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Dylan Tucker avatar image
Dylan Tucker asked

Understanding wire guages in terms of cable length

Hi everyone - I'm currently working out the gauges for my wiring and I'm super confused when it comes to length calculations. I've read that I should be doubling the length of the cable to account of both positive & negative when referring to cable guage charts, but does this only apply for 2 core cables?


For example, if I have a lighting run that is 6m from the source that is 2 core, is that 12m?


But if I have separate positive and negative cables that run for 6m do they both count as 12m or do I stick to a 6m calculation for each cable as they are separate?


Thanks!

wiring
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John Leslie avatar image John Leslie commented ·
When wired into a simple round-trip circuit, a 2-core cable 6m length is the same electrically as two 1-core cables 6m length. Assuming same gauge and quality, the only difference is the extra jacket around the 2-core to make running the cable easier for applications where 2 cores are required from point-to-point. So if your max physical distance from your load to your source is 6m, you would calculate your wire size for a 12m total, with inputs for voltage, max current, bundled or stand-alone, and and allowable voltage drop for your application (3% or 10%, etc)
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jkmann avatar image
jkmann answered ·

Wire gauge is determined by two factors: voltage drop and ampacity. Let's look at these two separately.

Voltage drop is caused by resistance in a wire to the passage of current through it. Let's say you want to limit the voltage drop in a circuit to 3%. For this purpose you need to consider the entire length of the circuit (+) and (-) sides. So it you have a (+) wire out to a load, and a (-) wire back to the battery the voltage drop will happen over the entire length of the wiring. That is why you sometimes hear that you should "double" the length of the (+) cable, while the correct way to think about this is that you should consider the entire length of the circuit.

A special case is when you have a (+) wire out to a load and a VERY low resistance return (-) path through a boat hull or vehicle chassis. In that case you will come close by just considering the length of the (+) wire.

Ampacity refers to the current that a wire can carry before the insulation will become compromised due to heat generated by the wire's resistance. Typically this will be a higher amperage than you would choose to limit voltage drop, but not always. If the circuit is only a meter or two in total, ampacity can be the controlling factor. You should consider the ampacity of a wire as the absolute maximum current that the wire should ever carry for more than a very short period, like a few seconds, and you should provide a fuse or breaker at or below the ampacity of the wire.

Ampacity is reduced when the ability of the wire to cool itself is diminished by being placed in a conduit, bundled with other wires, surrounded by insulation, or being located in a warm space, such as an engine room. If in doubt, choose the next fatter wire.

Once you understand the reasoning behind wire size I think it makes it much easier to understand what you should be doing.

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Kevin Windrem avatar image
Kevin Windrem answered ·

If you are using a table, that gives you resistance per unit length then you need to consider the round trip length as jkmann describes. However some on-line voltage drop calculators ask for the length of the run and it factors in the out and back length.

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ahtrimble avatar image
ahtrimble answered ·

You measure total distance between connections...then double it for round trip.

This calculator will give you the information you need...along with fuse requirements: https://www.explorist.life/wire-sizing-calculator/

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kevgermany avatar image kevgermany ♦♦ commented ·
@Dylan Tucker

Note also that at max current, the tables, may result in a cable temperature you consider too high because of risk of burns/melting plastic etc. 70C is one I've seen.

Your next question will be fuse sizes. This is for the cable protection, not the equipment.

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ahtrimble avatar image ahtrimble kevgermany ♦♦ commented ·
I am not sure if you replying to me or not, but in case you are...

1) I wasn't referring to any table. I prefer to use the calculator that I linked to because it does a better job than a table...in my opinion.

2) I also don't understand your reference to "70C" in regards to anything I talked about. If you are referring to wire/cable insulation, the standard is 105C. If you are referring to ambient temps, the calculator has an option for that...installed in an engine compartment...meaning a higher than normal ambient temp.

3) The calculator also provides fuse sizing information based on the wire/cable not on equipment.

I hope that clarifies any misunderstanding.


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kevgermany avatar image kevgermany ♦♦ ahtrimble commented ·

Just adding for the OP. In case he used other sources to double check.

BS7671 cable sizing chart 4D1A is conductors at 70C, ambient 30C. for example.

I don't want any of my stuff getting anywhere near that hot. Going over 100C is worse.

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