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Thread: How to wire a switched +12V distribution block in the engine bay?

  1. #1

    Default How to wire a switched +12V distribution block in the engine bay?

    So my question is just like the title says. What's the proper way to wire up a +12V switched distribution block in the engine bay? It would power my Sniper, Dual Sync distributor, Holley coil, MSD 6AL, and anything else I need. I have some weatherproof 30/40 amp 5 wire relays sitting on my shelf. What gauge wire should I use for everything?

  2. #2

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    The gauge of wire depends on the current & length of the wire you are using. I would find a good spot to mount it and then count the length of wire you need. After that figure out how many amps each accessory draws, and then you can find out what wire size you need. Is the distribution block you are planning to use, the kind with one main power input and branches coming off of it with spots for fuses? If that's the case, I would wire up the relay to feed the power to the main input using one of the old ignition wires to trigger the relay. Then you can have all of the accessories protected by fuses. Also, adding a circuit breaker or fuse to the stretch of wire between the battery and the relay for added protection would be a good idea. If you plan to add accessories in the future to it going with a larger wire size never hurts. If most of the circuits are low current, I would think either 10 or 8 AWG wire would most likely be acceptable.
    Last edited by SatherCS; 01-31-2018 at 08:44 PM.

  3. #3

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    Yes, I was planning on using the fuse block with one main input and six other fused outputs. So I come off the battery with a heavy 8 AWG wire with a fuse (what size fuse?) in it, going to my relay that is switched from a keyed +12V wire. From the relay to the fuse block using the same 8 AWG wire. Then out to my accessories. What size wire goes with each fuse size out to my accessories? Let's say I have a 20 amp fuse, what size wire would that require? Thanks.

  4. #4

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    Here's a wire sizing chart: http://www.wiringproducts.com/automotive-wire (scroll halfway down the page). This one shows how long a one way length of wire can be based on current & wire size. If you were running a 20 amp load on 8 gauge wire, the max length you could run to reach the object would be 18.9'.

    As far as fuse sizing, you would want to run a fuse that is at least 1.25 times higher than the current in that circuit. With 20 amps you would run a 25 amp fuse. This allows you to run the full 20 amps and give you good protection against current spikes & short circuits. I'm not sure what the switched side of each accessory draws in terms of current. The MSD draws only .7 amps per 1000 RPM on the main power side, which would only be about 5 amps at 7000 RPM. I would think that the small switched +12V wire draws less current than the heavy red wire. Maybe a 2 or 5 amp fuse? I think that the Holley Sniper pink switched wire goes to its own relay in the main harness, which means that a 2 amp fuse should be fine for that wire. The other accessories I'm not sure of.

  5. #5

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    Thanks for the info. I'll see how it goes.

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