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Old March 13th, 2018, 06:28 AM   #1
theripley
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OT: Star-Delta

Hi.

For a 100 hp (at 230 Vac) FLC is 248A (based on NEC table), is this correct:

1.) Branch circuit protection will be 248A x 250% = 620 A or 600 AT

2.) Main contactor is 248A x 58% = 145A, and main contactor = delta contactor = overload protection

3.) Star contactor is 248A x 33% = 85A

4.) Branch conductor size is 60mm2 (00 AWG). Ampacity of 60mm2 is 170A, this is > 145A.

Did I forget something? Please respond.
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Old March 13th, 2018, 07:24 AM   #2
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I never chose the contactors or the delta-star starter according to the intensity. I choose the contactor according to the manufacturer's catalog and selecting the immediate superior for the motor power and voltage

One example : https://www.schneider-electric.com/e...on-and-control

This would seem the exact model for 100HP and three-phase at 230V, but if you want more margin maybe it would be better to choose the next higher

The same applies for the circuit breaker, choose one suitable for motor protection according to the power and voltage.
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Old March 13th, 2018, 09:45 AM   #3
T Gibbs
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Branch circuit conductors have to be able to carry 310 amps (248 x 125%)
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Old March 13th, 2018, 08:32 PM   #4
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Quote:
Originally Posted by T Gibbs View Post
Branch circuit conductors have to be able to carry 310 amps (248 x 125%)
Hi T Gibbs,

I have attached herewith my diagram. I use 2 sets of 60mm2 as my branch conductor, is this correct?

As for the contactor, I will choose 1 size higher than my computation for safety.
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Old March 13th, 2018, 08:39 PM   #5
T Gibbs
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I’m not sure what 60mm is in AWG. If 60mm2 wire is good for 155 amps or more then you’re good.
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Old March 13th, 2018, 09:36 PM   #6
theripley
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Just wondering, what is the implication if I use 2 sets of 150mm2 wire (295A) instead of 2 sets of 60mm2 (170A)? Aside from cost concern of course.

I was asking because we have stock of 150mm2 wire from previous project, just wondering if I can use it or not.

Thanks.
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Old March 13th, 2018, 09:52 PM   #7
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Quote:
Originally Posted by theripley View Post
Just wondering, what is the implication if I use 2 sets of 150mm2 wire (295A) instead of 2 sets of 60mm2 (170A)? Aside from cost concern of course.

I was asking because we have stock of 150mm2 wire from previous project, just wondering if I can use it or not.

Thanks.
For those of us in North America, 150mm2 is roughly equivalent to 300MCM, 60mm2 is roughly equivalent to 1/0 (1/0 is 55mm2).

The only potential problem with running cable that is larger that necessary is that it will not fit in the lugs of the devices made for lower currents.

To your first post:
Yes, the MAXIMUM circuit breaker could be 248 x 250%, but because it is a Reduced Voltage Starter, you could likely get away with smaller. I've done plenty of that size using a 400A breaker, never had a problem with it.

The two main contactors should be the same, and yes, 58% of the motor FLA. The Star contactor is sized to 33% of FLA minimum, but also it must be capable of being mechanically interlocked with the Start contactor (M2 in your diagram), which sometimes dictates the minimum size. You have to make sure of that before buying.

Side note: I stopped messing with all of that years ago and just use Solid State Soft Starters. No muss, no fuss, nothing to remember.
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Old March 13th, 2018, 10:05 PM   #8
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Quote:
Originally Posted by jraef View Post
For those of us in North America, 150mm2 is roughly equivalent to 300MCM, 60mm2 is roughly equivalent to 1/0 (1/0 is 55mm2).

The only potential problem with running cable that is larger that necessary is that it will not fit in the lugs of the devices made for lower currents.

To your first post:
Yes, the MAXIMUM circuit breaker could be 248 x 250%, but because it is a Reduced Voltage Starter, you could likely get away with smaller. I've done plenty of that size using a 400A breaker, never had a problem with it.

The two main contactors should be the same, and yes, 58% of the motor FLA. The Star contactor is sized to 33% of FLA minimum, but also it must be capable of being mechanically interlocked with the Start contactor (M2 in your diagram), which sometimes dictates the minimum size. You have to make sure of that before buying.

Side note: I stopped messing with all of that years ago and just use Solid State Soft Starters. No muss, no fuss, nothing to remember.
Hi jraef,

Your side note is the best. I learned my lesson now, thank you very much.
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