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Discussion Starter · #1 · (Edited)
I understand use of a relay to directly power a device from the battery. Example: powering a device that has a 14 gauge power wire. The relay is wired to the battery using a 6-8 gauge wire, then a power wire (6-8 gauge) is run to the device. Since the power wire attached to the device is a thinner gauge (in this case 14 gauge), does connecting the thicker gauge wire to the thinner wire from the device have any concerns?

My specific issue is when my AC evaporator fan kicks on the voltage drops from 14.3 to 13.6-13.7. Then when the compressor kicks in the combined voltage drop results in a reading of 13.1-13.2. My thought is to connect the evaporator fan directly to the battery using a relay to avoid that stress on the electrical system.

Thank you

Allen
 

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Not knowing how much each device draws in current, a 10 gauge wire (30 amp load) or a 12 gauge wire (20 amp load) that is running from the relay pin 30 to the +Batt could work for you. Connecting your 14 gauge wire from the device wire and using a thicker gauge wire to relay pin 87 should work for you. This device wire probably come from the device manufacturer like this.

I agree with you adding relay's to these systems to avoid draw on you ignition+. Be sure to place a fuse between the battery and the relay pin 30 for safety.

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Wire gauge is sized to safely carry the current load and in automotive applications the runs are relatively short and the resistance minimal. Electro-mechanical relays are used as a low-cost solution to shorten the run of large gauge wiring or to isolate a switch or other component from high current loads.

If your A/C evaporator fan draws 15 amps it's going to draw 15 amps whether connected using a relay or not.

Remembering that the voltage, in a circuit, will reflect all the resistance in the circuit what you are seeing is the reflection of the load on the electrical system of all the devices in use at the time. I doubt you'll see more than a few 10th's of a volt improvement by adding a relay and "heavier" wiring, at the expense of another electrical component to potentially fail.

Assuming you're checking voltage at the battery, what you are seeing is relatively normal and, under heavy load, a charging voltage of 13.2 is perfectly fine.
 

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Discussion Starter · #5 ·
Wire gauge is sized to safely carry the current load and in automotive applications the runs are relatively short and the resistance minimal. Electro-mechanical relays are used as a low-cost solution to shorten the run of large gauge wiring or to isolate a switch or other component from high current loads.

If your A/C evaporator fan draws 15 amps it's going to draw 15 amps whether connected using a relay or not.

Remembering that the voltage, in a circuit, will reflect all the resistance in the circuit what you are seeing is the reflection of the load on the electrical system of all the devices in use at the time. I doubt you'll see more than a few 10th's of a volt improvement by adding a relay and "heavier" wiring, at the expense of another electrical component to potentially fail.

Assuming you're checking voltage at the battery, what you are seeing is relatively normal and, under heavy load, a charging voltage of 13.2 is perfectly fine.
I have the Dakota Digital gauge setup. The voltmeter reading is taken off of a key on power source. Before switching back to manual brakes, the vacuum pump I had connected to the power brake booster was connected to a key on source would drop my voltmeter reading significantly when it turned on (12 or so). When I connected it to the battery with a relay, the voltmeter reading did not drop when it turned on.
 
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