Wire sizing help

[Lancie1]

That what we ended up doing. #6 sized for the motor. We also switched to a 480V setup...

I am pleased that it worked for you. Because your drive was rated for more than the motor Full Load Amps, be sure that you change the default motor parameters in the drive to match your actual motor. Change the drive default FLA, voltage, and frequency to match yours, or your drive may not activate its electronic trip on a motor overload.

 

[g.mccormick]

Will do. We actually paid to have someone for ABB to come do the start up on it. I don't know why, other than something about warranty.

 

[Lancie1]

Good deal. I am sure that ABB set up your drive to match your motor size.

 

[johnkbaum]

The number #6 THHN-THWN-2 will be fine for the 40hp 480vt motor meeting both terminal ratings of 75 degrees, along with the requirements of 125% for continuous loads.(assumming you wont have more than three current carring conductors in the raceway eliminating the need to derate for heat dissipation.) 60amp fuses on the line side of the drive may hold depending on the type. But for drives you need a very fast blowing fuse that would require a size off roughly 3-500% of the drive rating.

The fast blowing fuses protect the drive, and you!
The overload setting protects the motor and the conductors.

Providing overcurent protection on BOTH sides of the transformer?

 

[Lancie1]

The overload setting protects the motor and the conductors.

It does, unless and until the drive electronic current trip fails, burns out, or goes kaput. Then you are back to depending on the fuse.

 

[g.mccormick]

Ok. I'm coming back to this for some more help.

We are building test cells. As part of the test cells, we are putting in hotwell, coldwell, 2 cooling tower, test cell ventilation. There are multiple vfds for all of this. I will be using Automation Direct GS2 and GS3 drives for all due to cost.

For the cooling water system, we are going to have
1 100hp drive/pump
1 75hp drive/pump
1 30hp drive/pump
2 40hp drives/fans

HP Drive Rated Input Rated Output
100 GS3-4100 160 150
75 GS3-4075 130 110
40 GS3-4040 60 60
40 GS3-4040 60 60
30 GS3-4030 49 45

I planning on probably putting all 5 of these drives inside 1 large NEMA enclosure being fed by 1 source.
According to NEC 430.122,
430.122 Conductors — Minimum Size and Ampacity.
(A) Branch/Feeder Circuit Conductors. Circuit conductors
supplying power conversion equipment included as
part of an adjustable-speed drive system shall have an ampacity
not less than 125 percent of the rated input to the
power conversion equipment.

So that means that wiring from the distribution block to each drive should be sized to not less than 125% of the drives rated input.
If I look at the 100hp drive, its rated input is 160A. 125% is 200A. I don't have NEC to look up that conductor, but it should be easy to figure out.
Next, the fuse that is specified by Automation direct is a 600A fuse. Do I need to size the wires feeding the drive for 200A or 600A??

On the load side of each VFD, I will size it for 125% of the motor FLA, correct?

Next question:
After I figure out the fusing and conductor sizing for each individual drive, how to I figure out the conductor and fusing rating for MCCB and conductors feeding the power distribution block?
If total vfd rated inputs is 459A, do I size the conductor and MCCB for 125% of that? 573A?

Thank you for any help in this.

 

[Lancie1]

Next, the fuse that is specified by Automation direct is a 600A fuse. Do I need to size the wires feeding the drive for 200A or 600A??

Something is wrong here. Are you sure that you are talking about the SAME rating - the motor branch-circuit short-circuit and ground fault protective device as described in NEC Article 430.52?

A 100 horsepower, 460 volt motor would have about a 124 Full Load Amps (FLA) nameplate rating. Even with an NEC allowed factor of 175% of FLA for dual-element time-delay fuses for an AC polyphase motor, that only gets you up to a 217 Amp fuse rating (225 next standard fuse size), or 250% of FLA for an inverse time circuit breaker, still only 310 Amps (from NEC Table 430.52). The NEC table is based on starting a motor directly across the line (no electronic drive in the picture). With the VFD, the starting current is not a problem, so the fuses can be much smaller, probably only 125% of FLA or 200 Amps. Remember, you want to use as small a fuse as possible for safety, but without creating nuisance trips. With the VFDs, your fuses can be smaller than normal, because the VFD provides a soft start for the motor, requiring less start-up current. I would probably use about a 225 Amp fuse because this is a 100 HP pump with higher start-up current. That allows you to use 4/0 cables (ampacity of 230 Amps).

On the load side of each VFD, I will size it for 125% of the motor FLA, correct?

Well you can set your VFD internal overload trip for 125% of 124 FLA, or 155 Amps and get by with a 2/0 wire size (rated 175 Amps). What if your pump needs more current to get started? You might have to raise the VFD trip setting to get the pump started, then you could be bumping against the wire size current rating. I think I would go up at least one size to a 3/0 (rated for 200 Amps). For a 100 HP motor, you don't want to take a lot of chances on having a set of hot conductors.

Remember that the NEC and other standards only specify the minimum required equipment. Good engineering often means using larger than the minimum to avoid problems down the road. This is specially true in plants (assembly lines) where equipment is changed out often. Having larger conductors in places may mean not having a huge added cost of tearing out the old and pulling in new cables.

After I figure out the fusing and conductor sizing for each individual drive, how to I figure out the conductor and fusing rating for MCCB and conductors feeding the power distribution block?

Basically, you want everything to be big enough to do the job, but not so big to be unnecessarily expensive. The NEC requires the equipment that handles the entire load to be rated for the largest expected continuous current. See NEC Article 430.53 for a multi-motor system. The main fuses need to have a rating for the sum of the largest branch-circuit device (200 or 225 Amps for the 100 HP pump in your case), plus an amount equal to the sum of the FLA ratings of all other motors AND any other loads connected to the circuit (without any other factors).

Largest motor:

100 HP Pump fuses = 225 Amps

   75 Hp Pump FLA =  96

   30 HP Pump FLA =  40

 40 HP Fan #1 FLA =  52

 40 HP Fan #2 FLA =  52

Other Loads(VFDs) =  ??

  Control Xformer =  ??

 TOTAL MAIN RAING = 465 + ??

The 125% factor is applied only to the largest motor in a multi-motor group running off one common circuit, assuming that will be the largest starting current. The other Full Load Currents are only added straight together to the total.

The VFD loads (power used by the VFD itself) would be the differences between input power and output power.

 

[Lancie1]

One big advantage for using 225 Amp fuses and 4/0 cables for the 100 HP Pump is that the 100 HP VFD could fail, and someone might have to temporarily wire in a motor starter to keep that pump running. With 225 Amps fuses and 4/0 cables, no problem, just wire the starter in to the existing circuit. With a 175 Amp or 200 Amp circuit (possible due to the VFD reduced motor start-up current), some fuses and cables would have to be replaced before the temporary starter would be code-compliant.

 

[g.mccormick]

Lancie1,
I've read your response and have some questions. I don't have the time for it tonight. I will get back with more questions. Thank you for all of the help

 

[Bit_Bucket_07]

Well, sort of, but that does not provide a clear picture of the entire situation. If the maximum 120 Amp rating of the drive is less than the rating of the wire size used, then the fuses will be much smaller than the allowed size strictly "to protect the wire". In fact, where the wire must be oversized due to the voltage drop in long runs, the fuses are generally always much smaller than the ampacity of the wires.

Now, if the drive can produce 120 Amps, it may be smart to use a #1 AWG 3-conductor cable, if there is ANY possibility that the motor at some years in the future might be swapped out for a larger size. On the other hand, if the budget is tight, you could size the wire for the motor FLA current, about 52 amps, and get by with a #6 3-conductor cable or three 1/c #6 wires, in which case your fuses would need to be only about 60 or 65 amps (three #6 wires with 75 degrees C insulation in a conduit is rated for 65 amps).

You're off-base here, Lancie. The fuses aren't intended to protect either the drive, or the motor.

 

[OkiePC]

When you get up above 50-100HP VFDs and soft starters, spend an extra few hundred dollars on semiconductor fuses. This will also reduce the arc flash rating and actually stand a chance saving the VFD from self destruction if there is a sudden massive ground fault.

I think Lancies wire size recommendations are spot on. Assume DOL operation and size accordingly. You may not need that much current to run the VFDs so you can reduce breaker current settings per branch, but have all you need, wire-wise, to upgrade, and upgrading never seems to mean less power...