Quick fuel pump wiring question

[talon187]

Ok Im going to be running two pumps. My battery is in the trunk. I have the relay and understand how to wire it. My frist question is can i wire both pumps to the one relay? or do i need one per pump? Second question if i run 16 gauge wire from the relay to the pumps will that be ok? Im going to run 8 gauge between the relay and battery. Thanks for the help!

 

[steve]

Both questions depend on how much current the pumps draw. I doubt 16 gauge wire is a good idea for the pump to relay. I wouldn't go any smaller than 12 gauge but again it depends on current, length (round trip) and acceptable voltage drop.

 

[talon187]

the pumps are external and both draw 12volts. the length of the wire is going from the trunk and to pretty much of the back of the tank im mountin under the back seat one on each side.

 

[steve]

They may run on 12v but you need to know how much current they draw when running.

 

[talon187]

two bosch 044 pumps that help ? thanks for the reply

 

[Gigaah]

quick search yielded this:
Bosch 044/ Walboro Fuel pump/ Delphi/Lucas Injectors - Honda-Tech

I think thats the right pump. Says max 15.5amps.

 

[steve]

Yep, 15.5A max so worst case for the two of them is about 31A.

This is too much current for the typical 12v bosch style relay. You'll need two or find a less common 40A relay. A 35A or 40A fuse on the battery is needed for the feed.

The wiring from the relay(s) to the battery will need to be sized to carry the 31A.
The wiring from the relay to the pump will need to be sized to carry the 15.5A.

In a car with a common ground half of the round trip distance is via the body but I still calculate the voltage drops assuming two wires. Actual voltage drop may be half the calculated or more than calculated depending on the quality of the common ground. If you haven't upgraded the battery grounds to the body they may become the bottleneck.

The relay to the pump distance in the hatch area can be as much as 5' so assuming a 10' round trip at 15.5A at 12v the voltage drops are:

14ga - 6.7% (0.804v)
12ga - 4.2% (0.506v)
10ga - 2.7% (0.318v)
8ga - 1.7% (0.2v)

The hatch to the battery distance in a DSM can be as much as 15' so assuming a 30' round trip at 31A at 12v the voltage drops are:

14ga - 40.2% (4.824v)
12ga - 25.3% (3.036v)
10ga - 15.92% (1.91v)
8ga - 10.01% (1.201v)
6ga - 6.3% (0.756v)

If you have shorter runs, good body grounds you can use the smaller gauge wires but this gives you some ballpark ideas based on real numbers.