# Just Wired Up A Vfd.



## Bob Sorenson (Jan 19, 2016)

The last couple of months I've read every thread relating a vfd. Having a grizzly 10 x 20 lathe, I decided to change over to 3 phase one horse d80 frame ( exact replacement ) size motor and a KBMA adjustable frequency drive. 
I have all of the parts here now, so I set it up on my backbench to test.
Adding to my materials list was 20 feet of 14/3 so wire and eight feet of 14/4 so cable. The 14/3 is for power from the outlet to the control box, and the 14/4 wire is to go between the motor and control box. Also used two 20 amp twist lock connectors to be used on the 3 phase, wiring (so I could remove the motor if I wanted to).
Wired it all up using a very good KBMA manual. On the bench, every thing works good.
The manual does not mention a breaking resistor.

My question to anyone who might know about my setup, and adding a breaking resistor. Really don't know even if I need the quick breaking system except for 1. An emergency stop and 2. to assist in thread making.

Thanks in advance for any help.
By the way my background is putting out fires, and electrician.


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## JimDawson (Jan 19, 2016)

Assuming you have the KBMA-24D, it looks like the only option for breaking is DC injection (factory programming option).  There doesn't seem to be any provision for a breaking resistor.  Normally you would find an additional pair of power terminals that would be labeled B1 & B2 or something like that.


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## Eddyde (Jan 20, 2016)

A 20 amp circuit 1 hp motor should use #12 wire. #14 is for max ¼ hp motor.


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## AaronD (Jan 20, 2016)

Eddyde, on 120v your figures are close but it seems that OP is using 240v single phase input.  He is fine operating his 1hp motor and VFD on the wire size and connectors purchased.

Helpful rules I have always followed

Watts/volts=amps

Approx 750w per HP


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## Eddyde (Jan 20, 2016)

The calulations don't matter, if its a 20 amp circuit code says it must be #12 wire. It's probably fine to use #14 from the VFD to the motor, provided its a short run, but the wire from the mains to the VFD should be #12, if its on a 20 amp breaker.


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## ch2co (Jan 20, 2016)

My G0602 with 1hp 3ph through a VFD is wired with 10ga. wire, coming off of the mains box. About 60 ft. of wire running through conduit. My VFD manual recommended
using a #8 ground for the circuit why? I don't know, but thats what I used. The rest of my house is severely underwired, being built in the late 1940's. The recommended wiring is overkill to be sure, but who knows what else might be added to that circuit in the future. These aren't production lathes running under full power for hours (days) at a time, so lighter wire would probably do just fine, but if I'm going to put the time into routing conduit, pulling the wire, adding a sub box etc. I happily spend a few extra bucks to not have to do it all over again in a couple of years when the 10-22 becomes a 12-48 or something to that ilk. 

CHuck the grumpy old guy


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## JimDawson (Jan 20, 2016)

Bob Sorenson said:


> Also used two 20 amp twist lock connectors to be used on the 3 phase, wiring (so I could remove the motor if I wanted to).



Guys, this is the only mention in the OP's post about amps.  He doesn't say anything about the circuit protection, or the system voltage.



Bob Sorenson said:


> By the way my background is putting out fires, and electrician.



The last sentence in his post makes me assume Bob knows ampacities and circuit protection.


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## ARKnack (Jan 20, 2016)

Eddyde said:


> The calulations don't matter, if its a 20 amp circuit code says it must be #12 wire. It's probably fine to use #14 from the VFD to the motor, provided its a short run, but the wire from the mains to the VFD should be #12, if its on a 20 amp breaker.



Actually flexible cords are a bit different. Table 400.5 in the NEC show 14awg good for either 15 or 18 amps. If the cord only has 2 current carrying conductor its good for 18 amp. From the plug to VFD in this case (hot & neutral). I do not know what the input amperage requirement is for this vfd. If it is less the 18 amps he good. If more then he needs a larger cable.
14awg with 3 current carrying wires is allowed 15 amps. This would be from vfd to motor. A 1hp 3ph motor only draws about 3.6 amp @ 220vac.


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## AaronD (Jan 20, 2016)

I did a little bit of...I know...assuming in my reply.  Although it was based on info provided by OP.

14/3 SO contains 3 wires.
The choice of wire told me 240v single phase input VFD, although I guess we could make an assumption that this would also work to carry the neutral to 120v input VFD...
14/4 SO carries an additional wire (3 hots, plus ground)

I guess we really need to know more info from Bob.  Specifically the drive model


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## ARKnack (Jan 20, 2016)

True. One mistake that is often made is the number of wires. 14/3 has three wires as you stated but it only has 2 current carrying conductors. A 1hp vfd I believe comes with both 120/240v input. Mine did. I had to switch mine to 240v because the vfd kept tripping the GFI breaker. I am also making a lot of assumption. So Bob will have to chime in but I think we are discussing something completely different from his question. Funny how we can get off on a tangent.


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## mksj (Jan 20, 2016)

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.


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## abrace (Jan 20, 2016)

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.


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## Bob Sorenson (Jan 20, 2016)

abrace said:


> 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.
> 
> ...


Now this is Bob into the conversation.


abrace said:


> 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.
> 
> ...


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## Bob Sorenson (Jan 20, 2016)

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.


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## abrace (Jan 20, 2016)

Bob Sorenson said:


> 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


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