Main Breaker sizing

[mastlog]

Good day Guys,
I have an MCC with 11 starters (DOL) each 7.5KW and as per datasheet LRA = 102.85 A and FLA = 14.4 A.

I need to size the main breaker for those starters.

So what i did actually. simply calculate the total FLA which will be 14.4 x 11 = 158.4 A. And keeping in view the 120% rule which makes 189.6 A.

So i selected 200A breaker for the mains incomer.

Can some body validate the calculation. Or i need to consider other factors too.

Thanks in anticipation.

 

[augie47]

Art 430 Part V provides you with the specifics for sizing Motor Feeder Short Circuit GF protection.
If you follow that Section I believe you will arrive at the same answer.
Other factors that might influence the sizing would be anticipation for future growth, buss size on the MCC and consideration if all motors could start simultaneously.

 

[mastlog]

Thanks sir,
Matter of fact that all starters can't start simultaneously as Mains feeder can't afford it. So the logic is already there in the PLC to start them progressively.

Thanks again.

 

[Smart $]

What we here on the forum refer to as the 120% rule is with respect to "backfeeding" a distribution system bus or feeder with an alternate power source, such a solar.

Perhaps you mean the 125% factoring for continuous or largest motor load...? Note only the largest motor load gets factored... and that is with respect to conductor ampacity determination.

With respect to short circuit and ground fault protection, you'd determine the rating per Article 430 Part V. Basically it'll say to not exceed the largest permitted protection rating for the largest motor plus the full-load currents of the other motors.

However, the determined value can be exceeded under 430.62(B).

 

[wsbeih]

Good day Guys,
I have an MCC with 11 starters (DOL) each 7.5KW and as per datasheet LRA = 102.85 A and FLA = 14.4 A.

I need to size the main breaker for those starters.

So what i did actually. simply calculate the total FLA which will be 14.4 x 11 = 158.4 A. And keeping in view the 120% rule which makes 189.6 A.

So i selected 200A breaker for the mains incomer.

Can some body validate the calculation. Or i need to consider other factors too.

Thanks in anticipation.

Practically, I would size the Main breaker to match the MCC bus rating. Typically, MCC sizes are 600A minimum. Specifying 600A breaker and 600A cable would allow taking advantage of the MCC in the future. Trust it will grow over time. Also, Clients would appreciate this approach especially the cost is not overkill by doing so.

 

[topgone]

Good day Guys,
I have an MCC with 11 starters (DOL) each 7.5KW and as per datasheet LRA = 102.85 A and FLA = 14.4 A.

I need to size the main breaker for those starters.

So what i did actually. simply calculate the total FLA which will be 14.4 x 11 = 158.4 A. And keeping in view the 120% rule which makes 189.6 A.

So i selected 200A breaker for the mains incomer.

Can some body validate the calculation. Or i need to consider other factors too.

Thanks in anticipation.

Lemme help.
A 14.4A motor load will need 2.5 x 14.4a = 36A, say 40A (Article 430.52), assuming you are using an inverse time breaker.
Your main breaker size will be = 1.25 of largest CB + sum of the FLAs of the rest of the motors = 1.25 x 40 + 14.4 x 10 = 194A, choose 200A.

 

[Jraef]

Given that you are in Saudi Arabia and you are giving your motor sizes in law, codes for North American are not going to be relevant. You really need to involve someone familiar with your local codes or at least IEC.

 

[Smart $]

Lemme help.
A 14.4A motor load will need 2.5 x 14.4a = 36A, say 40A (Article 430.52), assuming you are using an inverse time breaker.
Your main breaker size will be = 1.25 of largest CB + sum of the FLAs of the rest of the motors = 1.25 x 40 + 14.4 x 10 = 194A, choose 200A.

You got it all right except the 1.25 (125%) part.

 

[Smart $]

Given that you are in Saudi Arabia and you are giving your motor sizes in law, codes for North American are not going to be relevant. You really need to involve someone familiar with your local codes or at least IEC.

What if he is familiar with local and IEC codes but the project spec's say installation must be compliant with the NEC?

 

[Tony S]

Given that you are in Saudi Arabia and you are giving your motor sizes in law, codes for North American are not going to be relevant. You really need to involve someone familiar with your local codes or at least IEC.

As far as I know Saudi Arabia electrical codes are an amalgam of US and EU standards. Picking the best (?) of each.

Saudi Arabia is in the purple bit.