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Well yesterday’s post got a response and good point made from Karl in NY. Karl is concerned with a specific remark I made in the post relative to wire ampacity.

Comment text: “(Note: use of higher voltage allows the use of smaller wire and cabling to carry the same amperage. In other words, at 12 volts if you need a 4 gauge wire to carry a certain amount of amperage safely, at 24 volts, you could carry the same amperage with a much smaller gauge wire.)”

I really believe that is incorrect. I think the ampacity of a wire is independant of the voltage. Did you perhaps mean to say “wattage” rather than “amperage”?

Karl raises a good point, but in fact he is only partially correct in actual practice. To Karl’s point let’s take a look at the basic equation used for calculating power:


power equation

The clip above comes from my book “The Powerboater’s Guide to Electricity” which is available here (click on link in left column). What is being shown here is the mathematical relationship between volts, amps and watts. It is Karls correct assumption that if voltage increases and amperage is a constant, wattage or power will increase. Mathematically this is absolutely true. Plug some numbers into the equation above and try it for yourself. Volts times amps equals watts. Watts divided by amps will equal volts and watts divided by volts will equal amps. This is an absolute folks, there is no maybe about it.

So, with this knowledge at hand, Karl is correct in his thinking that if we increase voltage from 12 volts to 24 volts and amperage is a constant, power, or wattage will increase. But, where Karl is in error is from a practical perspective is in his assumption that a wire’s ampacity is totally independant from the voltage involved. The clip below, which comes from the ABYC E-11 Standard and is a partially displayed table used to size wire for a pre-determined amount of voltage drop and nominal system voltages at different circuit lengths given in feet and various amperage values as displayed in the far left hand column illustrates all that I was trying to say in my post yesterday.


voltage table

So, using an example and the table above, let’s say we have a 12v circuit that needs to carry 15 amps and the circuit is 20 feet in length. Based on the table, you will have to use 10 AWG wire to maintain a maximum 3% voltage drop throughout the circuit. Now look at what happens if the voltage gets upped to 24v. Same circuit length, same 15 amps, but you’ll be able to use 12 AWG wire to maintain the same level of voltage drop. I’ve described this to others in the past as the “adjusted ampacity” of wire. For the technical purists out there, my apologies for slaying the important term,,,,,,,,,ampacity.

Higher volts= smaller wire for the same level of current or amperage and yes, more power as well. Thanks for your note Karl and I hope this clears things up for you and others who may have had the same thoughts.

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