Cable Sizing for Abnormal Startup

[adamscb]

All,

We have a 200hp motor in our plant that has a pretty unique starting characteristic. We use a soft-start to start the motor. It draws 800A for about a second and a half, and then draws 500A for 10-12 minutes. After that time period, the motor is up to speed, and then drops down to full load (~196) amps.

My question is, I should not be sizing the feeder cable for this as I would a regular motor, correct? Or can I get away with doing that? A co-worker pointed out to me that no-where in the code does it say you have to size cables differently for a soft-start, and I can see where he's coming from, but in my mind it's wrong to have your cables overloaded for 10-12 minutes when starting up.

Thoughts?

 

[Jraef]

All,

We have a 200hp motor in our plant that has a pretty unique starting characteristic. We use a soft-start to start the motor. It draws 800A for about a second and a half, and then draws 500A for 10-12 minutes. After that time period, the motor is up to speed, and then drops down to full load (~196) amps.

My question is, I should not be sizing the feeder cable for this as I would a regular motor, correct? Or can I get away with doing that? A co-worker pointed out to me that no-where in the code does it say you have to size cables differently for a soft-start, and I can see where he's coming from, but in my mind it's wrong to have your cables overloaded for 10-12 minutes when starting up.

Thoughts?

Here's the way I would look at it. Undoubtedly you must have over sized that soft starter to have it pull 200% current for 10 minutes, right? So you should size the conductors the same way.

Secondary issue: this is (would have been) a good candidate for using a VFD as a glorified soft starter, you could set the VFD to current limit at FLA and take as long as you want to bring it up to speed without over sizing anything. The difference is, at FLC from a VFD, the motor would be producing full torque but with a soft starter current limiting at 200%, you are going to be getting about 33% torque. It would have actually accelerated FASTER with the VFD and not stress the motor or cables.

 

[topgone]

All,

We have a 200hp motor in our plant that has a pretty unique starting characteristic. We use a soft-start to start the motor. It draws 800A for about a second and a half, and then draws 500A for 10-12 minutes. After that time period, the motor is up to speed, and then drops down to full load (~196) amps.

My question is, I should not be sizing the feeder cable for this as I would a regular motor, correct? Or can I get away with doing that? A co-worker pointed out to me that no-where in the code does it say you have to size cables differently for a soft-start, and I can see where he's coming from, but in my mind it's wrong to have your cables overloaded for 10-12 minutes when starting up.

Thoughts?

Please check your cable manufacturer data. Look for short-circuit capacity ratings. IIRC, a 250MCM cable can withstand a short-circuit current of 1,000kA for 12 minutes!

 

[sii]

Just curious, why such a long start up?

 

[Wyo721]

I would question your PF for this installation

I would question your PF for this installation

Do you know the PF of the system?
I agree VFD is a better way to handle the gradual speed increase.

 

[kwired]

Assuming you have some high inertial load that takes time to accelerate there.

See 430.35 - it allows shunting overload protection for such applications - does not mention increasing conductor ampacity.

The conductors will take this temporary over loading if the motor can take it.

 

[Julius Right]

It depends on a few factors:
1) how many times you may start the motor.
2) if you start shortly after the motor operated -at rated temperature
3) what is the ambient temperature
4) the type of cable raceway-air-in conduit or without conduit-underground directly or in duct.
NEC Table 310.15(B)(16) 200 A #3/0 75oC insulation.
Let's take the calculation way of IEEE 242/2001:
[IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems]
9.5.2 Overload capacity [ pp. 313]
and write the emergency equation-in excel language:
IE=In*sqrt(((TE-To)/(TN-To)-(Io/In)^2*exp(-t*k))/(1-exp(-t*k)*(230+TN)/(230+TE))
where:
TN is conductor normal operating temperature,[let's say 75oC]
TE is conductor emergency operating temperature[ 130oC for XLPE],
To is ambient temperature[Let's say 40oC air ambient],
In is normal current rating[A],
Io is operating current prior to emergency[200 A],
K is a constant, dependent on cable size and installation type (see Table 9-5),
K=1[No-conduit, in air] #3/0
t [hour]=720/3600=0.2
230 is zero-resistance temperature value (234 for copper, 228 for aluminum),
IE is emergency operating current rating[allowable in A],
one single start ,full load [200 A] before start.
Ie=200*sqrt(((130-40)/(75-40)-(200/200)^2*exp(-0.2*1))/(1-exp(-0.2*1)*(234+75)/(234+130))
IE=479.456 A.<500 A. It seems you'll need a small derating and take the next 4/0 [ 230 A rating]. and if In=230 then IE=479.456*230/200=551A >500 A.

 

[adamscb]

Assuming you have some high inertial load that takes time to accelerate there.

See 430.35 - it allows shunting overload protection for such applications - does not mention increasing conductor ampacity.

The conductors will take this temporary over loading if the motor can take it.

Bingo, this is a very high intertial load. The 800A for two seconds is just to break the shaft free, and then it's programmed to allow 200% FLC to ramp the motor up to speed, no matter how long it takes. Just happens that it takes 10-12 minutes.

 

[Jraef]

Assuming you have some high inertial load that takes time to accelerate there.

See 430.35 - it allows shunting overload protection for such applications - does not mention increasing conductor ampacity.

The conductors will take this temporary over loading if the motor can take it.

Yes, but be careful here. Motors that are specifically designed for starting high inertia loads are often customized to take the extended heating time (more iron, cooling blowers, etc., so just because the motor can take it does not mean the conductors are OK with it.

Buying and installing larger size conductors up front is one HECK of a lot cheaper than doing it twice, then STILL up-sizing them the second time after all.

But...
It COULD be that rather than buying a custom motor, they just bought one bigger than they needed, i.e. the machine only needed 100HP but because they knew it would take that long to accelerate, they bought a 200HP motor. In that case, sizing the conductors for the motor you have might be, in effect, up-sizing the conductors for the actual connected LOAD. It's worth checking into if you really need to save the $$ on the conductors. I for one would likely just increase the size and do something else with my time.