Wiring Clarification

[disco_al]

When i wire up the lights on the light bar, i plan to use 2 switches and 2 relays (split the load, and wire two pair).

I plan to mount the switches and relays in the overhead *cough* storage in the Disco, in the middle, by the interior light.

Now, i need to run a fused feed from the aux fusebox in glove comp, to power the relays/switches/lights, rather than have to run approx six wires up the pillar (2 x to feed the relays, 2 x for the switches and 2 x earth wires), can i just use one higher rated fuse at the fuse box, and split the power feed to the relays/switches, thus doing away with 2 x wires straight away, if i then earth through one of the screws holding the binnacle up to the roof, i will only need 2 x wires to the fuse box.

In other words - share the power feed for the relays that would normally run to terminals 30 & 85 from one wire, with say a 20 amp fuse, instead of using 2 wires?

I have 28/0.30 cable (rated to 17.5 amps approx) and switches rated to 20 amps - my calcs give me a load of about 9.5 amps for 2 x 55w lamps.

Does this make sense?

[TheRecklessEngineer]

Can I ask why you want to split the load? If you are using cable rated at 17A and total draw is only 10A then you will be fine to run the whole load through one cable...

Again, if your switches are rated at 20A then you don't need the relays either...

[disco_al]

i would rather use relays to lessen the voltage drop, and ease the loading on the cables.

the main question is if i can just use one cable to provide the power to both relay terminals, rather than two cables

[Mark]

Al, How many spots? If I read right you are talking about 2 pairs at 55W per light. that means 220W at 12V will be drawing ~19A with all 4 lights on...

to my mind this means you will need to power the relays separately using the cable you have.

Have you thought about mounting the relays under the dash with the fusebox? Or, as I did, in the engine bay? This means you then only have to run the low current cables up the door pillar.

You should be able to get away with only running 2 low current cables up to the roof pod, if you can find a decent earth up there - at worst 3 if you bring the earth back down again.

That isn't explained too well, but I can do a quick diagram if you like?

Cheers

Mark

[Mark]

having re-read your post - you want to use the main power feed to the relay to also provide switching power to the low current side? That should be fine, as the current draw of the relay is pretty minimal.

I would however still be inclined to site the relays lower, and just run the control cables up the door pillar....

hth

Mark

[disco_al]

yep, it will be 4 lights, but i was going to wire them on 2 switches ie 2 lights per switch and 2 relays.

looking at it a bit more, could i use a twin output relay ie 87 & 87b rated at 2 15 amp outputs, i would only need one switch and relay

[Mark]

Bearing in mind lights tend to be left on for a little while, and things get warm, I would be inclined to use something that was rated at about twice the steady state load - that is at least a 40A relay. I think I used a 70A cos that is what I had....

No harm in running one relay with one switch. much simpler after all.

May be just my paranoid over engineering though...

Cheers

Mark

[disco_al]

can see the reason for using twice the current loading, however, running 55w bulbs in the lamps it is only going to be pulling 9 ish amps for each feed, so a twin 15amp output relay may be enough.

i will see if i can get a higher rated twin output relay.....

[disco_al]

right, have sorted a twin output 40 amp (2x20amp) relay for not a lot from the local auto electrical place

so will use that with the 55w bulbs and reduce the wiring complications.

cheers for the help Mark

[FridgeFreezer]

Put the switches on the ceiling, put the relays down low in a reasonably direct line from battery to lights. The switches could be run in doorbell wire or similarly thin/cheap stuff if they're only switching a relay coil on and off - for future proofing you could run either skinny 7-core trailer wire or even CAT5 network cable up to the switches - it sounds dodgy but it gives you 6 spare wires for more switches in future, and is more than capable of carrying the tiny current a relay needs.

Earthing to bodywork is OK if you've either got a small load or a very good connection (EG bolt to chassis) but I tend to run earths back to the battery - after all, LR bodywork is very rarely all nice pure clean steel so you're liable to lose the odd volt here and there

[disco_al]

CAT5 network cable

never thought of that....

[red90 driver]

never thought of that....

Don't!!!

CAT5 cable although having 6 separate cores, each core has only a single strand of solid wire. They do not like the vibration in a vehicle as the cores will snap in time, giving you intermittant operation. Plus solid core cable is a sod when soldered or crimped for snapping.

Thinking aloud mode....... You could use external grade CAT5 cable, but that starts getting pricey.

If you must use a multi core cable, use one in which the cables have more than one wire, e.g 7/0.2 which is 7 wires of 0.2mm in each core.

I have used 4 core RS-232 comms wire in the past, with 7/0.2 cores for relay operation, and also some 4 core artic spec mains cable, that has 30/0.2 cores.

Confused? You will be!

[FridgeFreezer]

Don't!!!

Strange, none of the kilometres of Cat5 I've got here or ever used has been solid-core, although normal telephony wire for IDC terminals is. It's a good point though, don't use single solid-core wire in cars.

[Outrage]

"can i just use one higher rated fuse at the fuse box, and split the power feed to the relays/switches, thus doing away with 2 x wires straight away"

I assume if you're using a single relay you've upped the size of your wires?

I assume that you will want one fuse feeding all four of your lamps? 4x 55w @12V = 18A, so you will no doubt end up using a 20A fuse. Bear in mind that your current carrying capacity of your cable is 17.5A.

If you take 2 individual feed from the output of a single fuse to power 2 sets of lights each circuit will pull around 9.5A which is fine as the cable will accept 17.5A - However in the event of a short circuit in order to blow the fuse the current has to exceed the 20A rating of the fuse, when it does this it's already exceeded the rating of the cable prior to the fuse.

Your cable has to be >= your protection.

Also agree with fridge on the earthing - if you've got a carp earth you undo any of the benefits associated with sorting out your voltage drop.

[monty]

there are two types of cat5 (or indeed most network wiring) - stranded and solid.....stranded cable is typically used for patch leads where the cables are likely to me moved around, solid is typically used in infrastucture type jobs, where cables are run in conduits and are not likely to suffer any stress during their lifetime....