Dedicated fuel pump circuit.

[DeLorean03]

Hey everyone,

So I am getting rid of the wiring that goes from the RPM relay --> washer pump --> fuel pump. I can't stand how frail this wiring seems to be. I am going to run a set of new wires - a red power wire to the RPM relay and a black ground wire to the horn bracket. I am fed up with how beat up my wires to the fuel pump are, and I want eliminate any power issues I may be having there.

So here's the question - I have 16 gauge wire. It looks nearly identical in terms of diameter as the stock purple/white wire that goes from the RPM relay --> washer bottle harness --> fuel pump molex connectors. My understanding is the max current load for this gauge of wire is 13A - 22A dependent on conditions relative to the wiring. Now, my understanding is the pump pulls 5A-7A - so I would think that this wire should be sufficient.

Am I missing anything in regards to my numbers and conclusion? Can 16 gauge wiring handle the fuel pump's current draw with no issues?

Thank you for your time.

[Ron]

Note factory fuse is 20 Amps.
For 12V at 20 Amps, 16 gauge is good for 10 feet TOTAL circuit length.

http://dmctalk.org/showthread.php?5894-Wire-Selection-Chart&p=84421#post84421

[Bitsyncmaster]

Wire gauge selection is not just how much current the wire can safely carry. Most of time with high current loads it's more of voltage drop over the wire. You can calculate the voltage drop if you know the current and length of the wire.

Use a table or calculator to get the resistance of your wire.

http://hyperphysics.phy-astr.gsu.edu...es/wirega.html

Here you will see a 1000 feet of 16 AWG wire is 4.016 ohms. So if you use 16 AWG for your fuel pump and it's 10 foot from the RPM relay to the pump and 10 foot from the pump to the inertia switch then that 20 foot of wire will be 0.08032 ohms. Now use ohms law "E = I x R to get the voltage drop at 7 amps.

16 AWG drop would be 0.56224 volts.

I think there is more than 20 foot of wire but you get the idea.

[DeLorean03]

I would strongly assume this was created in response to my thread about current load and gauge wiring, so thank you for that. I totally forgot to attach the link I had for the chart I was using as a reference.

So, the pump does pull 6A-7A - correct? If so, then 16 gauge wiring should clearly be fine for the wire I am running from the terminal of the RPM relay to the pump.

I have verified that what I am doing is safe from two vendors - I just want to make 6A-7A is truly what the pump demands.

[DMCMW Dave]

Example (Red): A 12V system with a 20 Amp circuit can use 16 gauge wire up to 10 feet long (TOTAL length of circuit).

As Dave M mentions there are two figures at play - ampacity is the ability of a certain size wire to carry a current without melting, and resistance is the resistance the wire adds to the circuit based on its gauge and length. The two figures are somewhat related. And you don't want to design to be running right at the edge.

The fun part about this is "total length of circuit". This means the length both wires + and - added together. It gets a bit more complicated since the return path in most cars is the frame, and in this car it's often a black wire that is shared among many circuits. The return path on the fuel pump goes through the inertia switch (often resistive in itself) and then into the harness where it is shared with some other circuits on its way to the back of the car. This sharing does not effect the voltage drop (other than via the added tie points) but it does impact the ampacity.

In DeLorean03's case he is planning to run a hot wire from the RPM relay to the pump. That is probably 10-12 feet of wire. If he were to run a dedicated neg wire back to the bulkhead, that would be another 10-12 feet so he could calculate based on about 25 feet total length of circuit. Easy to figure out.

His ground path will probably be another wire connected to the front frame at one of the ground points (bypassing the inertia switch - save that for another discussion). The ampacity (current carrying capacity) of the return path (the frame) is not an issue at all, but the resistance of that ground connection and the connection from the frame to the battery are a bit more difficult to determine.

[DeLorean03]

In DeLorean03's case he is planning to run a hot wire from the RPM relay to the pump. That is probably 10-12 feet of wire. If he were to run a dedicated neg wire back to the bulkhead, that would be another 10-12 feet so he could calculate based on about 25 feet total length of circuit. Easy to figure out.

His ground path will probably be another wire connected to the front frame at one of the ground points (bypassing the inertia switch - save that for another discussion). The ampacity (current carrying capacity) of the return path (the frame) is not an issue at all, but the resistance of that ground connection and the connection from the frame to the battery are a bit more difficult to determine.

Based on this and other posts I've read, I feel confident that 16 GA should be sufficiently adequate for what I am planning on doing. The wire to the RPM relay looks to be very, very close to 16 GA - if not an exact match. There's no way the stock purple/white wire is 12 GA. I might be able to go up to 14 GA. The uninsulated female connectors I retrieved for the RPM relay can handle 16GA; 14 GA I don't know about.

I am planning to use 10-12 feet of wire going to the RPM relay and I would guesstimate 3-4 feet going to the horn bracket.

Where does the RPM purple/white wire run through the car exactly ? I know it goes from the RPM relay --> windshield washer bottle connector --> fuel pump. That seems WAY longer than what I plan on doing. I plan on going from RPM relay --> center console --> across from psgr side to drivers side through the driver's footwell --> up the fuel tank to the pump.

If I have to go get 12 GA wire and uninsulated female disconnects, that's fine. I just can't help but wonder why I should when it looks like 16ga factory stock wiring from 31 years ago seems to work just fine (short of blowing fuse #7).

[Ron]

I would strongly assume this was created in response to my thread about current load and gauge wiring, so thank you for that. I totally forgot to attach the link I had for the chart I was using as a reference.

Yes...I just put it here for future reference.


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Not to to take away from anything said so far (I agree), but, when it is a question of replacing, rather than designing (already done in this case), wire choice is easy -- Larger than existing is a waste and smaller is a risk, so you choose a wire that has the same or greater rating. If you can't tell what rate it is, or need to add length, note the 20A fuse is chosen for wire protection, and refer to the chart. (If the replacement wire is rated as high as, or higher than, the fuse originally used to protect the entire circuit, you are covered...!)

Most connectors that accept 16 gauge will accept 14 gauge... get the 14 and forget about it ;-)

[DMCMW Dave]

across from psgr side to drivers side through the driver's footwell --> ).

You might be able to sneak it through the same hole that the wiring for the A/C low pressure switch goes, (and the heater/freon lines) and it's very close to the pump at that point. Just seal it up good and avoid chafing points.

Some future dealer tech is going to hate you.

[DeLorean03]

Some future dealer tech is going to hate you.

Not if I never sell the car ....

So here's what I'm going to do:

1. Using all 12 Gauge wire

2. Examining the fuel pump circuit, I am going to run a hot wire with an in-line bladed 20A fuse from the battery directly to terminal 30 (terminal that is labeled "Hot/Live wire at all times). That will take some of the stress off fuse #7.

3. Running a wire from terminal 87 to the pump.

4. Running a ground from the pump to either the horn bracket or the ground that is located on the flat part of the frame near the cooling fans.

With 12 GA wiring, that gives me about 15 feet of wire to play with, BUT the power for the fuel pump will be taken directly from the battery, sparing fuse #7, so there should be a bit more of give there.

This will all also be with DMCH new fuel pump/sender combo.

I feel safe and confident with this setup. If 16 GA wiring could handle this (barely) with everything on fuse #7, then 12 Gauge wiring with a separate 20A fuse running the fuel pump circuit to keep fuse #7 safe with a possible maximum draw of 10A (assuming DMCH's fuel pump/sender combo demands as much current as John's powerpump), everything should be relatively safe.

Someone say something if you see a flaw with my idea/setup.

This is my way of keeping maximum uninterrupted power draw to my fuel pump as well as trying to keep fuse #7 safe. I'm tired of watching it try to melt down. Something needs to be done to fix this problem; this is my possible solution to both uninterrupted power draw and minimizes the stress on fuse #7...

Oh, and don't worry Dave - I am not cutting or hacking anything. This will be completely reversible if someone decides to reverse it after I die (: ... It'll also be documented and photographed.

[Bill Robertson's Sock Puppet #1]

IMHO Fuse #7 melts because it is already close to amperage limits in the best of circumstances, and fuse blade/terminal corrosion that is a very real threat for DeLoreans due to battery location pushes it over the edge.

There is a venturi tube that pulls battery gases into the passenger rear wheel well that does a very good job of keeping outgassing under control, but it only works when the car is moving. I suspect that is why DeLoreans that sit for protracted periods of time have so many more fuse box issues than DeLoreans that are driven regularly (I'm talking thousands of miles per year, not regular trips of a mile or two apiece).

Simply starting the engine and letting it idle while the car sits still definitely does absolutely nothing to vent battery gases.