Placement of relays?

Joined: 19 Mar 2008 Location: lost

When doing a light bar for the roof do you need to get the relays as close to the lights as possible or as close to the battery as possible? Or doesn't it really matter?
Thanks in advance.

Joined: 11 Jan 2004 Location: Adelaide

Off the top of my head, I don't think it matters a great deal.

Personally, presuming I'm using +ve switching, I'd position the relay as close to the battery as possible - that way, when the relay is off, there's a shorter distance of wire connected to +ve, so fewer opportunities for a short to earth. Of course, once the relay is on, that no longer makes any difference...

Joined: 25 Oct 2003 Location: Melbourne Australia

I'm with Scott, just three things I can think of:

1. The total wire length from battery to lights via the relay matters, so don't put the relays somewhere that makes a big detour in the wiring
2. Fuse as close as possible to the battery
3. One advantage of putting the relays close to the lights is that you can run a single heavy wire to ALL of the relays, then shorter and lighter wires to the lights from each relay. That's assuming you have more than one relay.

Joined: 19 Mar 2008 Location: lost

Thanks for the replies.
I am thinking of running 2x100 watt lights on each circuit. This would draw 16.66 amps on each circuit. I am thinking 15 amp fuse is too low but next size up is 20 amp is this too high?
I am also running plugs between relay and lights, to disconnect lights when not offroad (to avoid being booked) but don't know how to factor that into the equation or if I need to.
Thanks again.

Joined: 11 Jan 2004 Location: Adelaide

That 16.6A is nominal, and real world may be a little lower - because the 100W rating is typically calculated with voltage closer to 14V than 12V. Either way, 15A is too low, but 20A should be adequate. The fuse is there to protect the wire in the event of a short - the fuse is intended to melt before the wire does. If you're using wire rated for 50A continuous then a 50A fuse is adequate - but a 25A fuse is probably a better choice.

Plugs in-line are not a bad idea, but make sure they're up to the full current load. If Mr Plod becomes pedantic they may help your case, but I wouldn't be sure that you wouldn't receive a ticket anyway.

I think removing the fuses would achieve the same result - show Mr Plod that the fuses are pulled, and he'll either accept it or he won't.

Joined: 11 Dec 2004 Location: Queensland

Hi stockhorse, as far as roof lights go, check with your states regs. Most states now allow roof mounted lights and Queensland goes as far as having regs as to the laying out the position and operation of roof mounted lights.

As to the relay position, as posted above but make sure you use suitably sized cable. 6mm auto for both the positive and negative would be the minimum size for each 100w driving light and even larger cable for 130w lights.

The fuse size for 6mm auto cable should be either 20 or 25 amps, no smaller, as posted above but no bigger as 6mm auto cable has a continuos rating of about 35 to 40 amps and a 25 amp fuse has a 1 hour continuos rating of about 37 amps.

Joined: 19 Mar 2008 Location: lost

Thanks for the great help.