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Old 03-06-2016, 12:00 AM   #71
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Respectfully disagree. Lew is correct in using that table.

Wire size is properly chosen based on amperage, and acceptable voltage drop over the length of the wire, period.

Insulation thickness and type is chosen for voltage and temperature rise caused by the current

A 1500 watt heater in my AS would NOT have a 16 gauge cord. It's too light to be safe. It would be 14 gauge minimum or I would not buy it.

A degree in Electrical Engineering and about 40 years of designing power distribution and grounding systems leads me to this conclusion




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Old 03-06-2016, 06:51 PM   #72
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We are talking about two different things. In solar, you speak in watts as that is the amount of power.

Take a space heater, 1500 watts of power.

A 12v 1500 watt space heater draws 125 amps! That is some thick wire required.

A 110v 1500 watt space heater draws 13.6 amps. Now the wire doesn't need to be so thick.

Whether you think 16 gauge cord is no good for 110v at 1500 watts (6 foot cord), the device is UL certified. When you increase the voltage of the solar system (to a point), the thinner the gauge wire can be used.

I think the confusion is people are talking about those charts at 12v only. 12v = thick wire needed. Bump your solar array up to say 48v and much thinner wire needed as the AMPs are changed.

600 watts of solar at 12v = 60 AMPS. 20 foot run within 3% voltage drop = 4 gauge wire.
600 watts of solar at 48v = 12.5 AMPS. 20 foot run within 3% voltage drop = 10 gauge wire.

There is a big bonus for increasing the voltage of many electrical systems. Hence why 220v in Europe has less losses than our 110v system in the US.

My favorite chart:




Alano's point was that if you can raise the voltage of the solar panels, you can use the factory pre-wiring at much higher wattage than you could if you kept it at 12v. This is absolutely true.

Now of course every time you change voltage etc you incur a loss, so all of the losses have to be taken into account.
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Old 03-06-2016, 07:50 PM   #73
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Many other issues exist with higher voltage solar arrays in an RV installation other than being able to simply using thinner wire. After almost 30 years in the RV solar business, AM Solar has effectively been there and done that and has experimented with many different components and wire/panel configurations in real world situations.

While an electrically savvy DIY installer can expect some success in going 'big' with panel output voltage, there are other concerns that we have seen which make the use of higher voltage solar arrays challenging with regard to the expected consistency for commercial installation purposes.
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Old 03-06-2016, 09:06 PM   #74
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I've had very good success stacking modern panels in a series/parallel arrangement to create a 400 W (four 100 W panels) solar array using the existing #10 AWG pre-wiring with wiring losses less than 3%. This is largely made possible with newer technology MPPT solar controllers. All specs are well within limits and after a year I've had no issues whatsoever. I have approximately 200 Ahr of total battery capacity and find that even if I consume 50% of the batteries' capacity, the solar charger is back to 100% even on overcast days. I also store my trailer under a cloth tent and I'm surprised that even with the diffused lighting of the covered tent, the solar system keeps the system at 100%.

For those of you DIYers who have some electrical knowledge, don't be afraid to install a generous solar system for a fraction of the cost of a factory or professionally installed system. There's plenty of folks on this forum who can share their experience!
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Old 03-06-2016, 09:31 PM   #75
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I think around 800 watts of solar and 1,000 AH of battery (3000 watt inverter) you can become completely independent of shore power. Unless it's something crazy like 110 degrees outside and you need to run both air conditioners.
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Old 03-07-2016, 07:54 AM   #76
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A 1,500 watt heater is a resistive load so spikes in the power draw. That is approximately 13.6 amps (1500 watts/110 Vac = 13.6 amps). Wire in branch circuits must be derated 20% per the NEC for continuous loads, #14 wire is rated 15 amps. Derated it has a 12 amp continuous load capacity. #16 wire is rated 10 amps or 8 amps for continuous loads. #12 wire is rated 20 amps and must be derated to 16 amps for continuous loads.

The possibility of a short in the #16 wire or the appliance means the wire could melt before the 15 or 20 amp breaker opened.

Just something to be aware of.
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Old 03-07-2016, 10:49 AM   #77
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NEC defines "continuous loads" as those of 3 hours or more in duration. That usually only means electric heaters, air conditioners, and water heaters. Even water heaters in RV use rarely run for 3 hours at a time. BTW, the de-rating also applies to the circuit breakers used on those same circuits. But to be clear, a 20 amp circuit, can legally cary 20 amps, and will do so just fine. However, if it is supplying a load known to be be "continuous" it must be de-rated to only 80% of capacity. This gets to be a somewhat hard to understand area of the code.

Yes, a smaller wire running to anything like a radio or clock which is not rated for 20 amps could in some strange circumstances become overloaded and fail prior to the typical 20 amp interior circuit breaker opening. But to build things like clocks, TV's and radios with #12 wire to them would make them clunky and very inconvenient, so the UL listings allow for smaller wire to plug in individual devices.

I am not disagreeing with anyone here, only trying to clarify the somewhat difficult to understand National Electrical Code, and some of the UL regulations and real life considerations. As said in the post above, just something to be aware of.
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