Wire size & Watts - Honda Goldwing Forums
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post #1 of 12 (permalink) Old 10-10-2017, 06:53 AM Thread Starter
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Wire size & Watts

I have a set of fog lights that are using 35w bulbs.

I want to upgrade these to 55w or better 100w bulbs. I wonder if the stock OEM wires will handle 100w. I feel pretty sure the wires will handle the 55w bulbs. These are H3 bulbs.


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post #2 of 12 (permalink) Old 10-10-2017, 09:59 AM
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I'm not sure what gauge the OEM wire harness is so I'm not much help. Hopefully one of the other members that has more electrical sense than me can chime in.

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post #3 of 12 (permalink) Old 10-10-2017, 10:35 AM
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Consider this at 35 watts on a 12 volt system you are drawing 2.9 amps per bulb. at 100 watts on the same 12 volt system you are drawing 8.3 amps per bulb. So you would be almost tripling the current draw through the wires.

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post #4 of 12 (permalink) Old 10-10-2017, 02:07 PM
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Wire size (Gauge) is determined by the Amperes the wire will be asked to carry and the distance/length of the wire. To determine the expected Amperes use the formula: Watts (P) = Voltage (E) X Amperes (I). Simplified this is P=E*I, or in terms of the Amperes it is I = P/E. Do NOT use 12 Volts as the voltage. The normal charging system voltage range is 12 to 16 volts. Since Amperes increase with increasing voltage, you must use 16 as the voltage in your calculations. Use this formula along with the attached chart to determine the proper wire gauge/size. I would consider lighting to be a "critical" circuit for the purposes of this chart.

For example: To run two 100 Watt lights (200 Watts total) on a circuit that can see 16 volts; the amperage I would use for my calculations would be: I = 200/16 or I = 12.5 Amperes. Looking at this chart, to be on the safe side I'd use the 15 Amperes column. If the total wire length is 0-6 feet use a 14 Gauge wire, 6 to 10 feet go up to a 12 gauge wire, etc. While under-sizing the wire (higher gauge number) by a little bit, emphasis on "little," is not likely cause a safety problem, it will cause a voltage drop in the circuit that will lower the performance (light output) of the lamp.
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Last edited by Bluehighways; 10-10-2017 at 02:14 PM.
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post #5 of 12 (permalink) Old 10-10-2017, 04:16 PM
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Quote:
Originally Posted by Bluehighways View Post
Since Amperes increase with increasing voltage, you must use 16 as the voltage in your calculations.
For example: To run two 100 Watt lights (200 Watts total) on a circuit that can see 16 volts; the amperage I would use for my calculations would be: I = 200/16 or I = 12.5 Amperes.
Minor correction. Amperes DECREASE with increasing voltage. Using a lower voltage will be more appropriate. 200w/12v=16.7 amps. Your wires and connectors must be able to carry that load. Good luck with your mod, and hopefully Bluehighways can recommend the proper size wire. too
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post #6 of 12 (permalink) Old 10-10-2017, 06:00 PM
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post #7 of 12 (permalink) Old 10-10-2017, 06:05 PM
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Quote:
Originally Posted by NREMTP View Post
Minor correction. Amperes DECREASE with increasing voltage. Using a lower voltage will be more appropriate. 200w/12v=16.7 amps. Your wires and connectors must be able to carry that load. Good luck with your mod, and hopefully Bluehighways can recommend the proper size wire. too
I believe that you may confusing Independent and Dependent variables. Voltage and Resistance are Independent variables while Current and Power are Dependent. Wattage is the result of the Voltage applied and the Amperage flowing through the Resistance. When a device is listed as "X" Watts they will typically provide the voltage at which that wattage will be generated. While it is probable that the 100W lamp that the OP is looking at is/was rated at 12.0 Volts, in a forum such as this it is probably better/safer to assume that it may be rated at the normal high end of the charging system Voltage used by vehicle manufacturers. This can make for better advertising copy. If the OP uses a slightly larger wire than is needed, no harm, no foul. OTOH the opposite may not be the case.
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post #8 of 12 (permalink) Old 10-10-2017, 06:06 PM
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post #9 of 12 (permalink) Old 10-10-2017, 06:45 PM
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The wiring can handle 55W bulbs, but the switch on it’s own will not and the circuit will require a relay to carry this load. I would not use 100W bulbs that close to the roadway, half of the light will be wasted on the roadway directly in front of the bike and not projected very far down the road.

Resistance is constant and the power and current will vary with the voltage applied. Goodluck, don’t burn it down...
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post #10 of 12 (permalink) Old 10-11-2017, 07:00 AM Thread Starter
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I want to thank everyone for the help. Ron Robertson, Bluehighways.. thanks a lot for the chart and the writeup. I saved the chart and I'll be using it in the future too.

Nremtp - Techdude 2000 - Budoka - Chopin - thanks guys!

Looks like I'm not going to burn things up now! I'm going to measure the wire's gauge and go from there. My feelings without looking at the wires but at the chart I'll go no higher than the 55w.
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