Well Pump Amps, Wire Size and Breaker Size by Horsepower

How many amps a well pump draws

The table below is the NEC full load current for single phase motors (Table 430.248) and three phase motors (Table 430.250). The NEC requires these table values, not the nameplate, when you size the branch circuit conductors and the short circuit protection. The nameplate amps are used for the overload. Submersible pump motors also list a higher service factor amps figure, and that is the most the motor should draw when fully loaded. A pump on a 120 volt circuit uses the 115 volt column and a pump on 240 volts uses the 230 volt column.

NEC Tables 430.248 and 430.250 as reproduced in electrician study guides. For three phase motors below 2 HP the published 230 V and 460 V figures differ between NEC editions, so use the table in the edition your inspector enforces or the motor nameplate.

Breaker size for a well pump

A motor circuit is allowed a larger breaker than its running current because the starting current is several times higher. Under NEC Table 430.52 an inverse time breaker can be up to 250 percent of the table amps, rounded up to the next standard size when 250 percent falls between sizes. That is a maximum, not a target. The conductors must carry at least 125 percent of the table amps under NEC 430.22. Separate overload protection, in the motor, the control box or a starter, protects the motor itself. Many submersible pump manuals list a smaller fuse or breaker for their control boxes, and where they do, use theirs.

Computed from the table amps. Breaker is 250 percent rounded up to the next standard size. Conductor is 125 percent, with the smallest copper size whose 60 C ampacity in NEC Table 310.16 meets it. Voltage drop over a long run usually needs a bigger wire, see below.

Breaker and wire for three phase pumps at 460 volts

Farm, irrigation and commercial wells often run three phase pumps at 460 volts. The same rules apply. At 460 volts the current is a quarter of the single phase 230 volt figure for the same horsepower or less, which is why large pumps are almost always three phase.

Computed from NEC Table 430.250 amps at 460 V, the same way as the single phase table.

Well pump wire size by run length

On a well the cable runs from the panel or control box down the casing to the motor, often hundreds of feet, so voltage drop sets the wire size long before ampacity does. A motor starved of voltage draws more current, runs hot and struggles to start. The lengths below are the longest one way run that keeps the drop to 3 percent at the table amps, computed from the copper resistance at 75 C in NEC Chapter 9 Table 8. Stranded wire reads slightly higher resistance than solid, so treat the figures as the outer limit.

Computed, 230 V single phase, 3 percent drop, solid copper resistance 3.07, 1.93, 1.21 and 0.764 ohms per 1,000 ft for 14 to 8 AWG and stranded 0.491 and 0.308 for 6 and 4 AWG. Too small means the wire cannot carry 125 percent of the table amps at 60 C. A pump maker's cable chart for your exact motor overrides this table.

Run lengths for three phase pumps at 460 volts

Three phase at 460 volts reaches much further on the same wire, because the current is lower and the allowed drop in volts is higher.

Computed, 460 V three phase, 3 percent drop, same resistances, length equals allowed drop divided by the square root of 3 times resistance times amps.

When a well pump needs a transformer

The usual case is a 460 volt three phase pump on a site that only has 208 volt or 240 volt three phase service, or a single phase farm service where the utility will not extend three phase. A step up transformer from 208 or 240 to 480 volts solves the first. The running kVA of the pump sets the floor, and the transformer then needs headroom for the motor's starting current, so confirm the size with the transformer and pump makers using the motor's locked rotor data from the nameplate code letter. The table shows the running kVA and the current the pump draws on the 208 volt side of a step up transformer.

Computed. Running kVA equals the square root of 3 times 460 V times the table amps. Supply side amps ignore transformer losses.

Generator for a well pump

A generator has to start the pump, not just run it, and a submersible motor's starting current is much higher than its running current. Generac's sizing guidance treats submersible pumps as a special case outside the usual motor sizing method, and Grundfos notes that a generator rated in kVA rather than kW needs its rating multiplied by 1.25 for its submersible motor sizing table. Size the generator from the pump maker's minimum generator table or the motor's locked rotor kVA, and if the generator is borderline a soft starter or a variable frequency drive lowers the starting load.

2 wire and 3 wire submersible pumps

The ground wire is not counted in either name. A 2 wire submersible pump has its starting capacitor and relay inside the motor down the well, so it needs only two power conductors plus a ground, which is why 12/2 with ground pump cable is so common on house wells. 2 wire motors are made only up to about 1.5 HP, and if the starting parts fail the pump has to come out of the well. A 3 wire pump keeps the capacitor, relay and overload in a control box above ground, so it needs three conductors plus a ground between the box and the motor, a failed capacitor can be changed without pulling the pump, and the motors go up to about 15 HP. Three phase pumps also use three conductors plus ground, with a starter instead of a control box.

Constant pressure and VFD pumps

A constant pressure system runs the pump through a variable frequency drive. The drive soft starts the motor, which cuts the inrush that trips breakers and bogs down generators,. Size the breaker and the supply wire from the drive's input current in its manual, not from the motor tables, because the drive is the load the circuit sees.

How many amps does a 1 HP well pump draw

By NEC Table 430.248 a 1 HP single phase motor is 8 amps at 230 volts and 16 amps at 115 volts. A three phase 2 HP pump is 6.8 amps at 230 volts and 3.4 amps at 460 volts. The service factor amps on a submersible motor's nameplate are higher and are the most it should draw at full load.

What size breaker for a 1 1/2 HP well pump

A 1.5 HP pump at 230 volts has a table current of 10 amps. 250 percent is exactly 25 amps, so the largest inverse time breaker the NEC allows is 25 amps, on wire rated for at least 12.5 amps. The pump maker's manual may call for a smaller size, and if it does, use that.

What size wire for a well pump 200 feet away

For a 1 HP pump at 230 volts drawing 8 amps, 12 AWG copper keeps the drop under 3 percent to about 220 feet one way, and 14 AWG only to about 140 feet. At 2 HP and 12 amps, 12 AWG reaches about 140 feet, so a 200 foot run needs 10 AWG. Always check the pump maker's cable chart for the exact motor.

Can I put a well pump on a 20 amp breaker

A 1 HP pump at 230 volts with an 8 amp table current can be on a 20 amp breaker, since 250 percent of 8 is 20. A 1/2 HP pump at 4.9 amps can go up to 15 amps. Larger pumps can need more, so work it out from the table amps.

Does a well pump need its own circuit

In practice yes. A pump motor sized under NEC Article 430 is on its own branch circuit with its own breaker sized from its table current, and a 230 volt pump takes a two pole breaker.

Can I run a 460 volt pump on 240 volt service

Not directly. You need a step up transformer from 240 or 208 volts to 480 volts sized for the pump's running kVA with headroom for starting, or a pump and motor built for the voltage you have.