Fire Pump Wire Size Calculator
Size the supply conductors from the power source to a fire pump controller under NEC 2020 695.6(B)(1). Enter the fire pump motor by horsepower or full-load current, add the pressure-maintenance (jockey) pump and any accessory load, and the tool applies the 125 percent motor plus 100 percent accessory rule and returns the smallest Table 310.16 copper or aluminum conductor that meets it.
How to use this calculator
- Choose the supply phase and voltage; these read the motor full-load current from Table 430.248/430.250.
- Enter the fire pump motor by horsepower, or switch to full-load current for motors above 200 HP or a marked controller rating.
- If the jockey (pressure-maintenance) pump shares the supply, add it by horsepower or current; otherwise leave it off.
- Add any associated accessory equipment full-load current, which is counted at 100 percent.
- Pick the conductor material and termination column, adjust ambient or conductor count if needed, and read the required ampacity and the smallest qualifying conductor.
NEC reference
This calculator follows NEC 2020 Section 695.6(B)(1): conductors supplying a fire pump motor, pressure maintenance pump motor, and associated fire pump accessory equipment shall have a rating not less than 125 percent of the sum of the fire pump and pressure maintenance motor full-load currents, plus 100 percent of the associated accessory equipment. Motor full-load current comes from Table 430.250 (three-phase) or Table 430.248 (single-phase) per 430.6(A)(1). Conductor ampacity comes from Table 310.16, with Table 310.15(B)(1) ambient correction and Table 310.15(C)(1) adjustment when you enter non-base conditions, and the 110.14(C) termination cap. The 125 percent and 100 percent factors are unchanged across the 2017, 2020, and 2023 editions.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Why fire pump conductors carry extra ampacity
A fire pump is a life-safety load that must run no matter what, so the NEC treats its supply differently from an ordinary motor feeder. The supply is not permitted to open on running overload (695.4(B) and 695.6(C)), which means the conductors have no ordinary overload device standing behind them. 695.6(B)(1) compensates by forcing the conductor ampacity up front: 125 percent of the sum of the fire pump and pressure-maintenance motor full-load currents, plus 100 percent of the associated accessory equipment. The overcurrent device that is present must instead be sized to carry the pump locked-rotor current indefinitely.
The 125 percent sum, not the feeder rule
The most common mistake is to size a fire pump supply like a multi-motor feeder under 430.24, taking 125 percent of only the largest motor and 100 percent of the rest. 695.6(B)(1) is different: the 125 percent applies to the sum of every fire-pump-related motor, so the jockey (pressure maintenance) pump motor sits inside the 125 percent term with the fire pump motor when it is fed from the same conductors. Only non-motor accessory equipment, such as controller auxiliaries, cabinet heaters, and lighting, is added at 100 percent, and it is added after the 125 percent motor term.
Worked example: 100 HP fire pump with a 5 HP jockey
- A 100 HP, 460 V, three-phase fire pump motor has a Table 430.250 full-load current of 124 A.
- A 5 HP, 460 V jockey pump on the same supply adds 7.6 A, for a motor sum of 131.6 A.
- Required ampacity is 125 percent of the motor sum: 131.6 x 1.25 = 164.5 A (no accessory load in this example).
- In the 75 degrees C column of Table 310.16, 1/0 copper is 150 A, short of 164.5 A.
- 2/0 copper is 175 A, which meets it, so the minimum supply conductor is 2/0 copper.
- Size the overcurrent device to carry locked-rotor current per 695.4(B), and check 695.7 voltage drop separately.
Full-load current: table value, not nameplate
Per NEC 430.6(A)(1), conductor sizing uses the full-load current from Table 430.250 for three-phase motors or Table 430.248 for single-phase, selected by horsepower and voltage, rather than the motor nameplate current. This tool reads those tables for you. Fire pump motors are often larger than the 200 HP three-phase table ceiling, and a listed fire pump controller frequently marks the full-load current to use; in either case, switch the input to full-load current and enter the value directly.
Voltage drop and overcurrent are separate
This calculator sizes the conductor for ampacity only. Fire pump feeders are frequently governed by NEC 695.7 voltage drop instead: not more than 15 percent below normal at the controller line terminals while the motor is starting, and not more than 5 percent while running at 115 percent of the full-load current. On a long run, that limit can force a conductor larger than ampacity requires. Size for ampacity here, then confirm 695.7 with the voltage drop calculator, and size the overcurrent device to carry locked-rotor current per 695.4(B) rather than as a standard 240.6(A) device.
Why this tool may size one step larger than others
This calculator holds the required ampacity to its exact value and never rounds it down to the nearest standard conductor. When the requirement lands just above a conductor's Table 310.16 rating, the tool steps up to the next size. That is the never-undersize choice of the accuracy principle this site is built on, not a disagreement about the NEC method, and it matters most on a life-safety circuit where the conductor must not be marginal.
Frequently asked questions
How are fire pump supply conductors sized?
Take 125 percent of the sum of the fire pump and jockey pump motor full-load currents, add 100 percent of the accessory load, then pick the smallest Table 310.16 conductor that meets it. A 124 A fire pump motor needs 155 A, which is 2/0 copper at 75 degrees C.
Is the jockey pump added at 125 percent or 100 percent?
At 125 percent. The pressure-maintenance (jockey) pump motor is summed with the fire pump motor inside the 125 percent term, not added at 100 percent like the 430.24 feeder rule. Only non-motor accessory equipment is added at 100 percent.
Do fire pump supply conductors need overcurrent protection sized?
The supply overcurrent device must carry the pump locked-rotor current indefinitely per 695.4(B)/695.6(C), and standard running-overload protection is prohibited, so this tool reports only the conductor and no standard breaker or fuse rating.
Does the calculator check fire pump voltage drop?
No. NEC 695.7 voltage drop (15 percent starting, 5 percent running) is a separate calculation; size for ampacity here and verify 695.7 with the voltage drop calculator, since a long run can force a larger conductor.
Where do I get the fire pump motor full-load current?
From Table 430.250 (three-phase) or 430.248 (single-phase) by horsepower and voltage, per 430.6(A)(1), not the nameplate. For motors above 200 HP or a marked controller rating, enter the full-load current directly.
Related calculators
- Motor Full-Load Current Calculator: look up a motor's Table 430.248/430.250 full-load current by horsepower and voltage.
- Voltage Drop Calculator: check the 695.7 starting and running voltage drop on the fire pump feeder.
- Wire Ampacity Calculator: the Table 310.16 ampacity of a chosen conductor, with ambient and fill derating.
- Equipment Grounding Conductor Size Calculator: size the EGC for the fire pump circuit per Table 250.122.
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