Need Some Practical Electrical Advice

Old 04-27-14, 02:43 PM
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Need Some Practical Electrical Advice

I am thinking of replacing the 10 ga running to my A/C compressor and water heater with larger wire and want to know if it will actually save some money. The A/C compressor pulls 9.8A a distance of ~50 ft ,and the water heater pulls 19.6A over a distance of 62 ft. I measured that amp draw at the 240 terminal at my outside service panel, and the voltage was 248v while they were runnning.

What would the actual reduction in power usage be?
Old 04-27-14, 02:49 PM
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Welcome to the forums!

What would the actual reduction in power usage be?
Essentially none.

Why do you think using larger gauge conductors might reduce your power consumption - that depends on the load. IOW, if you want to reduce electrical consumption by any measurable amount you need to install more efficient appliances.
Old 04-27-14, 03:34 PM
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want to know if it will actually save some money
You will save zero money, and it will actually cost you more money besides the cost of the wire. Power is watts.

Using your water heater numbers:
240 volts @ 19.6 amps is 4704 watts and 12.2449 ohms

#10 copper wire has a resistance of .062 ohms
#8 copper wire has a resistance of .039 ohms
So deduct .062 from 12.2449 for the #10 wire and add back .039 for the #8 wire and you get 12.2219 ohms.
Plug that in our handy dandy ohm's law calculator and you get 4713 watts for a difference of 9 watts. Notice the wattage went up, not down. So since you pay per kilowatt hour, that larger wire will cost you more, not less.

This should not give you the idea to run smaller wire, unless you want to burn your house down.
Old 04-27-14, 07:08 PM
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Using the water heater numbers:

The number of volts lost in the 10 gauge wires is19.6 amps times 0.062 ohms, or 1.22 volts.

The number of volts lost in the 8 gauge wires is 19.6 amps times 0.039 ohms, or 0.76 volts.

The difference is 0.46 volts, times 19.6 amps equals 9.0 more watts lost in the 10 gauge wires compared with in the 8 gauge wires. In winter this heats up the house a tad more so it is not wasted.

It has not been stated how much less resistance the water heater element has because it did not attain quite as high a temperature as a result of receiving 0.46 fewer volts. So we are not ready to draw any conclusions as to the difference in amperes drawn with 8 gauge wires versus 10 gauge wires.
Old 04-27-14, 07:25 PM
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Save your money and time and resources. The wire to the AC is already oversized.
Old 04-27-14, 08:12 PM
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Insulate your water heater (even if the tank is already internally insulated, you can still make it lose a few less degrees by putting a blanket around it).

Insulate your hot water pipes if they are exposed in the basement.

Insulate your ductwork - ESPECIALLY if you have vents or returns running through the attic.

Make sure your outdoor unit is in full shade all day. If it's not, plant some tall bushes or install a trellis with vining plants near it to cast shade (don't get within 2-3 feet of it or you'll restrict airflow).

Have the system serviced.. If the gas charge is low, the unit will work harder.
Old 04-27-14, 08:55 PM
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IsqR Losses

Based on your water heater circuit at 62ft, going from #10 Solid to #8 Stranded, with a load of 19.6 amps you would save about 21 watts. It would take over 47 hours of operation at the full load, to save (your Kwh rate/hr). Not a good return on investment. Using the energy saving methods mentioned would save you more money.
Old 04-28-14, 05:14 AM
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I forgot to mention that if the water heater itself drew more power due to lower voltage (whether due to voltage drop in the wires or due to getting 208 volts from one kind of 3 phase hookup instead of its rated 240 volts) then all of the extra watts drawn go into heating water. That is, there is no waste there.

Then the water heater would have cycled off sooner making its own power consumption the same regardless of voltage or wire size.

Last edited by AllanJ; 04-28-14 at 05:30 AM.

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