Relay harness fuse size

[buck91]

Allow me to start with yes, I have read Mr. Daniel Sterns material. He does not discuss fuse SIZE.

I am working on a relay setup for my 1996 Mustang. Planning on running two seperate relays, one for the low beams and one for the highbeams (so one must handle 110 watts total and one 130 watts). Now I've seen some various talk on the net and based on the below I'm not sure what the more appropriate fuse size is. I am currently planning on running 14awg power wires from the ear relay to each bulb with 12awg power wires to each relay and for both bulb grounds (each bulb has its own ground which is shared by both filaments).

According to this: http://www.oznium.com/forum/topic15761
My max current is 11.8A since my smallest wire is 14awg. Now I figure that my current draw will be inversely proportional to voltage, which should range from 12.8vdc to 14.8vdc giving a maximum current draw on the highbeam side of 10.2A and a minimum current draw on the low beams of 7.4A. Now it looks like the factory fusing is a 20A common for pretty much all the exterior lights. Now, my load is more concentrated but since the wiring will be significantly stouter than OEM would a 20A fuse still be appropriate or should I keep it down to a 15A or less?

 

[Colt45ws]

20A is fine. I have a 30A breaker on my harness. Mainly you are protecting the wiring from a short. As long as your fuse blows before the wiring goes up in flame, its good. If you look at how thin the factory wires are, that is why they fuse lower.

 

[HangFire]

No...

130W lamps don't stay at 130W when you increase the voltage yeilding an inversely proportional current draw. They don't work that way, and Ohm's Law doesn't work that way.

Incandescent lights do change resistance with their temperature, but generally speaking the more voltage you provide the more current they will draw.

Long answer: look up the voltage and current draw curve for the bulb. Unfortunately, curves or multiple voltage/amp specs can be hard to find.

Short answer: Go with the 20A. That chart you linked doesn't determine the most possible amps per gauge; it lists the safest possible continuous amperage without undue wire heating. Current won't magically limit itself just because you used a finer gauge wire, the flow is limited by the resistance of the wire. A shorted 12 ga wire will blow a 20 amp fuse just as a shorted 16 ga wire will, it's just that the 16 will briefly get a little hotter in the process.

 

[JHZR2]

Ampacity and voltage drop on a conductor is a different story... For that, you want to go as big as you can that is practical, but within reason. You can calculate voltage drop if you know your conductor's resistance, but that's a different discussion.

Simply put though, 130W = V * I = 12 * I, so I=10.8 The 130W may be rated at a certain feed voltage, like 12.8V instead, and the resistor (bulb) will dissipate more energy at higher voltage, but its probably close enough when we do some handwaving and some derating...

SO if your system was running low and you had lights on, you would be pulling under 11A.

Therefore you can put in a 20A fuse, and in reality could probably push a 15A in there in a pinch.

Basis is a high underhood temperature, so a 20% derate. The fuse should be at least 25% oversized for the load as well.

So: Rating = A/(0.75*0.8) = 11/0.6 = 18.3

But the small conductor in the fuse that is supposed to vaporize is itself a series resistance (which is why Id avoid the 15A fuse). Reality is that a hard short, which is the big concern, would pull a LOT of amps... So in the interest of not pushing the fuse, Id put in a 20A fuse or even 25-30. 14 Ga wire can carry 20A continuously on a 6' run per this chart:

http://www.qualitypowerauto.com/Wire Sizing Chart 12V DC.htm

So Id protect it as defined. If the run is shorter, Id consider running as large a fuse as is allowed for a continuous loading on 14 Ga conductor, or at the max ampacity of the relay.

You could go crazy oversize with everything, but its all diminishing returns too, so dont go overboard.

If I was building it, Id get the appropriate type of underhood high temperature and fire resistant wire (GXL or SXL), and protect it with the 20-30A fuse, depending upon the run length.

Automotive wire info:

http://www.awcwire.com/faq-automotive-wire.aspx

 

[eljefino]

Your fuse should be sized for the wiring, eg, should blow just before the wiring overheats. Remember these are your HEADLIGHTS and if the fuse blows you can WRECK driving at night. 20 amp is entirely reasonable. Your looking at the theoretical max of 11 amps or whatever and going just slightly over that is the wrong approach. In fact if Ford put a 20 amp fuse on 16 ga wire I don't think it's unreasonable to put a 25-30 amp fuse on 14 ga wire. The rules for short car wire runs aren't the same as household electrical code and with cars you can expect a larger fuse for the same wire gauge.

I don't think it would be overboard to add two more relays for a total of four, one for each filament. Fuse the incoming left and right sides separately. Or use thermal breakers; they make them in ATC fuse size and they reset after they cool down. Common for GM power windows.

 

[buck91]

eljefino, you make some good points. Although I believe that wiring the high and low beams independent offers adequate redundancy even if not ideal. Plus I am also running Hella FF 75 fogs in place of the factory useless, I mean units; these offer very reasonable performance even as stand alone lighting.

 

[Snoman002]

As was stated, size the fuse for the wiring, the load is irrelevant. The fuse is ONLY there to prevent the wiring from catching on fire (ask me how I know...).

 

[SHOZ]

Automotive wiring is a bit different on amp draw depending on the location. Here's Hyundais recommended wire size and amp load.

 

[JHZR2]

Hyundai image doesn't seem to be working, but I already provided an ampacity chart above which uses the following assumptions:

This table was calculated for a 12V system assuming, a .36 voltage drop for starting and a .44 voltage drop for charging, a maximum operating temperature of 150F. A larger wire can be used to decrease the voltage drop or to operate at a higher temperature.

If selected correctly, the wire will be sized adequately but not excessively over, to support the connected load. In that case, the fuse would protect the wire but must also be considerate of operating load.

Everything goes hand in hand unless you're talking big feeder conductors. Here its a dedicated conductor to the load, so the load dictates ampacity, and the fuse protects that conductor, sized to at or a bit over its continuous rating.

 

[SHOZ]

It's a gif file. Seems most computers block it.

 

[buck91]

Got the relay harness finished last night. 14ga power wires, in pairs (each positive bulb connection has its own wire), 10ga supplying the relays and 12ga grounding wires straight to the negative terminal. Haven't measured voltage drop after the upgrades, but light output is quite a bit brighter with significantly "whiter" light.

Stock wires had 1.7vdc drop on the positive side, but I didn't measure drop on the ground. Will be interesting to see what this harness shows.

 

[eljefino]

 

[nipsip]

Originally Posted By: buck91
Allow me to start with yes, I have read Mr. Daniel Sterns material. He does not discuss fuse SIZE.

I am working on a relay setup for my 1996 Mustang. Planning on running two seperate relays, one for the low beams and one for the highbeams (so one must handle 110 watts total and one 130 watts). Now I've seen some various talk on the net and based on the below I'm not sure what the more appropriate fuse size is. I am currently planning on running 14awg power wires from the ear relay to each bulb with 12awg power wires to each relay and for both bulb grounds (each bulb has its own ground which is shared by both filaments).

According to this: http://www.oznium.com/forum/topic15761
My max current is 11.8A since my smallest wire is 14awg. Now I figure that my current draw will be inversely proportional to voltage, which should range from 12.8vdc to 14.8vdc giving a maximum current draw on the highbeam side of 10.2A and a minimum current draw on the low beams of 7.4A. Now it looks like the factory fusing is a 20A common for pretty much all the exterior lights. Now, my load is more concentrated but since the wiring will be significantly stouter than OEM would a 20A fuse still be appropriate or should I keep it down to a 15A or less?


http://goo.gl/UgffHp

 

[buck91]

Uh...what?