Wire Gauge and AC vs. DC Flow

In general yes. If the 20 amps AC is peak. If the 20 amps is RMS than the peak current will be 28 amps.

how far is the cable run???? i suggest you double the size of the conductor otherwise you might only have 10 volts when you get there

The wire gauge or cross sectional area will need to be the same for ac as dc when you refer to short runs but you must make the calculations for voltage drop on 12 or 24V (ac or dc) if you are accustomed to using 110V or 230V. It is one of the dangers of using "rule of thumb" guides for things like wire size.

I will relate an amusing story as a superb example of this.

A client of mine went sailing with a friend who was a successful electrical contractor. During the cruise the forward toilet pump failed and they purchased a new electric unit to replace it. They installed it in a couple of hours but found that it would pump out the water but as soon as the macerator impeller of the pump hit anything a little more solid (Paper or SH1t) it would stall the motor. I looked at the installation and immediately realised the wire was too light for the job. The electrician was surprised because his rule of thumb allowed 2.5mm² acceptable for the rated motor current of 15Amps. I did a voltage drop calculation for him which he immediately understood and accepted. Even though this was only a 10m (33ft) run the simple calculation gives about 9.5V at the motor. In fact I recall measuring about 6V because as the motor slows it draws more current than its rated 15A.

regards

Chas

'120 VAC',by convention, is sine wave AC with the same 'heating value' as DC of the same measured voltage. All our voltmeters are designed to incorporate this convention. The true peak voltage in '120VAC' is I believe around 170 volts

There is no longer a need to guess. A wire guage calculator is online:

http://www.powerstream.com/Wire_Size.htm

In a purely resistive load,there is no difference between DC and AC voltage drop calculations.Inductive/capacitive loads introduce impedance that is not present is DC circuits. Number 14Ga. copper wire is rated for 15 amps,#12 is rated for 20 amps,#10 is rated for 30 amps.The amp rating of the wire is determined (at least by the NEC) by the Temperature Rating of the insulation,Not the voltage drop.You must calculate voltage drop separately.This calculation will be the same for DC or AC of the same RMS. The wire manufacturer has no idea how long your circuits will be. An example:IF you have a #3/0 conductor with 90degree C insulation, and it passes through a boiler room ceiling where the temperature is 90C, how many amps can you SAFELY carry with this conductor?Answer: ZERO.ANY current flow will cause the insulation to surpass the insulation rating. When it comes to switches,contacts, and disconnects, it is harder to interrupt DC current than AC current,because AC crosses the zero point many times per second, depending on frequency,and DC does not.That is why switches, contacts, fuses,etc., always have a lower rating for DC than AC. The reason AC is used in modern power stations is because it is Easily transformed into different voltage levels for long distance transmission and back down for local use. DC, however is also used in high power,high voltage, high power transmission systems, such as the grids from Canada to the USA.The power is converted to AC by inverters, and phase-matching between grid systems is very easy by this method.DC actually has less line losses than AC, but inverters are too expensive for general use.

Greeting.

More people and contractors burn out or damage electrical saws, heavy drills, etc because they think that a 12 gauge 25 feet long cord that would work is the same as a 12 gauge cord that is 100 feet long. I like the double your wire size idea. That is how I try to do it.

When it comes to drop cords, use the biggest gauge you can afford.When bidding on a job, you must use the minimum safe requirement that conforms with the NEC or you will not win any bids.Competition is fierce.

There is a common misunderstanding that DC will not carry as far as AC, which is the reason we have AC everywhere instead of DC.This is not true.AC is simply easier to convert to a higher voltage and back.DC does not work on transformers, must use inverters.Inverters are very expensive, and did not exist in Tesla's era, when AC was invented. Edison fought for DC,and opposed Tesla at every move,Edison wanted to build generating stations every few miles.But AC finally won out because of the reasons just mentioned. In answer to your question:The amp rating is the same,however, as length goes up, so does resistance (R), as load (amps) (I) go up, so does voltage drop.E=IxR (Current times resistance;in this case (voltage drop), the wire resistance ) Same formula for DC or AC if a purely resistive circuit. Hope this clears up the confusion.

Hi

With lower voltage you get more powerloss per.meter. so with 12VDC you need bigger cable if you have big load and long way to go. Another thing to consider is that with DC system you need special breakers, relays etc. the DC system tend to burn more contacts than AC systems.

tomkoko,

People here have bounced around what I think are the two critical things you should take from your question.

1. The wire gauge for current draw ratio that several posters have touted here are "rules of thumb" values based on the power dissipated per unit length of this size wire. Following this guide will prevent failure of the wire's insulation.
2. The voltage drop created by your wire run will be identical for the same current whether AC or DC. If your AC voltage source is 120V a 6 volt drop will probably not be significant. But with a 12VDC source a 6 volt drop will be a very significant loss.