Mixed wire sizes

[Aurelia]

I have been reading and thinking but this question still leaves me unsure and I have not found additional good explanations at least in 12v situations thus far.

Lets use just one example set of wire charts as a reference, not saying they are the best or worst, and give it capacity number as a 30ft length.

http://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity

My theoretical three size 30ft length of 6,8, and 10 AWG wire could handle...?

Up to 60A at 10% drop or 35A at 3% drop? (smallest wire limit at 10ft length of 10AWG)

Up to 41A at 10% drop or 12A at 3% drop? (smallest wire limit at 30ft total run length)

Or is it really somewhere in between?

Un-hijacked,

Greg

[thataway]

We have to clarify if this is a 30 foot round trip--or 30 foot one way?

Assuming 10 foot one way (or round trip circuit 5 feet each way). The West Marine table is a very rough way to figure. Using this table, what you say is true for the single wire. If you add in the different resistances, which can be calculated for each length of wire, as per the formula I gave in my first post, then you could calculate how much voltage drop--figure that back to % and then come up was an answer which is somewhere in-between the two values as in your question.

Far better is the Blue Seas Circuilt wizard:

http://circuitwizard.bluesea.com

This is far better, because it takes into account wire size, run, voltage, temperature, insulation, fixed vs variable load, engine room temps, etc. It is also available as an app! This has been developed by Blue Seas Electrical Engineers and is also compliant with ABYC standards, which is where you want to be for safely. It is more than just voltage drop!

Taking 10 feet by pure voltage drop capability, 10' (round trip 5 + 5) at 3% #10 wire would be suitable. If you take all of the factors by Blue seas engineers, and ABCY it would require #8 AWG wire. (Variable, 60degree insulation, non engine room)

At 10% voltage drop only, 10 feet would be OK @ #14 AWG, but recommended by all other criteria would be #8 AWG!

If you take this out to 30 feet round trip--or 15 feet each way: it would be recommended at 4 AWG, and by some ABYC standards could be #8

If you take this out to 30 feet, round trip: 15 feet each way:
and 10% drop: Voltage drop only AWG 10, but all factors AWG #8…

So you see it is not as clear cut if you really want to go with the ABYC best recommendations:


Take this to 44 feet, which is the windlass length for most 22's--the
3% comes up with: #3 AWG
And 10% comes up with 8 AWG

The recommended #6 AWG wire allows for a 7% voltage drop at 44 feet.

By Blue Seas Calculations: Max amps allowed in any length of #10 wire is 60 amps. But this would only be for a very short run.

Blue seas puts out another nice table, Called finding the correct size of wire for DC circuits:

http://www.bluesea.com/resources/1437

[Aurelia]

"an answer which is somewhere in-between the two values as in your question"

Perfect Bob. My understanding was leading me to the same conclusion but the variables involved make precision tough and that I can live with. Too many things I read provide specific requirements of this or that but never address the why or gaps between the prescriptions. I feel more comfortable now in judging the safety of a curcuit and whether or not something installed in a given boat should get attention.

I have seen the Blue Seas chart before but it seems a bit conservative and does not for instance list 10g as even being an option for a 40a load although you and I know it could take at the 0-short distance the chart claims to include.

The wizard is great and I had not ran into that one. Thanks for the link and I will be using that one on occasion.

Greg