Just Wired Up A Vfd.

[mksj]

A few things to note, I have used the KB drives, and use this particular one for the Z axis lift motor on my mill, I run VFD gear head motors for X and Z directions. To answer your original question, there is no provision for an external braking resistor on this model. It does have some adjustment for acceleration and deceleration time via the trim pots. These KB drives have pretty limited braking ability, and some of there other KB VFD systems I have used, the braking module cost almost as much as the VFD. I ended up replacing my KB VFD on the X drive, because of limitations in braking and no programming ability other than tweaking the pots. That being said, on a small 10x20 lathe that you want variable speed, it will work fine.

VFDs trip GFIs not because of overload (A GFI does not trip for an overload, only ground fault), they trip because of the EMF caused by the VFD is picked up in the neutral wiring from the VFD to the wall. This trips the GFI, most VFD manuals I have read state not to use a GFI. Common problem when people try to run VFDs off of there garage GFI outlets.

 

[abrace]

Motors are special beasts and a lot of the sacred cows in the electrical code do not apply. The first is the rule about 20A on 12AWG wire and 15A on 14AWG wire. That concept primary comes from 240.4(d), and is generally known as the small conductor rule. It basically says that even though the 75 degree ampacity table 310.15(b)(6) tells us that 12AWG has an ampacity of 25 amps, and 14AWG has an ampacity of 20 amps we are not allowed to protect it with an OCPD (breaker/fuse) greater than 20A or 15A respectively.

This limitation does not apply to motors, welders, phase converters etc. The ampacity of the wire can come right out of the relevant table for those, and then follow any rules in their applicable section.

There are 2 calculations that need to be done with a circuit. The first is the ampacity...that is the current carrying capacity of the wire, terminals etc. The second is over current protection, which is the size of the branch circuit fuse/breaker.

As long as a given motor has overload protection, which is required for motors of any size, whether it be built into the motor, or accomplished via a magnetic starter with heaters, the OCPD (breaker) for the motor can be UP to 250% the full load current rating of the motor, assuming the use of a standard circuit breaker. This is in table 430.52.

So a 3HP 240V single phase motor has an FLC of 17A according to 430.248. Multiple that by 1.25% to account for the required continuous duty adjustment in 430.22 and we come up with a required ampacity of 21.25A, rounding up to 22A. Again, 310.15(b)(6) tells us that 12AWG can do 25A, assuming I am not using romex and am using MC or something similar. Therefore 12AWG flies for a 3HP motor.

Now we pick the breaker. Lets assume the motors nameplate has a full load amperage of 15A, and has the required overload protection integral to the motor. 430.52 tells us we can go up to 250% of FLA, so that means I could use a breaker anywhere between 15A and 35A for that motor, all while it only has 12AWG wire run to it.

The code basically allows us to upsize the breaker well past what would normally make sense to account for startup surges etc.

With a VFD it has its own rules that need to be followed between the panel and the VFD, and then the motor rules would take effect between the VFD and the motor.

However, it just isn't true to say that 20A = 12AWG and 15A = 14AWG in all cases.

*Edited to fix my 5000 typos.

 

[Bob Sorenson]

Motors are special beasts and a lot of the sacred cows in the electrical code do not apply. The first is the rule about 20A on 12AWG wire and 15A on 14AWG wire. That concept primary comes from 240.4(d), and is generally known as the small conductor rule. It basically says that even though the 75 degree ampacity table 310.15(b)(6) tells us that 12AWG has an ampacity of 25 amps, and 14AWG has an ampacity of 20 amps we are not allowed to protect it what an OCPD (breaker/fuse) greater than 20A or 15A respectively.

This limitation does not apply to motors, welders, phase converters etc. The ampacity of the wire can come right out of the relevant table for those, and then follow any rules in their applicable section.

There are 2 calculations that need to be done with a circuit. The first is the ampacity...that is the current carrying capacity of the wire, connectors etc. The second is over current protection, which is the size of the branch circuit fuse/breaker.

As long as a given motor has overload protection, which is required for motors of any size, whether it be built into the motor, or accomplished via a magnetic starter with heaters, the OCPD (breaker) for the motor can be UP to 250% the full load current rating of the motor, assuming the use of a standard circuit breaker. This is in table 430.52.

So a 3HP 240V single phase motor has an FLC of 17A according to 430.248. Multiple that by 1.25% to account for the required continuous duty adjustment in 430.22 and we come up with a required ampacity of 21.25A, rounding up to 22A. Again, 310.15(b)(6) tells us that 12AWG can do 25A, assuming I am not using romex and am using MC or something similar. Therefore 12AWG flies for a 3HP motor.

Now we pick the breaker. Lets assume the motors nameplate has a full load amperage of 15A, and has the required overload protection integral to the motor. 430.52 tells us we can go up to 250% of FLA, so that means I could use a breaker anywhere between 15A and 35A for that motor, all while it only has 12AWG wire run to it.

The code basically allows us to upsize the breaker well past what would normally make sense to account for startup surges etc.

With a VFD it has its own rules that need to be followed between the panel and the VFD, and then the motor rules would take effect between the VFD and the motor.

However, it just isn't true to say that 20A = 12AWG and 15A = 14AWG in all cases.

Now this is Bob into the conversation.

Motors are special beasts and a lot of the sacred cows in the electrical code do not apply. The first is the rule about 20A on 12AWG wire and 15A on 14AWG wire. That concept primary comes from 240.4(d), and is generally known as the small conductor rule. It basically says that even though the 75 degree ampacity table 310.15(b)(6) tells us that 12AWG has an ampacity of 25 amps, and 14AWG has an ampacity of 20 amps we are not allowed to protect it with an OCPD (breaker/fuse) greater than 20A or 15A respectively.

This limitation does not apply to motors, welders, phase converters etc. The ampacity of the wire can come right out of the relevant table for those, and then follow any rules in their applicable section.

There are 2 calculations that need to be done with a circuit. The first is the ampacity...that is the current carrying capacity of the wire, terminals etc. The second is over current protection, which is the size of the branch circuit fuse/breaker.

As long as a given motor has overload protection, which is required for motors of any size, whether it be built into the motor, or accomplished via a magnetic starter with heaters, the OCPD (breaker) for the motor can be UP to 250% the full load current rating of the motor, assuming the use of a standard circuit breaker. This is in table 430.52.

So a 3HP 240V single phase motor has an FLC of 17A according to 430.248. Multiple that by 1.25% to account for the required continuous duty adjustment in 430.22 and we come up with a required ampacity of 21.25A, rounding up to 22A. Again, 310.15(b)(6) tells us that 12AWG can do 25A, assuming I am not using romex and am using MC or something similar. Therefore 12AWG flies for a 3HP motor.

Now we pick the breaker. Lets assume the motors nameplate has a full load amperage of 15A, and has the required overload protection integral to the motor. 430.52 tells us we can go up to 250% of FLA, so that means I could use a breaker anywhere between 15A and 35A for that motor, all while it only has 12AWG wire run to it.

The code basically allows us to upsize the breaker well past what would normally make sense to account for startup surges etc.

With a VFD it has its own rules that need to be followed between the panel and the VFD, and then the motor rules would take effect between the VFD and the motor.

However, it just isn't true to say that 20A = 12AWG and 15A = 14AWG in all cases.

 

[Bob Sorenson]

This is Bob into the conversation, To more clearly explain my wiring, I used a two pole 15 amp.breaker I have a 15 amp receptacle below the electrial panel. It is 240 volt, 15 amp outlet. Then it is now 10 feet of 14/3 SO cable to the vfd.
Now I use 240 volts in this discussion because all electrical calculations must have common number to use to calculate amps, watts, etc. If you notice the voltage given in these texts and manufactures suggest their product is 115, 117,125, and 220, 230 . and if you notice 208 and 277 volts are rarely misinterpreted because mostly tradesmen work with these voltages.

 

[abrace]

This is Bob into the conversation, To more clearly explain my wiring, I used a two pole 15 amp.breaker I have a 15 amp receptacle below the electrial panel. It is 240 volt, 15 amp outlet. Then it is now 10 feet of 14/3 SO cable to the vfd.
Now I use 240 volts in this discussion because all electrical calculations must have common number to use to calculate amps, watts, etc. If you notice the voltage given in these texts and manufactures suggest their product is 115, 117,125, and 220, 230 . and if you notice 208 and 277 volts are rarely misinterpreted because mostly tradesmen work with these voltages.

Bob, that sounds good to me, depending on the motor size/vfd needs. VFDs are not my specialty, others will have to weigh in on that part.

240 is generally what single phase line-to-line is in the US. It has been going up over the years from 220 to 230 to 240...My place here I am about 245V line to line. It is fine to use 240V for your calculations, however, when sizing motor circuits you have to use the amperage in the table in the NEC to size the circuit. The NEC genies have invented numbers they like and we must follow.

15A circuit with 14AWG SJ cord sounds good from that standpoint, whether 15A is what your VFD needs someone more skilled in that area would have to weigh in.



---Aaron