rswolff001
Member
I have a 60 hp VFD.. rated current is 77a.. however we can run the inverter up to 150% (115a) for short periods.. Should I size the wire (supply and motor) for the 77a or for the maximum 115a? Thanks, Bob 
Wire gauge
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Frank DuVal
Senior Member
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- Electrical Contractor, Electrical Engineer
What is the motor nameplate? Or is it multiple motors on one VFD? Or is it not motors being controlled?
rswolff001
Member
its a 60hp inverter with a 60hp motor.. motor is rated at 480vac 77a (or close to it).
rswolff001
Member
ok you got me.. what am I looking for in 430.112? The inverter is RATED at 77a but can within its specification supply 150% for 1 minute. Lesser percentages between 100% (77a) and 150% can be supplied for periods ranging from continuous at 100% to 1 minute. I'm not looking for the overcurrent protection specification.. I just need to know if the wiring needs to conform to the motor FLA or the maximum current that can be supplied during normal operation. The inverter is protected by a 125a circuit breaker which is the specification from the inverter manufacturer. So the supply wiring needs to be sized for the cb size. Does the motor wiring need to do the same?
Speedskater
Senior Member
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- retired broadcast, audio and industrial R&D engineering
Wire and many breakers are spec'd for 3 hour continuous current.
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- Illinois
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- retired electrician
The motor circuit conductors need to be sized at 125% of the value found in Table 430.250. See 430.122(B).
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- San Francisco Bay Area, CA, USA
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Assuming the VFD is UL listed (and not some cheap Chinese junk bought off the internet), it is required to have the motor circuit overload protection built in. So to that end, using the regular motor branch circuit conductor sizing rules is going to cover whatever the drive can dish out, because anything within the thermal damage curve of the motor circuit components will fall under that thermal protection curve.
As mentioned, the LINE side conductors must be sized per 430.122, for which you must find the rated INPUT amps of the VFD. Input amps is always different from the rated output amps, which is how drives are sold. So you usually have to look that up in a manual or technical document, or often it is listed on the VFD nameplate if you already have it.
MOST of the time, the input amps are actually slightly LOWER than the output amps, so if you use the output amps, you are OK. But it’s not guaranteed to be the case because some Asian based drives (Mitsubishi for example) design their drive sizes on IEC motor kW ratings, then RE-rate them for HP, but the IEC motor current ratings don’t match up to standard HP ratings, so they have to go larger, yet the input amps don’t change so you end up with the input being HIGHER than the output. Just RTFM…
As mentioned, the LINE side conductors must be sized per 430.122, for which you must find the rated INPUT amps of the VFD. Input amps is always different from the rated output amps, which is how drives are sold. So you usually have to look that up in a manual or technical document, or often it is listed on the VFD nameplate if you already have it.
MOST of the time, the input amps are actually slightly LOWER than the output amps, so if you use the output amps, you are OK. But it’s not guaranteed to be the case because some Asian based drives (Mitsubishi for example) design their drive sizes on IEC motor kW ratings, then RE-rate them for HP, but the IEC motor current ratings don’t match up to standard HP ratings, so they have to go larger, yet the input amps don’t change so you end up with the input being HIGHER than the output. Just RTFM…
Jpflex
Electrician big leagues
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Wire and over current protection is sized per horsepower based on FLC table but you did not provide enough info. Phase and voltage values?I have a 60 hp VFD.. rated current is 77a.. however we can run the inverter up to 150% (115a) for short periods.. Should I size the wire (supply and motor) for the 77a or for the maximum 115a? Thanks, Bob
2 phase 67 amperes at 460v
3 phase 77 amperes at 460 volts
I think your source is 3 phase? If so you size at 1.25% of this 77 FLC 77 amperes x 1.25 for wires and over current protection
Overload protection use nameplate amperes x 1.25%
As much as I know don’t forget wire temperature termination and derating
sorry, incorrect !
Jpflex
Electrician big leagues
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- Victorville
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I think it is you who are wrong. This is straight from NEC code
Jpflex
Electrician big leagues
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- Electrician commercial and residential
Overload multiplier is based on temperature rise and service factor on name plate. I do not have book on hand at the moment so I don’t remember if it was 1.20 or 1.25 fla or whatever
rswolff001
Member
ok... not sure why this is so difficult.. yes I indicated the inverter amps and voltage.. 3ph 480vac rated at 77a with the capability of supplying 150% of rated FLA for 1 minute. Does the wiring from the inverter to the motor need to be based on the FLA or the maximum amps available from the vfd?
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- Illinois
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- retired electrician
The code does not require the conductor between the VFD and the motor to be larger than 125% of the motor full load current as found in the tables at the end of Article 430. See 430.122(B) that sends you to 430.6.
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