Panel breaker sizing for a VFD (yet another question on that topic...)

[maxime.levesque]

I have a 3 phase 2HP motor that draws 5.8Amps at 240v

I got HuanYang VFD to drive it, with the following specs:

• VFD rated power: 4kW
• 1-phase input: 36 Amps
• 3-phase output: 18.5 Amps

The VFD is of course oversized for the motor, the reason I got it was to have the option to upsize the motor in the future.

(The motor will power a lathe, and while I could go into more details about the motor upgrade option, I would prefer if the thread stayed focused on the sizing question).

In the VFD manual (page 14), it is written: size the input breaker at 1.5x to 2x the rated current of the inverter.

Which translates into a breaker between [54 and 72], i.e. a 60Amp breaker.

From wire gauge tables I have found on the internet, the wire size for that breaker is from 4 to 6 AWG.

It's an awfully big wire.

First questions: did I get the breaker and wire sizing correctly ?

If so, is it possible to downsize the breaker (and wire), if I abandon the option of upgrading the motor ?

Downsizing the breaker is based on the assumption that I would only ever run the 2HP (5.8 Amps) motor on it.

The VFD 3-phase output at 18.5 Amp, is 1/3 the current draw of the motor (5.8 Amp).

Does this mean I can size the breaker at 1/3 of it's capacity (36 amps) between x1.5 and x2 ?

This would give me:

• min = 36*(1/3) * 1.5 = 18
• max = 36*(1/3) * 2 = 24

And a 20Amp breaker would fall in between these two values.

If I ever feel need to upgrade the motor, then I would change the wire and breaker, it turns out the lathe is only 5 feet from the panel.

 

[matthewsx]

Just gonna spitball this, but I think you'll be fine with a 20 amp breaker.

If you're running a new circuit you could up the wire size so you're good to use a 30 amp if you run into trouble, but I've used 20 amp circuits for most of my stuff and never had any issues.

Welders being the exception of course....

John

 

[Shotgun]

The cost of power increases exponentially.
2Hp is very effective for most hobbyists. Until you start on Abom sized projects, it's going to do more than you'll ever really need.
I'd go with a 20A breaker, 12AWG wire and be done with it.

 

[Beckerkumm]

Are you running wires in conduit and can swap them out easily or will they be buried in the wall ? If conduit, I'd make the conduit size large enough to eventually go with 3 lines of #8 which would take you up to a 7.5 hp motor with single phase delivery. For now you can run #12 and a 20 amp breaker or even #10 and 30 amps which would allow a motor swap up to 5 hp in the future. A machine with 2 hp probably isn't built to handle a larger motor than 5 hp anyway.

If you do upgrade to an expensive motor, I'd also consider a better quality vfd but that is another issue. Dave

 

[mksj]

US electrical code typically is 1.25Xthe VFD rated input amps, it is not dependent on what it is driving/motor amps. Motor wiring is rated at 1.25X the motor FLA. So if someone comes along and changes the motor, the breaker/wiring is rated VFD input for the at the full motor rated output. Type of breaker and fusing also effect the amperage rating. Semiconductor fuses require higher amp rating than dual element which are less. So technically 50A breaker and #6 wire, but you could get by with a 30A breaker and #10 copper wire single phase input with the premise that the breaker will trip if a higher current is pulled, and the wire is technically protected by the breaker. Even in that scenario, the wire/breaker are not rated at 30A continuously, only about 80%.

I have not seen any information as to how a 3 phase input VFD used with single phase power (derated mode) should be fused/breaker rating. I typically use the same fuse/breaker size manual recommendations as would be used with 3 phase power. The premise being that with single phase the THD is increased and this will trip the VFD input overload detection circuit and trigger an error code. Most 3 phase input VFD's when running on single phase are derated by a factor of ~2 these days.

Motor wiring, 14AWG is sufficient.

 

[TooManyWrenches]

Just a thing to consider; breakers are there to protect the wire. The wire is sized for the amps drawn during continuous load. If you want motor protection, use fuses - you can get time delay fuses for motor control. Generally, breakers are not configurable enough to protect motor equipment. Your VFD may have over current protection too.

 

[Ironman2]

My Lagun FTV-3 has a 3hp 3ph motor on it and is powered by a VFD feeding off a 15 amp breaker. The inrush current on a single phase motor is 300% of run current, and that is not the case for 3 phase. Also the capacitor bank in the VFD will take care of most current surge issues, so the main thing is run current and I have yet to pop that breaker.

 

[MikeInOr]

Just a thing to consider; breakers are there to protect the wire. The wire is sized for the amps drawn during continuous load. If you want motor protection, use fuses - you can get time delay fuses for motor control. Generally, breakers are not configurable enough to protect motor equipment. Your VFD may have over current protection too.

Correct!

Specifically the breakers are there to keep the wire in the wall from overheating, melting the insulation, arcing and starting a fire! The breakers have nothing to do with protecting the VFD or motor! The gauge of wire required for a particular current actually varies. A higher melting temperature insulation can use a smaller gauge wire for the same current than one with a lower temperature insulation. How much current a specific wire/insulation is rated to carry will also depend on if the wires are tied together in a cable like Romex, are individual strands in a conduit or the wires in free air. Code books give different ampacities and breaker sizes for a particular gauge wire and particular insulation depending on how the wire is run.

A VFD will pull much less current when starting the motor than the same motor connected directly to 3ph. When connected directly to 3ph a motor will JUMP to life and go from 0 to 3600 in an instant. By ramping up the speed of the motor over a second or a few seconds the inrush current is GREATLY reduced. Part of the reason VFD's have a soft start feature is so larger motors can be started with smaller power transistors. I have VFD's which have inputs to shutdown the VFD/motor if the motor goes over a certain temperature.

Current doesn't kill electrical devices, the heat generated by the current is what kills electrical devices.

 

[verbotenwhisky]

I have a 3 phase 2HP motor that draws 5.8Amps at 240v

I got HuanYang VFD to drive it, with the following specs:

• VFD rated power: 4kW
• 1-phase input: 36 Amps
• 3-phase output: 18.5 Amps

The VFD is of course oversized for the motor, the reason I got it was to have the option to upsize the motor in the future.

(The motor will power a lathe, and while I could go into more details about the motor upgrade option, I would prefer if the thread stayed focused on the sizing question).

In the VFD manual (page 14), it is written: size the input breaker at 1.5x to 2x the rated current of the inverter.

Which translates into a breaker between [54 and 72], i.e. a 60Amp breaker.

From wire gauge tables I have found on the internet, the wire size for that breaker is from 4 to 6 AWG.

It's an awfully big wire.

First questions: did I get the breaker and wire sizing correctly ?

If so, is it possible to downsize the breaker (and wire), if I abandon the option of upgrading the motor ?

Downsizing the breaker is based on the assumption that I would only ever run the 2HP (5.8 Amps) motor on it.

The VFD 3-phase output at 18.5 Amp, is 1/3 the current draw of the motor (5.8 Amp).

Does this mean I can size the breaker at 1/3 of it's capacity (36 amps) between x1.5 and x2 ?

This would give me:

• min = 36*(1/3) * 1.5 = 18
• max = 36*(1/3) * 2 = 24

And a 20Amp breaker would fall in between these two values.

If I ever feel need to upgrade the motor, then I would change the wire and breaker, it turns out the lathe is only 5 feet from the panel.

If you'll go to Bing and select the Chat bot then ask it the question as you have asked it here it will give you a complete and detailed answer. I did the same for my 3 Phase compressor and it was very helpful...

 

[TooManyWrenches]

My Lagun FTV-3 has a 3hp 3ph motor on it and is powered by a VFD feeding off a 15 amp breaker. The inrush current on a single phase motor is 300% of run current, and that is not the case for 3 phase. Also the capacitor bank in the VFD will take care of most current surge issues, so the main thing is run current and I have yet to pop that breaker.

Nope. VFDs are not capacitive start, they are frequency starting. At Locked rotor, the VFD (variable frequency Drive) will VARY the frequency (in the US is60Hz) to a lower frequency, which changes the apparent power (PF). A capacitor stores energy for a sudden release. You find capacitors in large motors with heavy loads that have to overcome lots of inertia, think a/c compressors, and those caps are the size of a typical vfd.