Not everyone says to run a 10 gauge.

Wiring gauge depends on a few things and those are mostly the length and the amperage.

Basically the longer the wire, the larger the gauge. The shorter the wire, the smaller the gauge.

I don't have a chart in front of me but let's say I had a 10A circuit and I had to run it off of a 2' piece of wire the wire might be an 18 gauge but if I now have to run a longer wire to pass 10A over 20', the wire gauge would have to be increased to something like a 12 gauge.

Jim

 

I assumed that when they sell this pump they know that it would be put near the fuel tank and the battery is most often in the front . At least 10 feet away. Thus requiring a larger wire. Just seems odd that 2 small (14 gauge) wires were there.

 

As Jim said, the wire gauge is chosen primarily based on length and amperage.

With a long length of wire you want to use a heavier gauge than would otherwise be necessary for the same current in a shorter run. That's done to reduce the amount of voltage drop in the long run of wire. But since the wire leads on the pump are relatively short, they won't create a significant voltage drop across their length and therefore don't need to be oversize.

 

14 gauge will handle all the current you need for your pump. It's rated for 32 amps for chassis wiring, and a short section of it will cause no problems at all.

 

Mike, I hate to question you as you really seem to know your stuff, but where did you get this 32 amps for 14 AWG rating? Seems a little high on the amp rating to me, especially for a 12 volt system.

 

Here is a pretty good cable length/current flow/voltage loss calculator:

http://www.calculator.net/voltage-d...=12&phase=dc&noofconductor=1&distance=10&distanceunit=feet&amperes=32&x=58&y=15

 

Remember we are talking about the short leads that come attached to the pump. According to this ampacity table, the maximum current allowed for 14 gauge wiring in a chassis is 32 amps. If you were using it for power transmission over a substantial length you would limit the current to 5.9 amps.

http://www.powerstream.com/Wire_Size.htm

Let's use your calculator to calculate the voltage drop at the rated current draw of the pump. I can't find the current specs, but the similar Mallory 110 and 140 pumps are fused at 7.5 amps, so I'll bet that the current draw at full load is about 5 amps or less.

14 gauge wire, 0.5 feet in length, 5 amps of current @ 12v applied =

Voltage drop: 0.013
Voltage drop percentage: 0.11%
Voltage at the end: 11.987

A 10 foot length of 14 gauge wire only drops 0.25 volts at 5 amps.

If you use a fusible link on a 10 gauge supply wire for a high current application it's a short piece of 14 gauge wire. Many GM cars have these built in.

 

Mike, just to let you know, that is not the proper ampacity chart to be using. The OP needs a chart that bases the gauge by the amount of amps and length of wire. Such as this one, which is perfect for 12v cars and trucks. http://www.jkowners.com/tech/electrical/ampacity.jpg

 

I'm also unable to find the rated current for the Mallory 95P fuel pump. So, laser-red-nova, there is likely stamped somewhere on the pump where it says something like "max fuse size recommended". For example, my Walbro 255 says right on it, 20 amps max fuse. If yours says the same, then use the chart I provided above, figure out how long your power and ground wires are (added together) then plug that into the chart, so if it says 20 amps max then figure 23-25 amps and the gauge that it shows for the length of wire you will use and utilize a 20 amp fuse.

 

Thats not the same chart I used but the values are identical.

 

Let's pick a value out of the middle of this chart and do some calculating.

It says I can run 24 amps through 10' of 16 gauge wire. 16 gauge wire has a resistance of about 4 ohms/Kft, or 0.004 ohms/ft. Our 10' wire would have a resistance of 0.04 ohms. Mr Ohm, the law in these matters, says that if I run 24 amps through this 10' piece of wire I would drop 0.96 volts, and end up with 11 volts available at my load. Mr Watt the power guy tells me that I'm down to 85% of my power, sort of like running my V-8 with one plug wire removed.

Do you think this is a good result, or would you increase the size of the wire?

Increasing the size of the wire to 10 gauge would reduce the voltage drop to 0.24 volts and increase my load power to 96% of circuit power.

 

Mike, I have the utmost respect for your opinion in these matters as you understand the math much better than I do. When I wired my car (and others) I used this chart as sort of a starting point. I also took into consideration under hood temperatures as well as the intended use of the circuit. For example, I think full power to a fuel pump or engine cooling fan is going to make those vital components run much more efficiently. I first measured the actual load and substantially shortened these circuits through the use of relays and hopefully engineered my charging system to run between 13.8 and 14.2 Volts. And yes, I did wire my fuel pump with 10 gauge wire. GXL throughout. I took the approach of when in doubt go up one conductor size.

Just for giggles, what does it do to your calculations?

13.8V, 24A, 10’, 16 Ga.

13.8V, 24A, 10’, 14 Ga.

Steve

 

The applied voltage isn't going to change the voltage drop across the wire, just the voltage available at the load.

This voltage drop calculator is easy to use. Since it is a two wire circuit calculator you need to put in 1/2 the length of your power wire, since in a car you use the large chassis circuit for ground.

http://www.calculator.net/voltage-drop-calculator.html

Pick your wire size, applied voltage, length of circuit and amps.

13.8V, 24A, 10’, 16 Ga = .96v drop, 12.84v at the load, 86% of circuit power available.

13.8V, 24A, 10’, 14 Ga = .61v drop, 13.19v at the load, 91% of circuit power available.

Switch to 10 ga = .24v drop, 13.56v available at the load, 96% of circuit power available.

Power dissipated in your wiring is lost power. It's just as easy to run a length of 10 gauge as it is to run a length of 16 gauge. A little too many circular mils is always better than too few.

 

Thanks Mike. I saved the calculator and looking at my schematics I should be fine.