Fire Pump MCB Size

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gary b

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I need some help with a fire pump question.
I have a 100 HP/480V, nameplate FLA 115A and code letter C on a fire pump motor. The CB, controller, and disconnect is sized and provided by the equipment provider. I calculated the LRC at 480A. A seperate service size installed is 480V/200A. In Article 625.4(B)(1) the OCPD shall be selected or set to carry the LRC of the motor. Will the 200A MCB fill this requirement? The inspector says I need to install a 500A service for this fire pump. What is the proper answer and what am I missing?
 
Re: Fire Pump MCB Size

Gary,

Take a look at 695.6(A) and (C)

Section 695.6(C) gives you requirements for conductor sizing.

The way I read this section you size the service and conductors according to 430.22 and the overcurrent device has to be sized off the LRA.

Chris
 
Re: Fire Pump MCB Size

Gary
If the OCPD is provided by the equipment manufacturer, it should be "fire pump rated". If so, then even though it shows to be a 200A breaker, it will not trip until the current is well above the locked rotor current of the pump motor. So you may want to check and verify this.
 
Re: Fire Pump MCB Size

Be very careful here. The conductor size requirement (695.6(C)(2)) was changed in the 2005 code. It used to be 125% of the motor RLA. The 05 Code revised it take in to account voltage drop at the terminals.

Check the code cycle that you are on is all I'm saying.
 
Re: Fire Pump MCB Size

So from what I gather from the code, the OCPD must be sized according to the LRC of the motor calculated as per Table 430.7(A). Table 430.251(B) cannot be used because the notes do not apply to this situation. The calculated current is 480A based on Table 430.7(A). Therefore the disconnecting switch and the OCPD would be rated for 500A and the wire size we have, 3/0 THHN CU is what is used from the OCPD disconnect to the motor, would meet the intent of code.
 
Re: Fire Pump MCB Size

Here are the code sections:


quote:
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695.4(B)(1) Overcurrent Device Selection. The overcurrent protective device(s) shall be selected or set to carry indefinitely the sum of the locked-rotor current of the fire pump motor(s) and the pressure maintenance pump motor(s) and the full-load current of the associated fire pump accessory equipment when connected to this power supply. The requirement to carry the locked-rotor currents indefinitely shall not apply to conductors or devices other than overcurrent devices in the fire pump motor circuit(s)
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quote:
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695.6(C) Conductor Size.
(1) Fire Pump Motors and Other Equipment. Conductors supplying a fire pump motor(s), pressure maintenance pumps, and associated fire pump accessory equipment shall have a rating not less than 125 percent of the sum of the fire pump motor(s) and pressure maintenance motor(s) full-load current(s), and 100 percent of the associated fire pump accessory equipment.
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quote:
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695.6(C)(2) Fire Pump Motors Only. Conductors supplying only a fire pump motor shall have a minimum ampacity in accordance with 430.22 and shall comply with the voltage drop requirements in 695.7.
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The conductor size is based on 125% of the sum of the FLC of the fire pump motor and the jockey pump motor plus the additional loads provided by accessory equipment. For fire pump motors only conductors shall comply with the minimum ampacity listed in 430.22.

Trevor
 
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