Voltage drop with mixed wire sizes

[leecea]

Hi All,

I'm replacing the wires that power the starter solenoid on my 3GM30.

I've run Ancor 10awg red and white wires to the starter switch but the only 30amp fuse holder I can find has 12awg wire tails.

So my new circuit will have ~40 feet of 10awg combined with a few inches of 12awg for the fuse.

According to a voltage drop calculator, the difference in voltage drop between 1 foot of 10awg and 12awg at 12v/15amps is 0.17%.

Does this mean my circuit will be almost unaffected by the short length of 12awg?

Thanks,
Andy

[SaltEnjoyer]

AWG ratings are for current. Wires are a bit like pipes and if they aren't big enough to pass the current, they will heat or ignite.

If the system calls for 10AWG you need to remain 10AWG for the entire circuit.

[SailingHarmonie]

Originally Posted by SaltEnjoyer

AWG ratings are for current. Wires are a bit like pipes and if they aren't big enough to pass the current, they will heat or ignite.

If the system calls for 10AWG you need to remain 10AWG for the entire circuit.

No, it’s not that simple.

DC wire sizing calculations are a two step process. First, you calculate the wire size needed to give the voltage drop you need, then you calculate the minimum wire size that can safely carry those amps. ALMOST always the voltage drop will give you the larger size wire.

Doing no review of the calculations as done, 12 AWG wires have sufficient ampacity (45 Amps) to carry 30 amps, and a few inches of them will have very little effect on the over all voltage drop in the circuit.

[ShoreFun]

So I did a quick check and what you say does not add up.

Per the chart a 12G wire is only good to a 25A fuse.

If you really have a 30A fuse holder then it should start at 10G wire.

So the statement it is a 30A fuse holder that only takes 12G wire means it is likely something is basically wrong. You need to get a correct fuse holder that takes 10G wire as there should NOT be any 12G wire in a circuit with a 30A fuse.

https://astrolabesailing.com/2017/05...ng-wire-fuses/

[SaltEnjoyer]

If you run 10AWG then splice in a 12AWG in series, you might as well run 12AWG. Its the same thing. You don't get better voltage drop by mixing wire types.

In DC circuits long wire runs wires are resistors, and only as much current can flow through the smallest part.

e: If you need 10AWG to handle the current over the distance, then ALL of the wire must be 10AWG.

The voltage drop over the distance subtracted from the supply voltage must just be higher than what is required by the equipment at the termination.

[Wolfe10]

Another solution to voltage drop in high amp/long wire run circuits is to use a RELAY with the switch/key serving to close the relay. The power IN and OUT of the relay in the appropriate gauge wire and much shorter wire run.


With Relays, bigger is better. 40 amp 12 VDC relays are under $10.

[markcouz]

You can absolutely mix wire sizes so long as the fuse/breaker is protecting the smallest wire in the circuit and your total volt drop is within spec.

A few inches of 12 awg will have negligible effect on your circuit.

If your insulation rating is 75C (167F) or higher then 12awg/30amp is fine, both inside or outside the engine bay (high temp area).

[GordMay]

Originally Posted by SaltEnjoyer

If you run 10AWG then splice in a 12AWG in series, you might as well run 12AWG. Its the same thing. You don't get better voltage drop by mixing wire types...

Absolutely WRONG.
His total voltage drop will equal that of the #10 run + that of the #12 run.
His #12 pigtail WILL make a slight difference; but not much.

[SaltEnjoyer]

Originally Posted by GordMay

Absolutely WRONG.
His total voltage drop will equal that of the #10 run + that of the #12 run.
His #12 pigtail WILL make a slight difference; but not much.

It is not absolutely wrong. Unsure why you would phrase it that way - but I can see your point.

For voltage drop yes, it will make a miniscule difference - but it does not matter as long as the drop subtracted from the supply voltage is greater than what the load needs to operate.

This thread is talking about fuse protecting wires for a starter motor which is a very current thirsty piece of equipment. As long as voltage is high enough its fine, but current is the main focal point for safety.

[boatpoker]

Originally Posted by leecea

Hi All,

I'm replacing the wires that power the starter solenoid on my 3GM30.

I've run Ancor 10awg red and white wires to the starter switch but the only 30amp fuse holder I can find has 12awg wire tails.

Thanks,
Andy

So use a 30amp breaker instead of a fuse.

[team karst]

Originally Posted by SaltEnjoyer

…
This thread is talking about fuse protecting wires for a starter motor

.

Starter solenoid. Per post 1.

[SaltEnjoyer]

Originally Posted by team karst

Starter solenoid. Per post 1.

You are absolutely right - I misread that. Solenoid switches do require a constant current to stay in the on position. Not mixing wiring, install proper fuse/breaker for wiring chosen is still the right choice.

[fourlyons]

Originally Posted by SaltEnjoyer

If you run 10AWG then splice in a 12AWG in series, you might as well run 12AWG. Its the same thing. You don't get better voltage drop by mixing wire types.

In DC circuits long wire runs wires are resistors, and only as much current can flow through the smallest part.

e: If you need 10AWG to handle the current over the distance, then ALL of the wire must be 10AWG.

The voltage drop over the distance subtracted from the supply voltage must just be higher than what is required by the equipment at the termination.

This is simply untrue. The length of the smaller wire, as well as the gauge, determines the resistance. Have you ever looked at the size of the conductor in a 30 amp fuse? It's far smaller than the 10 gauge wire it protects, that works because it's very short.

[wholybee]

Since a 30Amp fuse is selected, it is safe to assume the current will be less than 30Amps, and either 10AWG or 12AWG is ok for 30Amps. So that part of wire selection is ok.

The OP's question, will the short piece of 12AWG matter. Let's look at that. If my math is correct:
40 feet of AWG 10 at 30Amps will have a voltage drop of 1.24V.
6 inches of AWG 12 at 30Amps will have a voltage drop of 0.025V

Further analysis requires knowing what current the solenoid actually uses. It is probably closer to 1A than 30A.

12V-30A, 40ft of 10AWG just barely exceeds 10% voltage drop, at 10.3%. ABYC tables for 30A show that for 40ft, AWG8 is needed, But it is close enough for me I would say it is ok. *IF* you consider your starter solenoid to be non-essential equipment. Essential equipment needs to meet 3% voltage drop, which 10AWG at 40 feet most certainly does not meet.

At 12V-5A, 40ft of 10AWG easily works. As small as 14AWG would work according to ABYC tables, which also answers you question about the short bit of 12AWG.

Reference w/math formulas.
https://shop.pkys.com/Voltage-drop-tables_b_46.html

[Bowdrie]

Originally Posted by fourlyons

This is simply untrue. The length of the smaller wire, as well as the gauge, determines the resistance. Have you ever looked at the size of the conductor in a 30 amp fuse? It's far smaller than the 10 gauge wire it protects, that works because it's very short.

True enough.
A reasonable analogy can be made with hose.
Blow thru a 6in long piece of 1/2 inch hose.
Now try and blow thru a 50-foot length of it.
Nevertheless, one has more peace of mind when ALL wires are oversize for the job they do.
An old Naval Architect I knew told me "There is NO place on a boat for any wire smaller that 14 gage".
Before someone gets their knickers in a twist, he was not talking about all the data cables people string all thru their boats.