Overcurrent protection

[Alex_rg15]

What size inverse time circuit breaker should be installed for a group installation to protect the following 208 V, three-phase motors? One 7.5-HP motor, two 10-HP motors, and one 15-HP.

Step 1
Find the FLC for each motor
From NEC Table 430.250:
• 7.5 HP motor: 24.2 A
• 10 HP motor: 30.8 A
• 15 HP motor: 46.2 A
Step 2
size
Calculate the maximum allowable circuit breaker
Apply NEC 430.52:
• Maximum breaker size
= (2.5 X FLC of largest motor) + sum of FLCs of otl
• Maximum breaker size
=(2.5×46.2 A) + 24.2 A + 30.8 A + 30.8 A
• Maximum breaker size
= 115.5 A + 24.2 A + 30.8 A + 30.8 A
• Maximum breaker size = 201.3 A
Step 3
Select the standard circuit breaker size
Solution
The size of the inverse time circuit breaker should be 225 A.

But in the answer they give me 200 A

May be I’m doing something wrong I just copy and paste the question to google, and google give a 200 A answer, But google give me 42 A for the 15 HP 208V 3-phase motor, I check the table 430.250 and shows 46.2
Thank you

 

[ptonsparky]

I didn't do the math but look at 430.52(C)(1) exception 2 (3). See if that fits for you.

 

[infinity]

May be I’m doing something wrong I just copy and paste the question to google, and google give a 200 A answer, But google give me 42 A for the 15 HP 208V 3-phase motor, I check the table 430.250 and shows 46.2

Google used 230 volts for the 15 HP not 208 volts.

 

[david luchini]

Step 3
Select the standard circuit breaker size
Solution
The size of the inverse time circuit breaker should be 225 A.

But in the answer they give me 200 A

May be I’m doing something wrong I just copy and paste the question to google, and google give a 200 A answer, But google give me 42 A for the 15 HP 208V 3-phase motor, I check the table 430.250 and shows 46.2
Thank you

430.52 is for individual motors. You want 430.53.

The rating of the OCPD for the largest motor would be 46.2Ax2.5=115.5A ==> 125A OCPD (next size up)


Per 430.53(C)(4):

125A + 30.8A + 30.8A + 24.2A = 210.8.A. A 200A OCPD would be the correct size (225A exceeds 210.8A)

 

[ptonsparky]

Interesting question. I've never calculated for a branch circuit with multiple motors. Always feeders.

 

[Tulsa Electrician]

The key is the word group. Can not round up. Must round down.

What size inverse time circuit breaker should be installed for a group installation to protect the following 208 V, three-phase motors? One 7.5-HP motor, two 10-HP motors, and one 15-HP.

 

[Tulsa Electrician]

Interesting question. I've never calculated for a branch circuit with multiple motors. Always feeders.

I read it as sizing for feeder SCGFPD.

 

[Tulsa Electrician]

Step 2
size
Calculate the maximum allowable circuit breaker
Apply NEC 430.52:
• Maximum breaker size
= (2.5 X FLC of largest motor) + sum of FLCs of otl
• Maximum breaker size
=(2.5×46.2 A) + 24.2 A + 30.8 A + 30.8 A
• Maximum breaker size
= 115.5 A + 24.2 A + 30.8 A + 30.8 A
• Maximum breaker size = 201.3 A
Step 3
Select the standard circuit breaker size
Solution
The size of the inverse time circuit breaker should be 225 A.

As mentioned the (115.5 A) should be a

The rating of the OCPD for the largest motor would be 46.2Ax2.5=115.5A ==> 125A OCPD (next size up)


Per 430.53(C)(4):

125A + 30.8A + 30.8A + 24.2A = 210.8.A. A 200A OCPD would be the correct size (225A exceeds 210.8A)

SCGFPD number in this case 125 amp breaker.
Then add motors.
Then round down accordingly via 240.6

 

[Alex_rg15]

Google used 230 volts for the 15 HP not 208 volts.

I know it, and that’s is wrong

 

[BillyMac59]

In your solution, you have the answer. You state: max size =201.3 . Breaker must be less than that . So 200A is your answer.