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I am going to install a GOULDS 2 HP 13GS20 pump with a FRANKLIN submersible motor (sorry could not get a specific model on this one)

In our area deep well pumps must have their own electrical meter. Since we have to provide everything after the meter, i want to make sure the wire size im going to buy is the correct size.

Electrical Meter -> 200 meters aluminum wire -> deep well site -> 75 meter copper submersible wire -> submersible pump

As an added constraint the 75meter submersible wire comes only in 12 AWG (4 sq. mm) and anything other than that would require a backorder and waiting about 1 - 3 months so if possible i would like to use that guage for the copper submersible

So from what i know the challenge i have to overcome is voltage drop, i must not let it exceed a certain percentage.

so first question what is the difference between a 5% and 3% voltage drop ? why should i choose 3% when the motor can run fine at 5%?

I am using this calculator for my computation and using 240V @ 10A

75 meter copper submersible will require 10 AWG for 3% drop and 12 AWG for 5% drop

In order to not bring the voltage much lower if we want 12 AWG for the copper wire, i decided to go with a 2 AWG for the aluminum that has a voltage drop of 3.5 volts.

In total i have 14.5 V of drop ( 11v from 12AWG copper and 3.5v from 2AWG aluminum)

So am i right to use these sizes?

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  • In my personal experience, the pump&motor documentation includes considerable information specific to the exact pump motor installed regarding wire size .vs. length, and being from the manufacturer may be more applicable than using a random calculator for that. Significant voltage drop is common in deep well pumps, to the point that you can even find lower voltage pump motors that are not a "standard voltage of the country they are being sold in" as a way of compensating. The same "pump" is typically available configured with different motors, so ideally you get that detail sorted.
    – Ecnerwal
    Jul 18 at 16:16
  • Voltage drop recommendations in the US are just that in other countries they are actual limits so knowing where you are at in the world will change the answer. The NEC recommends the maximum of 5% at the end of the line. This is only a recommendation can you have a larger drop? Possibly 10-15% at start up and the motor will usually have a normal service life. I have installed both deep wells single phase and multi phase wells well beyond the normal voltage drop but not exceeding the manufacturer’s minimum. So the operation range is needed if the motor is rated 208-240 then a 190v at startup ok
    – Ed Beal
    Jul 18 at 16:36
  • @Harper-ReinstateMonica its is in a remote area, and the well area is 200m from the nearest electrical post. I do not even think they can give me ay higher, the nearest transformer is nowhere to be seen , the meter will be installed next week so it is only next week i will see if the voltage supplied to us is even near the supposed 240Vac . Im currently scouring the internet for the manual of the motor to see its operating voltages
    – DrakeJest
    Jul 18 at 17:35
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    You won't be able to meaningfully measure the voltage until you put a well-pump-sized load on it. Are 240V/480V transformers in the 5 KVA range something that are readily available in your country? (Which it would help to tell us something about, by the way). Jul 18 at 17:39
  • @Harper-ReinstateMonica its in a somewhat remote area in malaysia, we basically just order parts from the city. I do have some 2HP motor that we can use to test it out. What im worried though is if i dont get the wire size correctly(even after allowances) it might just be wasted. Will i need 2 transformers 240/480 then back to 480/240?
    – DrakeJest
    Jul 18 at 17:52

1 Answer 1

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Voltage drop is proportional to current. So at zero current you'll see zero voltage drop.

A 2-horsepower motor draws about 12 amps at 240V, according to the NEC table. (I know that doesn't make any sense when you take 1 horsepower as 746 watts, but motor horsepower is funny.)

Anyway, plugging that into a voltage drop calculator, your #12 wire works out to 4.46% voltage drop going down the 75m of pipe shaft.

So you don't have a huge margin to waste on the 200 meter run from the utility. That 200m is no joke, you're going to have to massively enlarge the wire - fortunately you're using aluminum.

Now in North America, 2 AWG aluminum is extremely common and has a pricing "sweet spot". That's my go-to for a lot of things, and that would work alright here, giving 1.93% voltage drop for the 200m run at 12 amps. The next sizes up are:

  • 1 AWG: 1.59% drop
  • 1/0 AWG: 1.37% drop
  • 2/0 AWG: 1.07% drop

So it comes down quite a lot to what your electric distributor has in stock, and how it's priced. Pricing gets weird because some sizes are commodities (like North American #2) and other sizes are slow movers.

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  • In the calculator you have linked, is the "length of cable run" round trip length or just 1 way>?
    – DrakeJest
    Jul 18 at 19:11
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    @DrakeJest any calculator aimed at practical installers will be round-trip. I have done the raw calculations and compared it to this calculator, and it's definitely accounting for round trip, with an extra allowance for imperfect power factor, which you need on a motor. Jul 18 at 19:18
  • Turns out you cannot get lower than 2AWG anywhere here.... There will be guys doing a pump test so we will see if these wires are enough, will update agai
    – DrakeJest
    Jul 21 at 6:34

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