light bar wiring

[colin14]

hi all, can anyone give me ideas on best way to wire 4 roof lights up to the main beam but obviously with a swith on them to, i know the basics but when it comes to near the fuse box/chasing the right wires i'm stuck.
 thanks

oops its a m reg 300tdi

[lambert]

Where about are you in relation to yorkshire? Only its easier to do than to explain really.

[colin14]

down in milton keynes bud. thanks

[Saffy]

Usual convention is a low current switched feed off the full beam circuit that powers a 30amp automotive relay for each pair of 55w spot lamps. The power feed to the spots is via large as possible cabling directly from battery via the relay.
How people install on a Disco I have no experience of. My Defender..... I located the high beam fuse and examined the loom leading to the fuse panel, check wire colour with manual, made sure it was correct wire with a 'pin' probe inserted into the insulation of the wire and measured for volts with the switching of highbeam.  Stripped backed 10mm of insulation and spliced in my low current feed wire - re-insulated the joint. The feed goes through a conviently located single pole single through switch (low current so can be small) and then onto the relay(s) coil.

[tack43]

Depending where you are mounting the relays you could pick up the feed by one of the lights, the main fuse box or the main beam/dip switch. I would use WP relays positioned near to the battery you are using and splice into the wire feeding the nearest light. Think it may be blue with red trace but as Saffy said check first. Wire it so one relay powers the outer lights and the other the inner lights with separate switches. This way you can have a pair of flood and a pair of spots if you wish.

[colin14]

thanks guys, plan was to to run the 4 off the main beam and fit another couple on the nudge later on with seperate switch. whats the problem with powering all 4 on 1 relay, will be using 27amp wire and 30amp in line fuse. thanks

[Saffy]

If you or someone happen to fit 4x 100watt bulbs in the future than that will be very close to 30amp current draw on one hot relay.
Long as you stick with 55w bulbs then 1 relay on the heavier gauge wire that you state will be fine.
About 16amp for 4 x 55w, two relays would allow for lower gauge wiring runs which *may* be easier to route. 
Two relays with the heavier gauge wire might be less volt drop on the longish run and therefore brighter lamps. (maybe do that math to see if relevant in real world)

[tack43]

It's always best to "over-engineer" for any later upgrades than have to do it all again!