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post #4 of (permalink) Old 08-20-2015, 12:44 PM
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Re: Choke, Fuel Pump, and Coil...same relay?

Be aware, I'm not picking on you for the large amount of info to follow. I have had several requests to outline how I determine the wiring in projects I do. So, I'm using your situation as an excuse to outline my process to others as I answer your situation, with how I would figure it. Remember, this is just how I do it on my stuff, and everyone must use their own judgment when it comes to electrical stuff.
Originally Posted by daginsu View Post
I believe I understand what you are saying about the relay, but I think the image you linked is being blocked from work because I didn't see anything. Is the attached image wired the way you are recommending?
Almost. The trigger wire for the relay would normally be (+) on 86, and ground 85. Otherwise, yes.

Here is the photo I linked to at another hosting site, or the direct link is <>. Let me know if you can see it here:

Originally Posted by daginsu View Post
Also, from the charts I'm looking at since I'm only drawing a max of 15A I should be able to use 14 gauge wire instead of 12 gauge correct?
That depends on your wire length and whether it is intermittent (choke) or constant duty (fuel pump). Only the primary wire to and out of the relay needs to be heavy, and then the wires to each device can be suitably smaller. So, I figured 12A max, and you're saying 15A, so let's go with 15A.

While trying to avoid info overload; here are a couple helpful aids and guidelines I use for good, safe and durable wiring. Remember, when designing a circuit, you are looking to satisfy three primary requirements:
  • Safety
  • Performance
  • Reliability
Safety The gauge size is commonly selected to limit the wire heating, and while this is not the only concern, it certainly impacts safety. The photo below of FAA recommended practices is a very handy reference. If in-doubt, I follow what it says. Note that the top section is for temperature, and a 14 gauge wire will max-out a wire rated at 105C at 12.7 amps. That's right - with less than 13 amps, that wire will be over 221F. While that's not enough to melt the wire or start a fire, it is enough to burn you or affect sensitive things touching it. Hotter means the insulation may melt and allow a short and possible fire. Of course, you could use higher-rated wire at 150C/302F and it would hold it fine, but the wire will be that much hotter as the amps get higher.

That gauge choice is up to you, but the chart indicates to me that 12 gauge would likely be a better long-term choice for the primary connection when I think about what that wire may be running near, or in a bundle, or under my carpet. If it's running in free air in the engine compartment, and it's rated for compartment temperatures with the added load temperatures, then it may be fine from a temperature and safety standpoint. If I'm out of 12 gauge wire, then perhaps I'll pair two 16 gauge wires to handle the load instead. Again, a personal judgment call, based on the specific situation and requirements.

Performance Voltage drop due to wire resistance means whatever you attached will get less voltage than it might need for best performance. The length of your power path in both directions causes this resistance to increase. A calculator to help determine voltage drop can be found here: Voltage Drop Calculator

One rule-of-thumb I use is that intermittent-duty devices can often get-by with 5% voltage drop. Constant-duty devices usually perform best with less than 2% drop. A looser set of rules some folks use are 1-volt and 0.5 volt drops. I use the percentage rules so that the system is still within the looser rules as the wiring ages, to maintain reliability and performance. Your horn volume probably isn't a concern, as much as your headlights dimming by 22% with only a 1-volt drop from the rated 14 volts to 13 volts.

While most spec's call for a 15.5 volt max system voltage, typically the highest you will see in normal operation is about 14.7 volts. That's the number I use in the calculator. Running the calculator with the following numbers give these results for performance:

As you can see, the results are over my limits, and very close to exceeding the looser rules with 14 gauge at 14.7 volts and 15 amps. My choice would be the next larger gauge.

Reliability The same process is used to choose the wiring for the smaller runs to each device. All wiring should only be automotive stranded wire or better - never solid wire. For simple robustness in daily handling, vibration, etc., minimum wire gauge for automotive use is typically 18 gauge. For anything. Lighter gauges are avoided as too easily damaged or broken in normal use, just due to their small size and low relative strength.

Hope that helps!


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Last edited by PSIG; 08-20-2015 at 12:48 PM.
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