Wiring size for fuel system

Hi All,

After reading wiring size chart upon wiring size chart I've confused myself on what I should be aiming for in terms of AWG for my fuel system wiring.

My setup will be something along the lines of the below:

- Relays mounted in passenger footwell

- Battery in engine bay

- 2 Walbro 525 in surge tank

- 1 Walbro 255 (not locked in) as a lift pump

My question is in regards to the wire size from the battery to the relays and then from the relays to the pumps. The distance from the battery to the relays is around 1 to 2m and the distance to the fuel pumps is around 4-5m. From what I can tell I would be a 10 AWG wire from the battery to the relay box and then 8 AWG from each of the relays to the pumps.

Does this sound right? Not sure if I'm over speccing wire sizes here.

AWG size works opposite of logic, so an 8 awg is actually bigger then a 10 awg. unless that was a typo you'll want the smaller size after your bus/splitter from the battery.

There is so many variables to wire sizing; ambient temp, insulation type, run length, bundling, etc. It can be hard to figure out.

For the pumps fuel pressure and flow also roughly determine amp draw. Keep in mind in a forced induction application the boost pressure and the base pressure add up (and a small amount of pressure drop over the run length) and that's what the pump is actually pushing against.

Based on the information given, I'd say an 8 awg from battery to fuse box and then 12 awg to each pump should be sufficient.

Apologies I've just typed it poorly but to clarify I said 10AWG from battery to relay/fuse box as the distance was short and then the 8 AWG to the pumps.

I was using the chart linked below which was how I came to the conclusion of the wire sizes for which I realised I made an error and agree on the 8 AWG as you suggested to the fuse box. Which sorts that out.

My main concern was the run to the pumps mainly the 525's as according to the chart a 12 AWG wire would be too small for the roughly 5m (16.ft [this is on the generous side for the distance]) and instead 10 or 8 AWG would be needed. Not trying to say you're wrong at all, just trying to understand if I'm interpreting the chart wrong.

Thanks!

which chart are you looking at?

I don't refer to a chart anymore, I use a milspec table (STD 339) and cross check with with a voltage drop calculator. The charts are usually simplified examples of these, but you don't usually know what the person who wrote the chart decided on for de-rating factors.

My math including the 8 awg run's voltage drop puts the 12 AWG as just the right size. No harm in upsizing to a 10 AWG (other then weight and cost but both are insignificant here really)

We can break this down pretty far, for the exercise.

A Walbro 525 (F90000285) draws around 20A at 70PSI of pressure (don't know what engine and boost you intend to run but base 40psi + 20 psi of base pressure, have 10 psi of wiggle room for other losses), otherwise less.

I'm also assuming the grounds will be as short as possible and since your wire lengths are a bit on the generous side I'm going to ignore the ground wires. Usually you do factor that into the total circuit length, when grounding to the chassis the 'wire' size becomes large and the resistance very low.

2 walbro main pumps and a lift pump (lift pumps in my experience are usually lower then 10A but lets keep the number round) add up to 50A total.

For the wire from the battery to the fuse box, which is 2m or 6.57 ft, and capable of 50A. Mil standard 339 allows this wire to be good for 53.8 amps (uprate for bundling, derate for engine bay ambient temp. Voltage drop calculator says this will result in a 1.6% voltage drop (using: 50A, 14.4V and an 8 awg wire with 0.7 ohms per 1000') (max acceptable voltage drop is 5%) or 0.23V.

Now the wire from the fuse box to the pumps; 14.4V - 0.23V = 14.17V

Mil standard 339 says a 12 AWG wire is ok for 28 amps after de-rating for ambient temp and bundling. Voltage drop (14.17V, 20A, 5m (16.42') and for a 12 awg 2.02 ohms per 1000') nets us a 4.68% voltage drop, or 0.66V (result 13.51 which is a 6.5% change from the 14.4 source voltage)

10 awg does math out a bit better (about 4.5% VD from source), but both are close. We are also assuming worst case scenarios for amp draw and distance, which is the safer way to go about it, especially if you're unsure.

Thank you for all the info and spending the time to explain it, really appreciate your help. As for the chart I've been using: https://mgispeedware.com/wire-size-chart/

Base pressure will be 40 psi and we're aiming for around 35-40 PSI of boost pressure. But that doesn't change the draw a whole lot ~21-22A. Correct in your assumptions for the grounds. They, at worst will be 450mm away.

Could you possibly link the mil spec table that you use? The only one I could find was this: https://cdn.glenair.com/wire-and-cable/pdf/b/m22759-16.pdf

Just out of curiosity if I were to have the fuses and the relays mounted in the boot meaning it would need a single power run of around 5.5m from battery to boot using the MIL STD 339 document I found and the logic you used I would need a 6awg wire? At which point I can have 12 or even 14 AWG to the pumps?

if you google MIL-STD-339 there's a few sites that have it downloadable as a pdf. I don't remember where I got the version I have, some of those sites do look a bit suspicious lol

The table you linked is wire spec's for M22759/16, which is relevant but the one I was referring to.

I haven't run the math again for those lengths, but sounds right off the top of my head. Voltage drop will normally limit you before amp rating does.

Apologies for the dodgy looking links, was all I could find.

Will have a suss to see if I can find the chart. Thanks!