At the large place that I retired from they had over 500 VFD'S from 2 to 1,750 HP. Not one drive had shielded cable from the drive to the motor. Almost every drive was within 50' from the motor. All had line side reactors. All wires were in EMT and meta! flex ( sealtite or greenfield ) and copper ground wire from drive to motors. I believe that for normal applications the shielded cable is not necessary & a waste of money. I asked service tech from two drive companies and they said the same thing. In a new building every drive 40 HP & larger were 18 pulse w/o shielded cable. Some of the drives fed plain Jane NEMA frame three phase motors that were over 30 years old and drives had over 120,000 hours of running time.
Motor branch circuits on VFDs
- Thread starter Isaiah
- Start date
Isaiah
Senior Member
- Location
- Baton Rouge
- Occupation
- Electrical Inspector
Our run length is only about 150’.
But; it’s run in PVC conduit. Will this become an issue with EMI?
Also the motor windings have integral thermostats and the motor service factor is 1.0. Does this mean I can size to output cable to motor based on its FLA without the 125% adder?
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ptonsparky
Tom
- Occupation
- EC - retired
Only if it bothers something.
FLA is determined by 430.250 for 3 phase motors.
Do you see anything in the code that does not require the 125% for your motor? If so, quote it.
Read in its entirety 430.21, 430.22.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
This is from a yaskawa drive but is typical. Note that the input rated amps is less than the output rated amps.
- Location
- Illinois
- Occupation
- retired electrician
It will if you have any sensitive electronic equipment.
Nothing changes the code rule that the conductors to the motor must have an ampacity of at least 125% of the full load current as found in the tables at the end of Article 430.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Like a lot of things, it depends. Seen a fair amount of vfds run in tray cable in trays not real far from cables for t/C's and rtds and other low level signals without any real problems.
Isaiah
Senior Member
- Location
- Baton Rouge
- Occupation
- Electrical Inspector
ptonsparky
Tom
- Occupation
- EC - retired
Isaiah
Senior Member
- Location
- Baton Rouge
- Occupation
- Electrical Inspector
If the CB is rated 100% you don’t need to add 125% to the branch circuit
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ptonsparky
Tom
- Occupation
- EC - retired
A breaker rated at 100% of FLA may not allow a motor to start.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Which is extremely unlikely for breakers under 400A.
I think 99.5% of the branch circuit molded case breakers I have dealt with in my career have been standard ones and subject to the 125% sizing rules.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Most 100% rated breakers have electronic trip units so it may be easier to match their curves to that of a motor.
Isaiah
Senior Member
- Location
- Baton Rouge
- Occupation
- Electrical Inspector
I don’t think it would trip in this case with a VFD.
The CB is for short circuit. The overloads protect against overcurrent plus this motor has integral thermostats in the windings
Overloads must be sized to the nameplate service factor usually at 125%
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- Location
- Illinois
- Occupation
- retired electrician
Still does not change. The rule requires the motor conductors to be sized with an ampacity of at least 125% of the full load current as found in the Article 430 tables.
ptonsparky
Tom
- Occupation
- EC - retired
Overloads are sized at nameplate FLA. The many newer blocks have an option for setting them at class 10-30 but the overload is set at FLA.
I agree for drive it most likely won't trip, but the conductors after the drive still need to be 125%
Isaiah
Senior Member
- Location
- Baton Rouge
- Occupation
- Electrical Inspector
Good to know.
I’ve got an engineer here telling me that “under engineering supervision” the 125% rule can be superseded.
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Dale001289
Senior Member
- Location
- Georgia