whenever i keep my lightforce 170s on for about 5 continuous minutes, they go out becuase a fuse blows. it's always the same 15A fuse in around the middle of the wiring.

can i simply put in a 20A fuse in its place? i mean i think that would help, but how do i know what's too much, what's too high an amp rating for a fuse?. it's a hella wiring harness that lights my lighforces via the stock fog light switch.

 

I'm assuming its the fuse plug thats built into the wire? If it is and it came with a 15amp fuse then I wouldnt upgrade it. Although the wire can probably support more then 15amp its not designed too. If it doesnt blow the fuse it will melt and burn the wire, which could cause a fire in an extreme case.

I would upgrade the wire to something a little beefier....

 

I, personally, would run a 20 amp in there. There is enough extra leeway in the wire to handle it. It's going to get very hot though.

 

i have 3 hella 500s, some with replaced bulbs... and i pull 17.5 amps ....

 

Those have 100W bulbs, right? I assume you have 2 lights on the same fuse, which means you are running 200W of lights through a 15A fuse. That's right at the rating and makes sense that the fuse blows after the bulb gets hot (the resistance increases with heat and so then would current also increase).

If you know power and voltage, you find current by:

P (in watts) = V x I (current in amps)

So

I = 200W / 13.5V = 14.8A

A 20A fuse might not blow, but the harness is designed for a 15A fuse and no one can tell you that's safe to replace. There is certainly a chance that replacing a 15A fuse with a 20A can cause a wire to melt and start a fire. You have to decide if the wire in the harness is sufficiently sized to allow a 20A fuse.

I have Hella 500 fog lights and mine came with 55W H3 bulbs, so 2 lights should be 110W, which is about 8.2A at 13.5V.

 

At 14.8 amps, a 15 amp fuse should NEVER blow.

The bulbs are typically rated at 12 volts, so the actual current for 2, 100 watt bulbs would be 16.7 amps. If you look at the time-current curves for a standard 15 amp automotive fuse you'll find that it takes awhile at small overcurrent for the fuse to blow. It's not uncommon for a fuse to maintain 110% current (in your case, 16.5 amps) for a long time. Check out http://www.bussmann.com/library/bifs/2009.PDF and look at the curve for the 7.5 amp fuse which will maintain 10 amps for over 100 seconds (top-of-scale).

Try to figure out what size your wiring is (the smallest size in the circuit) and use that as your judge in going to a 20 amp fuse. You can find wire size current ratings on the web. I'd be leary if the wire is #16 AWG or smaller. You should be ok if it's #14 AWG or larger, provided the wire is standard automotive wiring.

 

thanks for the replies guys. yeah, each one of the two is 100 watts.
i'm not sure what the wiring gauge is in the harness is, but in the wires from that harness to the lightforces, it's either 14 or 16, i forget, i'll have to check when i get home. i really hope i don't have to re-do those wires again if it's 16 just cause it's such a process, but i will if necessary. then again, i gotta make sure the rest of the wires are up to the task of a 20a fuse too.

 

that's a really cool chart...thanks for posting it!

 

Yup, bulbs are rated at 12V for 100W, but they are actually getting something else. Are automotive bulbs rated at some temperature? I mean a bulb sitting in Fairbanks in winter is going to draw less current than one in Phoenix in July. Aux lights are usually wired directly to the battery with a relay, so why I picked 13.5V, which allows for some V drop through connections and wire. That was just a guess, plain and simple. I wanted to make the point that the lights are drawing right around 15A

A 15A fuse should allow 15A always, yeah, I agree with ya 100%. But it should blow at some current above 15A. I do see that the TCC curve you link indicates that Bussman fuses will tolerate 20A for long periods. His fuse opens after 5 minutes, so the chart does not even consider this period of 600 seconds. Subjected to current over its rating at some time the fuse will open. It could require hundreds of hours continuous at something like 105% of rating, but it should /eventually/ open. Just like the bulbs themselves, the TCC curves are assumed for ideal conditions. A hot engine bay, fuse age and vibration will change just how accurately the fuse element melts.

I just yanked the Littelfuse catalog off my shelf, looking at their ATO fuses. Their data show they these will tolerate 110% of rated current for a 100 hours min, but go up to 135% of rated current and they will tolerate that current for 0.75 second min and 600 seconds max. Assuming 12V current of 16.7A, you are looking at 111% over current, so that's somewhere between the 100 hours and 600 seconds, obviously more likely many, many hours. We're also assuming that a 100W bulb is right at 100W, could easily be more or less.

http://www.littelfuse.com/data/en/Data_Sheets/257.pdf
http://www.littelfuse.com/data/en/Time_Curve/257.tc.pdf

No matter how we split hairs, 200W worth of bulb is right around the 15A mark. And if we are sitting around 15A, any number of things could push the fuse over its melting point. Too small wire size, poor splices or connections, corrosion on the relay or bulb contacts, old filament in the bulb, ambient temp.

I do think the circuit needs to be sized for 20A. I'd personally put a 20A circuit on 12AWG wire, but that's really a swag. I could easily be over conservative and you may well be right that a fused 14AWG would be OK, too. Just dunno for sure. I'd be very careful about 20A on a 16AWG wire, even fused.

 

chuck,

are you well grounded? make sure that your ground point has no paint on the contact point---scrape it off w/ a dremel sanding bit or soemthing like that

bob

 

Wasn't really looking to split hairs, just helping him understand the time from turning his lights on until the fuse blows.

As a side bar, I work at a nuke plant and I used to specify and review destructive testing for fuses we bought. As far as the actual current draw on something like an automotive lamp, I agree, there are many variables to consider. And yes, temperature and contact/cable condition are major contributors. Suprisingly, vibration is not a factor for most fuses. We've seismically tested them and the industry has determined them to typically be seismically rugged. But I did see a lot of variance in as-found new part specs, especially out-of-tolerance items from supposed "quality" suppliers.

 

I know, didn't really take it that way. Just that the details sometimes obscure the big picture. There are a lot of variables that affect whether or not a fuse opens and these 2 lights are right on the edge for a 15A fuse. Really, a good example for nuisance fuse blowing. Fuse a circuit right around the expected load current, add wiring practices that aren't exactly stellar (we all do it, bad crimps, poor weather sealing, whatever) and relatively harsh conditions and you get a fuse that'll be a royal PITA.

Sounds like that'd be really cool. For what I design, fuses are not allowed. Maybe on the ISS or Shuttle (and then they'd likely be circuit breakers). But, on the stuff I do it's a bit difficult to swap them out. I have one board in a system on geo-syncronous orbit and a couple of other things straped to their boosters just waiting. We do occasionally design circuits with elements that will blow, but it is assumed in that case that you never use the circuit, or very potentially, the PBA ever again. Did not know that vibration wouldn't affect a fuse, those blade fuses just seem kinda fragile to me.

 

I agree. In my experience, all the electronic circuits I have built with fuses will blow them if the fuses are right at what the circuit will pull. I usually get a fuse at least 5 amps over. Just like most every electric circuit, it can pull way more than 15A at startup, and that can certainly lead to a fuse blowing prematurely. Try switching a 15A continuous load on and off a bunch of times on a 15A fuse and you will probably blow it.

 

If it were me I would probably rewire the lamps with wire the next size up (keeping in mind the lower the number the the bigger the wire gauge). I would also highly recommend going for a breaker fuse (won't blow but it will reset), double checking connections and grounds. I'm not as up to speed on DC current as I should be, but in the AC world a fuse or breaker should never exceed 80 percent of it's capacity.

 

Don't know if this has been mentioned but I think the Hella wiring harness is for (2) 55W lights, not (2) 100W lights...thus the reason for the heat and blowing fuses. You need to upgrade wire size and fused capacity IMO to the PROPER size and then some, no need to burn down your rig by putting in a 20A fuse and taking the lazy way. A guy burned his RX7 to the ground in Walmart several weeks ago due to his lazy half arse stereo install...wire burnt up cause he didn't fuse it properly....inferno at Walmart.

 

^ yeah. i just checked, the wiring i used fron the lights to the male end of the plug is 14ga. the wiring on the hella harness (and the female end of the plug to the fuse box), i'm not sure what gauge that is, but i guess need to upgrade that to 14ga. i still need to check the grounding.

 

Chuck,

Use this to get the proper size wiring and fuses.

http://the12volt.com/info/recwirsz.asp

Yep, a bad ground, or one that is loose can blow fuses.