Have you ever wondered what size wire to use for DC?

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May 24, 2015
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Theoretically if you ran 20ft of 2awg for 100amp could run the appropriate size wire on the ground side for a 1-2ft run at 100 amp. In this case you'd run a 4ga for the short ground run.

What I'm unsure of...say you're powering a 15 amp light bulb direct off the battery. The light is 15ft from the battery, and it is decided to ground the light back at the battery. To determine wire size, is only a single 15ft run considered? Or is the circuit considered as needing wire big enough to run 15 amps at 30ft?

I'd reasonably assume it's only 1 run @15ft because 15 amps is travelling along the hot and being used by the light, the ground only has to support 15amps to the light? Whats the authority on this?
 
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Nope. It is calculated round trip. Resistance in the wire is causing voltage drop. Always, both sides. It depends also on how much voltage drop is acceptable for the consumer to work.

For DIN71522 voltage drops are factored in with wire lenghts appearing in a vehicle running 12V.

I am in my shack this afternoon and will do a pictured test for you to show, whether amps disappear or not ;)
 
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The simple formula is V = I x R

V = voltage
I = current
R = resistance

So, if you have 15A of current and 0.1 ohms of resistance (the TOTAL wire resistance + connectors/joints etc),

then you would see

V = 15 x 0.1 = 1.5V

That 1.5V is the total voltage drop due to the wiring and connectors/joints etc.

Just google for wire gauge current and you'll find online sites that give you the resistance per 1000' for various common wire gauges. You can then calculate the resistance for the length of wire you are using and knowing the current you can calculate the voltage drop.

The current flows equally through both the positive and ground wires and so the resistance of both wires impacts the voltage drop. Of course the current can drop as voltage drop increases - e.g. if the wires were powering say an incandscent bulb (this due to the bulb drawing less current with less applied voltage across it).

cheers,
george.
 
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So you're saying...and I'm gonna pull some arbitrary numbers out for the sake of making the scenario easier:

A light pulling 15 amps. 15ft on the hot and the ground side terminates immediately to a non restricted ground(aka frame) has a Vdrop of 1
Take that same light, using the same wire size, make the hot side 15ft and ground side 15 ft, the voltage drop is now 2v? (+/- a smidgen, but lets assume its simply that linear)

Suppose that would make sense I guess if you measured the hot side from batt to bulb and got 1v, adding a 15ft run on the ground side would also have a 1v drop measured from batt to bulb....therefore reducing the voltage the light sees by 2.

So to run a hot side of 20ft of 2ga for 100 amp service at a desired voltage drop, the ground has to essentially terminate immediately? And if the ground is to be equidistant to the power, now it has to be considered as a 40ft run at 100amps to meet that same voltage drop requirement?
 
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^ as far as voltage drop is concerned, the electrons don't care if they are on the 'hot' side wire or the 'ground' side wire. So, don't think of hot/ground wire, just think of total wire length. The current flows through all that wire length and that is what will introduce a voltage drop. Also, do NOT assume that the chassis/body is not introducing a voltage drop - it certainly is since it is iron/steel and has much worse conductivity than copper.

cheers,
george.
 
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May 24, 2015
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Interesting. Idk why I liked to think of them as a power/ground... suppose I kind of just imagined the current being used by the consumer so only the more restrictive side was a consideration.

In this bulb scenario then, does this 2v drop translate into heat in the wire, or does the bulb simply just see 2v less and only illuminates so bright?

Heres a question about fuse rating then....

Kinda working backwards now lets say we only wanted 3% drop (12v*.03=.36v) on that 30ft circuit and we wanted to find out what current it can support, knowing 30ft of whatever wire producing a 2v drop at 15 amps is seeing .133R.... v/r=I .36v/.133R=2.7 I....then the existing wire is to be fused at 2.6A in order to "protect the wire"?

Am I thinking of that right?

How does that fair with protecting the wire then if we choose to allow a 10% drop....now the wire is rated for 1.2v/.133R=9 amps

Is the fuse rating of a wire based on its allowed voltage drop? Beyond looking to satisfy a voltage drop requirement, how is it calculated when the wire is actually going to burn up? Say we allowed a 50% drop, is that 30ft circuit actually going to support 45 amps without making fire?

In a winching application where we know more often than not buddys 1/0 wire isnt going to feed a 400amp pull all day....is that simply a matter of controlled abuse to the wire? Aka we dont fuse it, we know it wont handle it, eventually it will produce heat and winching perfomance issues, at that point we back off and let it cool down?
 
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Bulb scenario.

The voltage drop equates to both lower light output, and heating of the cable/connections.

The fuse rating of a wire is usually related to the thermal capability of the insulation on the wire. A 70 degree C rated insulation gets fused at less than an equivalent size wire with 95 degree C rated insulation. It also depends on the depends on the method of cable installation, which relates to the cables ability to dissipate heat.

The fuse on a wire is there to protect the wire, primarily, not the connected equipment.

If I need to size a wire/cable at work, first thing I do is look at how much current the equipment draws. Then circuit length gets used to determine the minimum size required, then cable derating factors (allowable voltage drop/ambient temperature/installation method/phase of the moon/whether the beer fridge is empty/etc) get used to increase the cable size if needed. Last comes the circuit protection selection. Cable sizing is a huge topic, and fuse selection isn't as cut and dried as it may first appear.

With the winch using an undersized wire, you'll get a larger volt drop across the wire, but as it's only short term, you don't tend to melt things too often. There should be a fuse inline with it though, car audio ANL or MANL fuseholders for engine bays would be my choice. Huge range of fuze sizes, and commonly available (the common name for them is forklift fuses).
 
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Your exact scenario here is why I mount a smaller rear battery, which can be charged at a much lower amperage, and I don’t worry about intermittent high amp draw from accessories.
 
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