What is the maximum size standard size overcurrent device permitted for a 3-phase 460-volt 75 HP fire pump when the nameplate reads Code letter "J" and FLA of 93 amps?

What is the maximum size standard size overcurrent device permitted for a 3-phase 460-volt 75 HP fire pump when the nameplate reads Code letter "J" and FLA of 93 amps?

• A. 700A
• B. 250A
• C. 800A
• D. 125A

Answer: (C)

In fire-pump installations, overcurrent protection is sized using a nameplate code letter that acts as a multiplier to the motor’s full-load current. Multiply the motor’s FLA by the multiplier associated with the code letter, and that product sets the maximum OCPD rating you may use. You then choose the largest standard OCPD rating that does not exceed that calculated maximum. With a FLA of 93 A and code letter J, the multiplier gives a calculated maximum around 800 A. Since 800 A is a standard available size and does not exceed the computed limit, it is the largest permissible option. Therefore, the maximum standard-size overcurrent device you can use is 800 A.

In fire-pump installations, overcurrent protection is sized using a nameplate code letter that acts as a multiplier to the motor’s full-load current. Multiply the motor’s FLA by the multiplier associated with the code letter, and that product sets the maximum OCPD rating you may use. You then choose the largest standard OCPD rating that does not exceed that calculated maximum.

With a FLA of 93 A and code letter J, the multiplier gives a calculated maximum around 800 A. Since 800 A is a standard available size and does not exceed the computed limit, it is the largest permissible option. Therefore, the maximum standard-size overcurrent device you can use is 800 A.