Fuse ratings 101

[Olazz]

As most of you know I'm a bit of an buffoon when it comes to electrickery, but I'm learning slowly.

So the question.

If a fuse is in place & rated at 15amps, would one expect the draw through that fuse to be considerably less than 15 amps?

I ask as I have a fuse that keeps blowing on my fog lights (HID's fitted which I'm unhappy with). I measured the draw through the fuse and it measures 14.25- 15 amps. There are no shorts that I can find. The fuse also gets pretty warm.

Should this fuse be uprated to 20amps? If I do so, will I have fire waiting to start by overloading etc ?

Thanks guys

 

[Gary Stockton]

15 amps to HIDs is not right I would have thought? Is all the ballast etc ok? And what rating is the wire? Up rating the fuse will cause issues (lots of smoke) if there is an inherent fault, or if the wire rating is too low. What is the rated draw of the lights?


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[Graham]

Defiantly some thing wrong there.

I would suggest that the HID head lights would draw around 7 amps together.
They might peak at double that for a split second, like 0.1 of a second while they strike up.

Some thing wrong, that's for sure.

Gra.

 

[Jon Wildsmith]

The fuse should be the weak link, so as long as the rest of the circuit is rated for more than 20 amps you can increase the fuse size but if not then there is a risk of magic smoke being involved. What wattage are the fog lights? A pair of 100 watt HID lights would draw that sort of current.

 

[Olazz]

Thanks for the quick replies oh learned Gentlemen.

15 amps to HIDs is not right I would have thought? Is all the ballast etc ok? And what rating is the wire? Up rating the fuse will cause issues (lots of smoke) if there is an inherent fault, or if the wire rating is too low. What is the rated draw of the lights?

Gary I didnt fit the HIDS so no idea on wire rating supposed to be 55W but the balasts are 35W. No idea of the rated draw...how do I find out?

Defiantly some thing wrong there.
I would suggest that the HID head lights would draw around 7 amps together. They might peak at double that for a split second, like 0.1 of a second while they strike up. Some thing wrong, that's for sure.

Gra, initial peak at about 14.93 amps then settles on about 14.25amps

What wattage are the fog lights? A pair of 100 watt HID lights would draw that sort of current.

Jon supposed to be 55W but balasts are 35W.

 

[Jon Wildsmith]

Any chance the fuse in question has the power for the HID's and the headlights running through it? Do you see power draw with just the headlights on? (I'm assuming the fogs don't work without the headlights?). I'm just wondering if someone upgraded the wiring for the headlights and the HID's are on a spur off that wiring.

 

[Olazz]

Jon,

The fog lights are incorporated into the main headlight (inner) as its a Jap import. They are on a separate switch and work independently of the dip/main beam, provided the side lights are on.
The the wiring as far as I kow hasen't been upgraded, but has just has HIDs fitted.
I am unhappy with the whole setup and will probably remove the whole lot, but don't want to end up with the "magic smoke" or have a "chris moment" on the drive before I do that.

I havent tested the draw with just the healights switched on (doh) but will do later today.

 

[Graham]

Hi Olazz,

Do you suggest that the power for the headlights, and the power for the fog lights are through the same fuse?

The power for the head lights, should be one fuse, and the power for the fog lights through another fuse.

let us know.

Gra.

 

[Olazz]

Hi Olazz, Do you suggest that the power for the headlights, and the power for the fog lights are through the same fuse?
The power for the head lights, should be one fuse, and the power for the fog lights through another fuse.let us know.Gra.

Gra, I don't know that , but will find out later today. I didn't think about checking that.. told you I was a bit Dof !

 

[Graham]

OK,
No worries,
When the fuse pops, do the head lights go out also? or just the fog lights?

Gra.

 

[Olazz]

Ok, done some more investigating.

The headlights are NOT fused through the 15amp fuse in the cab. The headlights and fog lights work independently. I can turn on fog lights via the fog switch on the dash. The outer lamps are both dip/beam - inner lights on headlight unit are the fog lights.
The headlights are switched via the stalk. Removing the fuse does not disable the headlights only the fog lights.
So click picture to watch this vid and see the current

 

[Jon Wildsmith]

Something not right there but whether it's a fault or just poor quality parts is hard to say. Can you measure the voltage at the fuse as well?

 

[Olazz]

Something not right there but whether it's a fault or just poor quality parts is hard to say. Can you measure the voltage at the fuse as well?

Yes I can Jon. What voltage should I expect to see, and what will it indicate?

 

[Jon Wildsmith]

You should expect to see the same voltage as at the battery terminals. I'm just wondering why the fuse is getting pretty warm with just 15 amps going through it and if it's because of a poor connection causing voltage drop.

 

[Olazz]

Thanks for the information Jon.
Will pick it up in the morning and see what's what.

 

[BlackWidow]

Olazz,

Just a quick thought have the lights been wired up in series instead of parralel, the easiest way to find out is if you were to remove the bulb from one light does the other one go out. If it does then it has been wired up in series and this could be the problem and give you the reading of 14 amps (2 x 7amps).

Steve

 

[Olazz]

Ah, thats a good point. I will check it out and see.

Thanks

 

[Nuclear Chicken]

Except if that was the case each of lamps/ ballasts would only be seeing about 50% of the available voltage and probably wouldn't work at all. A 15A rated fuse should run at 15A quite happy, and will probably get warm, but should not fail at 14.??A. A typical 35W HID lamp is 35W so 2 of these will run at about 7A as somone else noted. You've a failing lamp or ballast or something. Anyway, let's hear what you find.... See if you can determine the type and brand of lamp too.

 

[Olazz]

Ok had a chance to look at this today.
Fog lights are not connected in series as one stays on when other bulb is removed.

Fuse has not blown now for a a week, with me switching on and off regularly. Still drawing 14- 15amps at the fuse end.

Now I've learnt that the import trucks are negative earth switching, but it would appear that my lights have been wired for positive earth switching. I assume that is what's causing the issues with the lights and the high current draw?

That being said, is this a fire waiting to happen? Is there a major issue if wired for positive switching when it should be negative?

 

[Nuclear Chicken]

Don't get too hung up on the pos vs neg switching thing: it's not that big a deal. It just means that one version switches the +12V onto the 'load' and the other switches the 'load' on to the -12V, the ground chassis. As long as the fuse pops as it does you shouldn't have a fire issue but I recommend finding out why the current draw is so high. It is possible that the connected ballasts for the HID lamps are very poor quality and draw more than 'normal' with losses and inefficiency but, even with that, it still sounds like too much. Did you check the lamp wattages? Are you sure you don't have a 55W where a 35W should be fitted? Until you are satisfied everything is OK don't fit the 20A fuse and you need to be satisfied that the wiring will cope with 20A too. You'll need a wiring diagram or consult and auto-spark. Depending on the wiring type and insulation it should be about 1.5mm sq to handle a 20A fuse.