Soft starts on 200 Hp Motors

[bdn2004]

We have some large 200Hp motors that are presently started across the line on some fan motors. We're thinking of putting in some soft starters in place of these.

1) Does the size of the load wires affect the soft start? I understand there is impedance there...but does this make any difference to what soft starter we purchase?

2) The existing motors are fed with 1000kcmil AL cable, which we want to re-use. However the soft start we are looking has a maximum wire size of 500 kcmil. I'm guessing there is a way to lug this down, but I'm not familiar how to do it...any suggestions?

3) Any other issues with soft starts? BTW the motors do feed fans that are moving air and don't require lots of starting torque.

 

[Mobius44]

1) If the wires are adaquate for across the line start they will be fine for soft start.

2) Similar ampacity can be obtained from 2-350kcmil copper cable, but it probably can't fit your 500 kcmil lugs either. It throws a red flag that the soft starter may be undersized if it can't handle the cable size. You can also check if your desired soft start supplier offers other connection options, if not you may want to look at other suppliers.

3) Follow the mfg.'s installation instructions carefully. Some soft start designs include bypass contactors, overload protections, while others need external parts. Some designs pass-through one leg of three-phase and only control the other two legs, this can surprise you with voltage at the motor when you think it's unpowered.

Good Luck!
Sean

 

[bdn2004]

Thanks for the answers.

2) The cable ampacity was derated because it runs outside and it is in a covered cable tray, thus the 1000kcmil size.

I'm asking can you lug this down legally...in some way legally. The current is no higher at this point, certainly seems you could. Maybe I need to go to the NEC forum.

 

[davidbeach]

Yes, if you find a suitable termination device, you can "lug down" you conductor size. You'll probably need a separate box with something like a power distribution block that accepts one 1000kcm and two 500kcm or something like that and just use one of the 500kcm positions. Or a box with short sections of bus and then lug onto the bus with the 1000kcm and off with the 500kcm.

Can you find something that works in the terminal space of the soft starter? Probably not.

 

[jtester]

You are permitted to lug it down, to the minimum value in NEC 430 Part II. The general rule is the motor circuit conductors must be at least 125% of the motor fla from Table 430.250. There are some exceptions, but that is the general rule. In this case 240 amps is fla, so 240x1.25 = 300 amps or 350 kcmil cu, 500kcmil aluminum. If you have to derate this portion of the circuit, then adjust these wire values.

Jim T

 

[jraef]

If you are in the US, there are rules about minimum wire bending space in the soft starter enclosure that are based on the maximum wire size the unit is designed for. Chances are about 99.99% that the soft starter mfr did not allow enough wire bending space for 1000kCMIL Al cable as it would be serious overkill for that unit. Your only option may be the intermediate box with a splitter block as mentioned by davidbeach.

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