Wire Guage Question

[Allen]

What guage wire for an 18 amp circuit?

Is a 20 amp fuse too close to the draw? Would 25 be better?

Thanks for saving me the trouble of looking it up.

Allen

[TObject]

How long the wire, and how much voltage drop can you afford?

[Allen]

Two runs of maybe 12-15 feet.....

How do I calculate the drop?

(Rocklights along the frame rails......)

Allen

[Jerry Bransford]

12 gauge wire, 10 if you want to be ultra-conservative. So far as the fuse goes, I would use nothing smaller than a 25 amp slow-blow because lamps have a high turn-on current when they're cold. I might even be tempted to use a 30 amp fast-blow fuse which will still protect them.

You are using a relay to actually turn the lights off and on with, right?

[TObject]

Originally posted by Allen
How do I calculate the drop?

There are formulas, or you can just use an online calculator, such as this one:

http://www.ncounty.net/users/lucky/e.../voltcalc.html

For 12 volts input, 18 Amps, over 15 feet of #12 wire, you'd loose about 1 Volt, according to the calculator:

Calculation Type: Voltage Drop
Material: Copper
Phase: Single Phase
Wire Size: # 12
Input Volts: 12.00 Volts
Actual Voltage: 10.93 Volts
Voltage Drop: 1.07 Volts
% Volt Drop: 8.89 %
Amperage: 18.00 Amps
Distance in FT: 15.00 Feet

The more voltage drop you have the dimmer your lights will be.

[Allen]

Yes Dad.....



I will use a relay.....

But a relay part number or something would be a big help.....

Cheapo lights don't supply relays.

1 relay for 4 lights.....can be done, right?

I hate wiring jobs.....the cutting.....the fishing.....the crimping.....the zip tying.....the looming.....ugh.....

Allen

[Stu Olson]

As for the relay part number.....I use the very common 30 amp relays that hang on the hooks at AutoZone....they run about $5. The pin out diagram and associated wiring for a typical lighting circuit is in the below link.

This should help fill in the gaps a bit....

http://www.stu-offroad.com/misc/lights-1.htm

Can 4 lights be driven off of one relay? If the combined current of the lights does not exceed the relay's rating, yes it can. I personally don't try to push the max current ratings but prefer to keep mine between 50% and 75% of what the relay claims it can handle. Call me conservative if you wish......but I've never had an electrical issue with any wiring that I have done myself.....Jeep can NOT make that claim for at least two of their vehicles I have owned!

[papromike]

Go to an electronic supply store, get a BOSCH relay



those chinese knockoffs at autozone, dont hold up and arent moisture proofed very well..


You can run 4 lights off 1 relay unless you want some redundancy

The relays by bosch come in a 30 amp and a 40 amp version..

I used to install very high end 12v audio and video systems and

only used Bosch relays, hardly EVER had a failure, you can use a

standard inline fuse for it, or you could use a thermal circuit breaker (which is very cool, cause no fuses to change)..

here is quick wiring diagram.






I also use nice neat "relay" sockets, you can get them prewired with pigtails coming off of them, it makes the wiring very neet, AND incase of relay failure, all you have to do is Unplug it, then plug a new relay in..

I wired up mine with an ON-OFF-ON switch by Carlingswitch (ARB style switch),

if you get a Diode across the two relays, you can have it work like this...

switch positon (on) 1 2 lights on
Switch postion middle (off)
switch positon (2) #2, all 4 lights on

here is a link with the part number on it

relay ....http://www.partsexpress.com/pe/showd...15175&CATID=47

relay socket...http://www.partsexpress.com/pe/showd...=9685&CATID=47

[Jerry Bransford]

I'd run no more than two big lights off a single relay. Like Mike, I also recommend finding the Bosch version, its quality is superior to most that you'll find on the store shelves. The schematic I emailed you this morning shows the relay connections for a Bosch relay but most any square relay that looks like the Bosch will use the same numbering scheme for the relay.

[papromike]

Yep, gerry I also use seperate relays for each pair of lights..


keep in mind the current draw...

a 50 w light draws 4.16 amps of current... round it up to 5 amps
a 100w light draws 8.33 amps of curret.....round it up to 10 amps


so basically if you use 30 amp bosch relays for a pair of 50 watt lights, you have some reserve..


Take into account that these measurements are at a constant 12V

You may be feeding the light a bit more, or bit less depending on your alternator/battery combination and wire length/thickness,

as the voltage drops, the lights actually require more current to operate..

for a 50w pair of lights 14 guage wire is sufficient, if you want a bit of overkill go to 12 guage wire and be done with it..

[mrblaine]

Toss the Bosch relays if you want to run lights in pairs and get the Hella relays that have two common output terminals set up specifically for running pairs of lights. Simplifies your wiring.

They have 2 87 terminals and no normally open side.

part number 4RA 003 510-26

I also have the relay sockets with the pigtails, but if your really after a neat look, I have the relay sockets and the crimp on connectors that snap into the socket to eliminate all the butt connectors at the pigtails.

[Jerry Bransford]

Originally posted by mrblaine
They have 2 87 terminals and no normally open side.

Did you by any chance actually mean to say no normally closed side Blaine?

[mrblaine]

Originally posted by Jerry Bransford
Did you by any chance actually mean to say no normally closed side Blaine?

Yep, looked at the word no and wrote normally open.

[Daless2]

Hi Allen,

What size lights are you installing and how many? Meaning what is their wattage rating? Are they 55 Watts?

If so, you can more then handle four of them off one 30 amp relay.
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How to Calculate Total Amperage and Relay Size

The math is quite simple.

Watts / (divided by) Voltage = Amperage Draw
.
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Plug the numbers in for one light.

55 Watts divided by 13.8 Volts = 3.99 Amps per light.
.
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Regardless of how many lights you have, the circuit for all lights wired through any single point(Be it a wire or a relay contacts) must be "Rated" to handle the Total (Cumulative) Amperage that all lights will draw.
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For example;

If you have Two 55 Watt lights then the amp draw would be 2 X 3.99 = 7.98 Cumulative Amps through any single point.

If you have Four 55 Watt lights then the amp draw would be 4 X 3.99 = 15.96 cumulative amps through any single point.
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Any good quality Relay (Bosch type) rated at 30 amps is More then enough to handle this current draw for either TWO or FOUR 55 Watt lights. (By far!)
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If you have Two 100 Watts lights you can indeed also use a Single 30 Amp rated Bosch type relay with plenty of safety factor built in.

100/13.8 = 7.25 amps per light X (TIMES) 2 = 14.5 Cumulative Amps through any single point in the circuit.
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If you have Four 100 Watts lights you can very safely use a single 40 amp rated Bosch type relay with plenty of safety factor built in.

100/13.8 = 7.25 amps per light, X (TIMES) 4 = 29 Cumulative Amps through any single point in the circuit.



What Wire Size?

Well if it is only Two 55 watt lights, you don't need much of a wire at all, provide you keep that wire length under 15 feet. You can use even a thinner wire if you are going to run them individually to each light from the Normally Open contact (87) on the relay.

Please understand, wire size has many variables built into determining the correct size, and amperage is only one factor. Wire length and temperature are others,as is where the wire will be installed. (In free open air or bundled up with other wires.)

That said, 16 gauge from the battery to pin 30 on the relay, and two 18 gauge wires from the relay (pin 87) to each 55 watt light (1 of these wires to each light at 18 gauge is way more then enough!!)

Yes I looked it up in the tables, but as supporting evidence I offer the following.

Wire size used by Manufacturers (who take Wire Length, Temperature and Wire Loom Wraps into consideration for their vehicles;

Hella 55 Watt Driving Lights = 18 Gauge

Jeep Low Beam Headlights = 18 gauge

Jeep High Beam Headlights = 16 gauge

Jeep Fog Lights (55w) = 18 gauge (single wire, not even two)

Headlight (low), Fog lights and Driving Lights for every vehicle I own a Factory Service Manual for (14 vehicles) = 18 gauge wire.


Fuse

I would use nothing larger then a 20 amp standard fuse for a two 55 watt light setup.



I don't believe any of the advice given in this thread is wrong or bad in any way. It is however way overbuilding a simple wiring system that in and of itself has significant safety factors already built in.


Please share with us more detail as to your lights, how many and what are they rated at. I know a lot of folks here can help out more specifically given that information.

Hope this is helpful,

Frank

[Jerry Bransford]

Ohhhh my head, my head... good stuff Frank but I feel like I'm back in the lab on a Monday morning after a weekend of rush parties.

[Daless2]

Originally posted by Jerry Bransford
Ohhhh my head, my head... good stuff Frank but I feel like I'm back in the lab on a Monday morning after a weekend of rush parties.

Hi Jerry,

I had to laugh at your comment. I certainly hope I am not getting too "anal" of late.

Have a great evening my friend,

Frank

[Allen]

Thanks Frank.....

That was a big help and explains why my box is full of 14ga wire. My book is up in my attic somehwere.....I was tempted to use 13.8 as my divider.....but stuck with 12 for a margin of safety. Reason being, I thought they were 12v systems, but just the alternator and battery hovered at 13.8.....

Right now I have two cheapy 55w lights on each frame rail, 4 total, and am kicking around adding one to the front and rear for a total of 6.....but I'm undecided on that. I'm thinking 4 is good.....

So using my 30 amp relay and 20 amp fuse, I can simply "Y" my lead coming from the relay with a leg running to each set of lights along the rail?

What if I decide to go with 6 lights? 40 amp? And how do I cleanly divide that one lead into 6?

Thanks,

Allen

[mrblaine]

Originally posted by Allen
Thanks Frank.....



What if I decide to go with 6 lights? 40 amp? And how do I cleanly divide that one lead into 6?

Thanks,

Allen

with 3 hella relays. Clean as it gets. Either that or run a small bus bar and connect the feed from the relay to it and the light leads to the bus bar.

You can also run a small stud style terminal and stack up the leads to the lights.

I prefer the relays. That way if a relay does die, you don't lose all your lights.

[Allen]

Originally posted by mrblaine
with 3 hella relays. Clean as it gets. Either that or run a small bus bar and connect the feed from the relay to it and the light leads to the bus bar.

You can also run a small stud style terminal and stack up the leads to the lights.

I prefer the relays. That way if a relay does die, you don't lose all your lights.

Duh.....

Thanks.

Allen

[Tim]

barely on the subject allen but i used to run more (4) rocklights on the frame and one up front and believe it or not two lights (one on each side behind the front tires) in the fender recess worked just as good for me. i have been using the rubber housed tractor lights under there for years.

i couldnt even see the light that was mounted up front. it may come in handy for a spotter though.

also, the lights are great for camping if you are setting up after dark. way better than headlights that are in your eyes half the time.