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# DC:Why is the positive wire thicker on the positive on some cables?

#1
05-05-14, 03:56 AM
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Join Date: Apr 2014
Location: South Africa
Posts: 13
DC:Why is the positive wire thicker on the positive on some cables?

Hello

I have a small solar power set up that powers our outside lights. Through it I have had to make myself very familiar with electronics LED lighting I constructed my own lights etc. for 12 V and all things DC. This little plant is the joy of my world.

As I do not have a comprehensive knowledge of DC power and the complexities of it.

I have noticed in the electronic shops, in the cable/wire section the cables for DC power the positive wire is much thicker often than the negative wire.

What is the rational behind that? Is the load actually carried by the positive cable in a DC system. What role does the negative then play.

I was wondering as I have more questions relating to DC power consumption, deep cycle batteries and DC in general and some ideas I have had. Is this the correct forum to ask these questions.

Regards

Godfrey

#2
05-05-14, 04:59 AM
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Join Date: Mar 2010
Location: USA
Posts: 4,460
Are you sure that the metal core is fatter or just the overall plastic covering?

Are you sure that some of the wire sizes and colors were not out of stock leaving perhaps a fatter red wire (for DC positive) and a thinner black wire (for DC negative)?

The current is the same on both wires going to and from a device or appliance or light or other load, for both AC and DC.

For 12 volts (AC or DC) in typical household wiring, you need much heavier wire for a given number of amperes compared with for 120 volts. For example, 12 gauge wire is good for about 3-1/2 amps at a one way distance of 50' from the power source, where you would lose about 4% of the voltage in the wires. Or you can draw 5 amps losing about 6%.

Numerous permutations of voltage drop versus distance versus wire size can be had including if you used different sized positive and negative wires. There is just one rule (Ohm's law): the voltage drop in a piece of wire equals the amperes flowing at that moment times the resistance of that wire.

While the power wasted in the wires produces a small amount of useful heat in winter, there is less voltage available to the load. Some equipment may draw more amperes to make up for the lower voltage. Other equipment may just run less efficiently and lights will be a little dimmer than expected.

Last edited by AllanJ; 05-05-14 at 05:20 AM.
#3
05-05-14, 05:18 AM
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Location: USA
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I would guess it is because of "Ohm's law", and that if the positive is larger, each device aling the circuit has its own ground. Positive has to be bigger to maintain the same voltage and amperage as the line gets longer.

#4
05-05-14, 05:25 AM
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Join Date: Mar 2010
Location: USA
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If the lights or other equipment do not have the benefit of a metal frame ground/negative such as in a car, then the negative wire has to be just as thick as the positive wire to avoid a higher voltage drop compared with in the positive wire.

Or perhaps a 2 conductor cable with all black molded insulation has the positive wire insulation thicker for easier identification by do-it-yourselfers. This is somewhat more expensive (more plastic material used) compared with just molding a rib on one side of the cable as is done for typical 120 volt lamp cord (by convention the rib is for the neutral wire).

For typical household and automotive and also industrial applications, DC behaves the same way as AC of the same voltage, subject to restrictions of the equipment or devices being used. For example you may not power a transformer with DC. (Some transformers and other devices are sensitive to the difference between 50 Hz and 60 Hz AC also.)

#5
05-05-14, 10:08 AM
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With that red/black type wiring - the red has a thicker jacket as it is used in a grounded environment and offers the positive wire additional protection.

The actual conductor should be the same.

#6
05-06-14, 06:30 AM
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Join Date: Apr 2014
Location: South Africa
Posts: 13
Firstly I would like to say a big thank you to every one who took the trouble to answer my question and give their opinion.

The facts remain Ohms law is Ohms law and I have been reminded of that in one of the posts. I some how thought that with DC power there was an exception or some other factor at play.

I returned to the store to look at the cable for sale and the positive core is thicker than the negative. The wire cable in question was for use in automotive applications.

The sort of voltages I am working with is 12 Volts the lights and everything is 12 volts. I have been toying with raising the voltage to 24 volts to get around voltage drop and other issues. But that is for another day and another time.

Thank you once again your assistance and time.

Regards Godfrey