Clarification on Voltage drop, 120 vs 240


Old 04-24-18, 11:20 PM
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Clarification on Voltage drop, 120 vs 240

Here is the situation:

I have a 30amp double-pole (240v) breaker in my main panel that powers a sub-panel in my well house. The well-house is 100 feet from the sub panel, and the wires feeding it are 10 gauge.

When I plug these numbers into the voltage drop calculator, it's all good: about 2.5% drop.

However: In the well-house sub-panel, the only device that uses 240 is the well itself, which is on a 15amp double pole breaker. Everything else (lights, and multiple exterior outlets for the yard) are 120volt.

So how do I calculate that? Do I just assume 240 volts even though everything that would get plugged into those outlets ---like a power saw, or other high-draw device----is running on 120v (effectively only one leg of the 240)?

Or do I have to do two separate calculations, one for the amperage draw of the 240v load (the well pump) and one for the potential draw of the 120v outlets, and then add those numbers together to get the total voltage drop?

Or am I just totally screwed-up and should probably tear-out the electrical all together and install one of those quaint old-timey wells with a little bucket on a rope? O_o
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Old 04-25-18, 06:17 AM
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If you want an accurate voltage drop you need to calculate using the voltage of the load.
Old 04-25-18, 06:33 AM
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Voltage drop depends on the round trip distance from the source to the load. Some calculators second guess for better or worse that when you specify the length of a cable the conductor length is twice that to account for the round trip.

You need to know and use the peak draw of the load, notably the starting amps draw of a pump or power tool or refrigerator, etc.

When the motor or other load offers a choice of source voltage typically manually selected via jumper wires, note that the amperes draw at 240 volts is about half the amperes draw at 120 volts. Voltage drop in a given piece of wire (e.g. measured from near the feeding panel breaker terminal to near the receptacle the tool is plugged into) depends on the amperes flowing and does not depend on the source voltage.

For your pump and other 240 volt equipment assign the amps draw to both of the hot lines making up the 120/240 volt feed. For each 120 volt item assign the amps draw to the one hot line to which the receptacle is connected. The neutral current is the difference between the currents on the two hot lines. .

Compute three one way voltage drops for one hot line, the other hot line, and the neutral respectively.

The significance of more or less balancing the 120 volt loads using both hot legs is that the draw on either hot leg will not get that high and the flow on the neutral will be very small resulting in a lower total voltage drop when you compute the voltage drop for the worse round trip (hot and neutral) for one of the groups of 120 volt equipment.

For DC (e.g. ordinary automotive) circuits and ordinary household AC circuits and also low voltage circuits (such as garden lights) you can use the formula: voltage (lost in a given piece of wire) equals the amperes flowing at that moment times the resistance of that wire. Several sources on the internet have resistance values for wires of various gauges.

Last edited by AllanJ; 04-25-18 at 07:08 AM.
Old 04-25-18, 06:45 AM
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The amperes drawn will change based on the voltage .
Old 04-25-18, 11:57 AM
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You have a sub panel predominately dedicated to your well pump. The well must take precedence over other loads. There should be minimal voltage drop when using a saw or other heavy starting load but continuous loads should be avoided so as to not reduce the voltage to the well pump.

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