Hi All,

Apologies for such a basic question....

Does the amps capacity (in terms of safety only) of wire change with length OR do you just get voltage drop??

So if my supplier has the folowing:
8 B&S - 74amps

Can I send 40amps safely through it (allowing for derating factors) over any distance and accept voltage drop? Or does the current become unsafe over longer distance as the wire heats up??

Thanks,

Jim.

 

In terms of safety you must look at the heating factor also with the assumption you want to keep voltage drops under a few percentage per connection. How that voltage drop translates to excessive heat in the wire depends of the voltage drop, current and the area of the wire that can safely dissipate that heat to the environment (insulation, wire conduits, etc...).

A typical car 12vdc wiring application guide:
https://jascoautomotive.com/automotive-wire-amperage-capacity-chart/

 

In terms of wire heating, the distance is not a factor.
The wire heats evenly over the distance, there is more power dissipation in longer wires, but it's spread out over the length.

 

In terms of wire heating, the distance is not a factor.
The wire heats evenly over the distance, there more power dissipation in longer wires, but it's spread out over the length.

That was what I was wondering? If the wire peaks at +30c over ambient for a short length, then I guess over a long length it would still peak at +30c over that entire length. So more watts lost, producing more heat and more voltage drop, but same +C degrees per metre. Is that correct?

 

That was what I was wondering? If the wire peaks at +30c over ambient for a short length, then I guess over a long length it would still peak at +30c over that entire length. So more watts lost, producing more heat and more voltage drop, but same +C degrees per metre. Is that correct?

Yes.

 

In general that's correct but boundary conditions like the terminal connections can upset that uniform heating logic. It's typical that when wire gauge is reduced the sizing for connections is also reduced so be sure that detail is understood.

 

Remember the prospective fault current.
If the wire gets too long, then the current that would flow in the event of a short circuit reduces. If the wire is too long, then the current will not be sufficient to operate the fuse and the wire would melt and things would catch fire.
If it gets too long, then you need a thicker wire.

 

If You are powering a Motor,
excessive Voltage-Drop in the wiring may cause the Motor to over-heat.

Too much Voltage-Drop also wastes Power in the form of additional Heat dissipation.
.
.
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Wire heating per unit length can be uniform if the wire is straight and surrounded by a uniform medium, but if the wire is partly coiled, for example. the heating will be non-uniform.