Wire sizing chart that you trust? (8 Viewers)

[jkeithw]

We're laying out the rear electrical right now for Julia's 80 (and my FJC, subsequently), and I'm finding a significant difference between various online wire sizing charts when researching length vs amperage.

Here are a few that I've seen linked here on MUD (and the one I've been using up to now, listed last.) How does one determine which is correct?

un-cited link from MUD discussion
(35-50a at 19-22feet - 6 or 4ga) (no temp reference)

enerdrive au chart
(40a at 20-25feet - 4ga) (no temp reference)

un-cited link from MUD discussion
(40a at 15-20feet - 10ga) (no temp reference)

blue sea circuit wizard
(40a at 20feet - 6ga) (221F)

quality power auto chart
(40a at 20-24feet - 4ga) (150F)

As far as I can tell, all of these assume at 3-3.5% voltage drop for starting.


Thoughts? (No one wants to run vehicle-length welding cable twice.)

 

[jonheld]

It's not just length/amps. The calculations depend on temperature.
I typically add 20% to any chart. I'm a big believer of oversized wiring, and correctly calculated fuses.

 

[jkeithw]

It's not just length/amps. The calculations depend on temperature.
I typically add 20% to any chart. I'm a big believer of oversized wiring, and correctly calculated fuses.

good point - I'll go back and add in the temp references for all those links.

- and while I agree, it's a lot different pulling a 4AWG cable than trying to get a 2AWG through some places (so it'd be nice to know with some accuracy what is actually needed in a specific situation)

also, 20% amperage? or length?

 

[NeverFinis]

I use Wirebarn's calculator (Wire Size Calculator - https://www.wirebarn.com/Wire-Size-Calculator-_ep_41.html)

Remember, there are different wire types, and I mostly use automotive grade (GXL/TXL). They have much higher temp rating compared to general purpose wire.

Wirebarn suggests 4 gauge for 40 amps at 20 feet.

Given that, I ran a 6 gauge welding wire from an engine bay battery to a 40 Amp DC-DC converter in the back. Both passenger side. I don't think it was even near 20 feet. The DC-DC converter recommended 8 gauge.

 

[HDJ80L]

In vehicle engineering you'd look at standardisation values in ISO or SAE for automotive wiring and get the specification from the wire manufacturer. Beside cross-section and temperature, current capacity also depends on the wire type (insulation, copper density) and how it is routed. For example, a cable routed separately will be able to dissipate more heat than one that runs in a bundle or harness. You'd also compensate for operating temperatures based on the installation area. A wire running in the engine bay might require a different cable type and/or more oversizing than one running through the trunk. Sometimes you also want a bigger wire purely for mechanical strength and durability even if you could get away with a smaller one current-wise.

Way too complicated for DIY but you might be able to get some manufacturer specs of the wire you are using instead of having to rely on random tables.

 

[jkeithw]

I use Wirebarn's calculator (Wire Size Calculator - https://www.wirebarn.com/Wire-Size-Calculator-_ep_41.html)

Remember, there are different wire types, and I mostly use automotive grade (GXL/TXL). They have much higher temp rating compared to general purpose wire.

Wirebarn suggests 4 gauge for 40 amps at 20 feet.

Given that, I ran a 6 gauge welding wire from an engine bay battery to a 40 Amp DC-DC converter in the back. Both passenger side. I don't think it was even near 20 feet. The DC-DC converter recommended 8 gauge.

yeah, good point - I do run welding cable for the big stuff/long runs. (I'd always assumed it was a better option than automotive wire)

I think I'm going with 4ga - it's from the driver's side battery, across the cowl, then back to the passenger rear - for our DC/DC converter and various lights. I'm guessing it'll good for 60a at that distance.


EDIT - I'm glad your link noted that it is for one-way runs (something I'd completely forgotten about):

NOTES:​

A) This is for a ONE WAY wire with the ground assumed to be the car chassis. If 2 wires needed to complete the circuit calculate for the total length of wire used for both legs.​

 

[jkeithw]

In vehicle engineering you'd look at standardisation values in ISO or SAE for automotive wiring and get the specification from the wire manufacturer. Beside cross-section and temperature, current capacity also depends on the wire type (insulation, copper density) and how it is routed. For example, a cable routed separately will be able to dissipate more heat than one that runs in a bundle or harness. You'd also compensate for operating temperatures based on the installation area. A wire running in the engine bay might require a different cable type and/or more oversizing than one running through the trunk. Sometimes you also want a bigger wire purely for mechanical strength and durability even if you could get away with a smaller one current-wise.

Way too complicated for DIY but you might be able to get some manufacturer specs of the wire you are using instead of having to rely on random tables.

all good points, but I can't imagine wire manufacturers giving any assurances for (or assuming any liability for advising on) our one-off installations?

 

[jkeithw]

thanks everyone!

 

[NeverFinis]

yeah, good point - I do run welding cable for the big stuff/long runs. (I'd always assumed it was a better option than automotive wire)

I think I'm going with 4ga - it's from the driver's side battery, across the cowl, then back to the passenger rear - for our DC/DC converter and various lights. I'm guessing it'll good for 60a at that distance.


EDIT - I'm glad your link noted that it is for one-way runs (something I'd completely forgotten about):

NOTES:​

A) This is for a ONE WAY wire with the ground assumed to be the car chassis. If 2 wires needed to complete the circuit calculate for the total length of wire used for both legs.​

Good point on the one way run.

Welding wire has a lower temp rating than quality automotive wire but I don't think one can get TXL or GXL in 6 or 4 gauge...

I have examined a ton of OEM (GM/Toyota) wiring while doing my LS swap. It is all smaller gauge than expected given the expected circuit load and yet it all still works...

Just look at the length of Toyota's fuel pump circuit and the wires used..

FYI: My 40 amp DC-DC only draws about 35 amps when charging the house battery.

 

[HDJ80L]

all good points, but I can't imagine wire manufacturers giving any assurances for (or assuming any liability for advising on) our one-off installations?

It's more about the manufacturer providing technical information or referencing industry standards so that the installer can look up essential things like a size/current chart to choose the right wire for the application. Not always easy to find or get that information though