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Allowable Voltage Drop

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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Hey Guys,

I have searched google for a while on this and have found no real answer. What is the allowable voltage drop percentage?

One forum has 2%, another has 5%.

Is there a standard to this or at least a target i should aim for? What happens if i go over the allowable voltage drop? Will it burn out my pumps over time?

Looking at using M22759/16-12 (12awg) for my 2 x 044 pumps but im calculating 4.74% and my intank Walbro 400 e85 comes up with 6.81%

Relays are planned to be under dash and pumps are in the rear with about 4.4meters of wire.

Would the intank walbro pump be ok with 12awg?

What matters is not really the % but the actual voltage drop.

2 x 044 pumps, each draw ~15A max, so a total of 30A.

12awg have a resistance of 5.2Ω/km, for 4.4m that's 0.023Ω

(side note: for the wire length, you have to take the whole length into account, from positive to the ground)

U = R x I, so 30 x 0.023 = 0.7 -> you will have a Vdrop of 0.7V with a 12 awg wire.

if you have 12v @ the battery, that's 11.3V @ the pump (5.8%)

if you have 15v @ the battery, that's 14.3V @ the pump (4.7%)

0.7V * 30A, that's 21W dissipated in heat !

So you also want to consider the temperature rise on the wire.

with 30A on a 12awg and a ambient temp of 20°c, your wire will get ~55°c (+35°c above ambient)

Personally I would use 10awg instead.


It's ok to use one 40A relay to feed 2x 044 pumps?

Ludo, do you have a AWG size vs resistance per foot/meter?

I've found this one:


Made an excel formula and came up with the chart attached. It is correct?

Attached Files

1000ft = 304.8m so yes your chart should be Ok. (the value I used above was for bare copper, not tin plated. My bad)

Attached Files

Cool, your chart is already in my personal database.

Any info available it's good to have.

Thanks again Ludo.

Thanks Ludo, Is that a single power feed for both pumps in your calculations? I was planning on each pump having its own circuit so i can stage them. Would you still suggest 10awg? What would you consider acceptable for the actual voltage drop, 0.2v?

Thanks Again

If you plan to use one wire for each pump, then yes 12awg is absolutely fine. (at least for the 044, I don't have the chart of the walbro)

and staging the fuel pumps is a good idea too. (PWM control would be even better, but that's harder)

As for the Vdrop, personally I try to stay under 0.5V (so around 3 and 4% depending of the battery voltage)

But for a "high" current draw, I'm more concerned about the heat.

You might have noticed that all the resistances are rated @ 20°C, there is a reason for that.

Copper have temperature coefficient. That mean that as the temperature rise, the resistance increase too.

For a 12awg, if you have 6.63Ω/km at 20°C, at 50°C that's 7.43Ω/km.

If the resistance increase, the vdrop increase and so does the heat too.

That can become a vicious circle. (the current draw is directly related to the voltage too, so you always reach an equilibrium)

M22759/16 is rated @ 150°C so that give you a good margin, but there is still a limit.

Also, the flow rate of your fuel pumps is dependent of the voltage. So that should also be considered.

(That's also true for injectors, coils, etc...)

Ludo, how do you convert Watts to temperature?

You said that 21W dissipated in heat should be 35ºC...

I've used your previous post to calculate the vdrop on my rear lighting wiring:

2 x P21W LED bulb, each draw ~1A max, so a total of 2A.

22awg have a resistance of 0,053Ω per meter

4 meters plus 0.5 meter ground, 4.5m total, but the bulbs ground its common for brake, parking and turn signal.

U = R * I, so 2 * (0,053 * 4,5) = 0,477 -> vdrop of 0,5V with 22 awg wire

12v @ battery = 11,5V @ bulb (4,17%)

15v @ battery = 14,5V @ bulb (3,34%)

0,5V * 2A = 1W dissipated in heat

I have 3 doubt's:

1- my tailight (pic attached) has a 5 pos connector (ground, parking light, turn signal, brake light, reverse light), so it's a common ground for all the bulbs.

parking light 5W @ 2W led / 150 mA

turn signal 21W @ 3W led / 200 mA

brake light 21W @ 3W led / 200 mA

reverse light 21W @ 3W led / 200 mA

I don't trust these specs on chinese bulbs, so I will consider all the bulbs as 1 amp.

My car is a euro spec with 1 side being reverse light and the other side rear fog light, I will build the harness as follows:

1 wire to both parking lights and license plate light, so 3A draw

1 wire to both brake lights, so 2A draw

1 wire to the right turn signal, so 1A draw

1 wire to the left turn signal, so 1A draw

1 wire to the reverse light, so 1A draw

1 wire to the rear fog light, so 1A draw

1 wire to ground right tail light

1 wire to ground left tail light

1 wire to ground license plate light

How to I properly calculate the ground awg size to support all these bulbs?

I know that LED lighting it's not a high current device but I want to make the harness the best way I can...

2- how to I calculate the temperature per watt?

3- I live in Brazil, tropical weather, it's very common to 30°C to 40°C ambient temperature, should I be concerned about it?

Thanks for all the helping

Attached Files

I've made a chart on excel based on some formulas and informations on this topic to help me calculate it faster


If you authorize me, I publish the file here.

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