MG Sports Cars

Ok, this seems obvious now, but it didn't hit me till today.... There's no reason you can't feed multiple relays with a single lead from the battery, right? And just jumper from one relay's +12V (#30) to the next & so on.

If you do that, presumably you size the main power lead (and circuit breaker) to handle the combined load of the devices being powered by the relays?

Does the wire used to jumper from one relay to the next have to be same gauge as the main lead?

I could have saved a bunch of wire if this had dawned on me earlier.

Rob,

Yes, you can jumper the relays, the limits being the size of the wire - it's hard to get two large wires into one terminal.

The first wire carries ALL the current for ALL the relays, the second carries the current for all but the first, etc. The wires have to be sized to carry the load they'll see. The last wire in the chain can be smaller than the first (see below).

The circuit breaker or fuse in the main lead must be sized to trip before the current rating of the SMALLEST wire in the circuit is exceeded.

Which basically means all the wires will have to be of the same size.

Or you use more than one breaker in that circuit (which will cost more than the wire). ;)

Circuit protection devices can be installed in series, and typically you would do this with downstream devices having smaller current ratings. In this way, you can protect circuits with varying current requirements and/or various cable sizes.

Circuit breaker? Be careful there. Fuses behave more predictably than circuit breakers. In any case, the higher the current rating, the more likely a thermal device will fail to blow when you want it to.

Will the relays you're planning be in a remote location (like up by the radiator fan and headlights) or will they be under the dashboard?

Daisychaining is entirely kosher, and often it's the most elegant way to go. However, if future vehicle modifications are likely, you might want to consider running your main "battery" lead to a power stud or buss bar. Here's an example power stud:


In fact, power studs are available that double as firewall passthroughs. Just one example:


The practical advantage of a power stud is that you can add a new circuit so easily by putting another eye-terminal under the nut.

Here's a nifty product I've used - a buss bar that hides inside a sealed MetriPack connector housing:


Here's the buss bar alone.

Note that if you snip the 6-way buss bar in half, you convert the device into a pair of 3-way buss bars. Delphi also makes these in both smaller and larger sizes.

Rob,

Dan, Carl and Curtis are all right on the money.

But there is no sin in running the extra wire.
And fusing correctly.
Two advantages come to mind.
One is safety. Suppose you daisy chain all your lighting relays to one breaker. That breaker pops and you lose all your lighting. Inconvenient at best.
Two is EMF or electrical noise. Not so much of a problem if you run a carb and points. But add a sound system or fuel injection and suddenly all of those relays on one line can become an issue. New cars don't run all those fuses because they want to, its because they have to.
Before I had my shop I was a FTS for Volvo and probably 50% of my work involved electrical interference issues.
So even the OEMs can't get it right all the time.
As far as breakers go at first blush they seem ideal, but they are load (heat) sensitive only. This is just fine in a static situation but we don't have that.
You start your car and drag the voltage down to say 10.5 volts then turn on the lights and heater and your at 11ish volts. Throw it in gear and drive away now we're up around 14.5 volts. You rate your breaker @ 12 volts. At 10.5 volts the amp draw goes way up and it may blow. At 14.5 volts the draw goes down and you might not have the protection that you need.
Fuses for our purposes react much more quickly and reliably.
As always "I don't know everything that works, but I know an awful lot about what doesn't"

Cheers
Fred

Great info as usual.

So it sounds like the consensus that it's better to use an inline fuse between the battery source and relay instead of a breaker. That Would make for a cleaner install too.

Right now I have my door poppers and headlights fed by the same wire for expedience but plan is to convert the headlights to the relayed ground method Fred shared this summer. Just going to get the car running 1st.

Had my 1st good day working on the car today... Figured out how to reroute the wiring and AC hoses to get everything to fit behind the dash. Had to move my fuse box - that was a chore but exciting to see the space problem solved. Once that stuff was solved I realized the wires for the gauges in the middle of the dash were hitting the square tube behind the dash - modified that too. Now everything fits.