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Fuse for aux lights gets hot

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Old 07-19-2012, 08:30 AM   #1
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Fuse for aux lights gets hot

Is this normal? It's the 15 amp fuse that runs from positive battery terminal to the relay (terminal #30). I have 3 Hella 700ff lights that run off one wire/fuse. The fuse casing (inline glass tube style fuse) isn't too hot to hold but the fuse itself is very hot. If this is not normal, what might I have done wrong? What would cause the fuse to get hot? I switched it to a 20 amp fuse since I am running 3 lights instead of the two that the kit came with. Do I need lower guage wire for the additional current. I belieive the kit came with 14 guage wire. Everything is wired as per the instructions and works just fine except the fuse getting hot is suspisious.

Lights
to ground
to relay #87

I have a 40 amp relay
30 - to battery with in line fuse
87 - to lights
86 - to switch
85 - to ground

Rocker switch in dash
to ground
to 2.5 amp fuse
to relay #86
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Old 07-19-2012, 01:43 PM   #3
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Might 86 and 87 be reversed? I don't think your lights would work then though.
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Old 07-19-2012, 05:27 PM   #5
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Poor contact on the fuse will cause that.
I do NOT like inline glass fuse holders for this reason... damned near burned out my '69 Camaro.

#1 - Switch to a blade type fuse and fuse holder.

#2 - 15a fuse should be carrying no more than 12 amps, wire should be 14ga or larger.
12 amps is about 144 watts.... Those Hellas are 55 watts each, you're pulling too much through the fuse and wire.

Three 55 watt lights is 165 watts, roughly 14 amps.
14 amp continuous requires a 20amp fuse and 12ga wire (80% rule). Anything smaller may run hot.
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Old 07-19-2012, 05:33 PM   #6
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Quote:
Originally Posted by Rich91710 View Post
Poor contact on the fuse will cause that.
I do NOT like inline glass fuse holders for this reason... damned near burned out my '69 Camaro.

#1 - Switch to a blade type fuse and fuse holder.

#2 - 15a fuse should be carrying no more than 12 amps, wire should be 14ga or larger.
12 amps is about 144 watts.... Those Hellas are 55 watts each, you're pulling too much through the fuse and wire.

Three 55 watt lights is 165 watts, roughly 14 amps.
14 amp continuous requires a 20amp fuse and 12ga wire (80% rule). Anything smaller may run hot.
well done sir.......... rep to you
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Old 07-19-2012, 06:10 PM   #7
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Quote:
Originally Posted by Rich91710 View Post
Poor contact on the fuse will cause that.
I do NOT like inline glass fuse holders for this reason... damned near burned out my '69 Camaro.

#1 - Switch to a blade type fuse and fuse holder.

#2 - 15a fuse should be carrying no more than 12 amps, wire should be 14ga or larger.
12 amps is about 144 watts.... Those Hellas are 55 watts each, you're pulling too much through the fuse and wire.

Three 55 watt lights is 165 watts, roughly 14 amps.
14 amp continuous requires a 20amp fuse and 12ga wire (80% rule). Anything smaller may run hot.

Well said. Thank you. Kind of what I thought but didn't know the technical stuff to back it up.

So I should run 12ga wire from lights to relay as well as the battery to relay?

Also, already planned to switch to blade type fuse. Have a fuse block on order for all my aux lighting. The glass tube type is what came with the kit.
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Old 07-19-2012, 06:18 PM   #8
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Yup. 12ga from the battery to the (20a) fuse holder to the relay to the lights AND to ground.
18ga or 20ga from a switched power source through a 5a fuse to the dash switch to pin 85 or 86 on the relay, same size from the other (86 or 85) to ground.

The three basic rules that will help ensure a safe build:

1 - The purpose of the fuse is to protect the wire... it must be sized to protect the smallest wire, switch, or relay.

2 - Nothing in the system may continually carry more than 80% of it's rated capacity.

3 - Often overlooked... What goes in must come out. The current at any point in a series circuit is identical. If you are drawing 15a through the relay, you are also pulling 15a through the ground wire.



#3 is one that bites people often. They'll run 4ga wires to a rack of amps... then they'll tie all 4 amps to a single 6ga ground wire. The ground wire just became the fuse.
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