KVA to the motor.
But the big advantage of a VFD in this area is that it will draw far less than the LRA while starting the motor.
Logically you should be close to actual loading if you take the VFD current for the largest motor plus the FLA of the rest.
But the code may not provide for that.

Sent from my XT1080 using Tapatalk

 

[MhsControls]

In most cases the motor FLA is more than the input amps of a VFD. Not by a lot, but that is usually the way it works out.

Bob, Thanks. The Allen Bradley PowerFlex 40P are quite a bit higher. For example a 5HP is 13A vs 7.6A for a motor. Now.... their ratings are for 380-480VAC, 3 phase and we typically are dealing with 480VAC.

 

[GoldDigger]

If you compare the output amps to the input amps you get numbers that are a lot closer. Maybe even close enough to be just from 380 versus 480.
My guess is that they are allowing for the most inefficient, lowest power factor motor at that HP.


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[MhsControls]

If you compare the output amps to the input amps you get numbers that are a lot closer. Maybe even close enough to be just from 380 versus 480.
My guess is that they are allowing for the most inefficient, lowest power factor motor at that HP.


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Thanks for all the input. GoldDigger, 430.122 states to use input for conductors. I am trying to see if calculating the main breaker for an MCP is we use the largest protective device and then the sum of the remaining FLC of the motor or use the remaining input of the VFD.

 

[Jraef]

Bob, Thanks. The Allen Bradley PowerFlex 40P are quite a bit higher. For example a 5HP is 13A vs 7.6A for a motor. Now.... their ratings are for 380-480VAC, 3 phase and we typically are dealing with 480VAC.

View attachment 9551

You must understand that the INPUT info in this chart is all about conformance to regulations regarding specific terms, i.e. "Maximum Input Amps", but then the OUTPUT is based on the CAPACITY of the devices, not the motor FLC. In most cases the motor FLC on the nameplate of a given motor in the stated HP range SHOULD fall within the maximum output amp ratings, but in several places, AB warns you (as ALL VFD mfrs do) to NOT use HP as a final value, you MUST use the motor NAMEPLATE FLC. That chart just means that under the WORST case scenario, being a 342V input, and controlling a maximum motor amp output current the transistors are rated to safely deliver, being 10.5A, the maximum input current will not be any more than 13.0A. That does NOT mean that the input current WILL be 13.0A under all conditions, i.e. 480V input.

The way Article 430.122 is worded, you MUST size the conductors based on that Maximum Amp rating, which is why the chart is worded that way. This is because regardless of what you may be connecting to the drive now, someone in the future may read the nameplate on the drive and go for the maximum. Requiring the conductors to be sized for the DRIVE maximum current ensures they will always be adequate.

Typically though, you size the conductors per the rules, then size the breaker to protect the conductors and per the breaker rules. All modern VFDs are now required by UL* to provide ALL of the motor branch circuit protection for each single motor below it, satisfying 430.53 (C). So once that is satisfied, your multiple drives application falls under the rest of the normal rules for 430.53.

By the way, you mentioned using an MCP, by which I assume you mean an "Instantaneous Trip" (Magnetic Only) Circuit Breaker. You cannot use those. Even VFD mfrs can no longer use them on VFDs, even when assembled by the factory. It used to be OK, UL recently changed their minds. But that was NEVER the case with regard to field assembly.


*UL listings for VFDs as "Motor Controllers" require this. There are cheap bottom feeder drives that still get away with calling their products "Power Converters" and UL listing them like they do for solar inverters, which gets around the need for having to provide motor protection. You can always tell if they did that by either looking at the label itself, or look at the installation instructions. If they say that YOU must provide motor branch short circuit and OL protection, you have been had...

 

[MhsControls]

You must understand that the INPUT info in this chart is all about conformance to regulations regarding specific terms, i.e. "Maximum Input Amps", but then the OUTPUT is based on the CAPACITY of the devices, not the motor FLC. In most cases the motor FLC on the nameplate of a given motor in the stated HP range SHOULD fall within the maximum output amp ratings, but in several places, AB warns you (as ALL VFD mfrs do) to NOT use HP as a final value, you MUST use the motor NAMEPLATE FLC. That chart just means that under the WORST case scenario, being a 342V input, and controlling a maximum motor amp output current the transistors are rated to safely deliver, being 10.5A, the maximum input current will not be any more than 13.0A. That does NOT mean that the input current WILL be 13.0A under all conditions, i.e. 480V input.

The way Article 430.122 is worded, you MUST size the conductors based on that Maximum Amp rating, which is why the chart is worded that way. This is because regardless of what you may be connecting to the drive now, someone in the future may read the nameplate on the drive and go for the maximum. Requiring the conductors to be sized for the DRIVE maximum current ensures they will always be adequate.

Typically though, you size the conductors per the rules, then size the breaker to protect the conductors and per the breaker rules. All modern VFDs are now required by UL* to provide ALL of the motor branch circuit protection for each single motor below it, satisfying 430.53 (C). So once that is satisfied, your multiple drives application falls under the rest of the normal rules for 430.53.

By the way, you mentioned using an MCP, by which I assume you mean an "Instantaneous Trip" (Magnetic Only) Circuit Breaker. You cannot use those. Even VFD mfrs can no longer use them on VFDs, even when assembled by the factory. It used to be OK, UL recently changed their minds. But that was NEVER the case with regard to field assembly.


*UL listings for VFDs as "Motor Controllers" require this. There are cheap bottom feeder drives that still get away with calling their products "Power Converters" and UL listing them like they do for solar inverters, which gets around the need for having to provide motor protection. You can always tell if they did that by either looking at the label itself, or look at the installation instructions. If they say that YOU must provide motor branch short circuit and OL protection, you have been had...

Thanks. We are using the AB 140M devices as the VFD circuit protection not a Circuit Breaker. When I say MCP I am referring to the Motor Control Panel and mean sizing the Main Breaker to this MCP which would be 430.62. It seems that 430.62(B) would size the breaker to protect the feed conductors and the feed conductors should be sized per 430.122. We are trying to determine whether in 430.62 do we take the largest protective device of one of the VFDs or other load and add the remaining FLC of the motors or the sum of the remaining VFD input amps. It seems that 430.120 states to use I to IX unless X applies.

 

[Jraef]

Thanks. We are using the AB 140M devices as the VFD circuit protection not a Circuit Breaker. When I say MCP I am referring to the Motor Control Panel and mean sizing the Main Breaker to this MCP which would be 430.62. It seems that 430.62(B) would size the breaker to protect the feed conductors and the feed conductors should be sized per 430.122. We are trying to determine whether in 430.62 do we take the largest protective device of one of the VFDs or other load and add the remaining FLC of the motors or the sum of the remaining VFD input amps. It seems that 430.120 states to use I to IX unless X applies.

OK, I see the subtle issue here: Is the 430.62 rule modified by Part X? Right?

The answer would be no then. The issues in Part X are related to the conductor sizing, not the OCPD sizing. Do it like you would otherwise do it; FLC of the largest motor x 1.25 + balance of all other FLCs.

 

[MhsControls]

OK, I see the subtle issue here: Is the 430.62 rule modified by Part X? Right?

The answer would be no then. The issues in Part X are related to the conductor sizing, not the OCPD sizing. Do it like you would otherwise do it; FLC of the largest motor x 1.25 + balance of all other FLCs.

Thanks. I just got off the phone with an engineer from NFPA and he said it's still a little gray area but that 430.62(B) is trumped by 430.122 where the branch/feeder conductors are based on the input current of the VFD and not the FLC of the motors.