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MG MGA - Wiring dual relays

I am in the process of tidying up the wiring on all my cars extra electrical gadgets.
These include- twin-electric fans, two-speed wipers, dip-flash relay, electronic rev-counter, electronic distributor, gauge- wizard fuel gauge controller, wiper delay unit, with more to come.
Most of these are wired directly from the battery at the starter switch and so they are not ignition controlled, which is not ideal.

So I have decided to fit an ignition controlled busbar so that all of these are controlled through a heavy duty 70 amp relay which is to be triggered by the ignition switch.

I have a few of these relays and I am wondering if it is ok to fit 2 of them side by side, wired in parallel so as to double the load up to 140 amps?
Can they both be safely triggered by the same ignition switch feed?
I know I could buy a single heavier duty relay but they are quite expensive.

My brother in law (who was an electrician), tells me that connecting 2 relays like this may cause some problems, but I can't see how this would be the case.
I have posted a pic of the wiring of a single relay.
My plan would be to connect the trigger feeds together (85) to the ignition sw.
Each relay would have an individual thick feed wire from the battery to (30) and each would have its own thick output wire from (87) which would then both connect together at the bus bar.
Would this work out ok?
Cheers
Colyn



Colyn Firth

I think it is OK - it is exactly how the rad fan on my Rover 75 is wired.
Chris at Octarine Services

Colyn
That would be ok provided you did not mean “connected in parallel” to mean connecting the two relay contacts in parallel to get 140 Amp switching capacity. Such low resistance parallel paths will not share the potentially 140 Amp load equally - one could well take much more than 70 Amp and the other much less. The problem being that each contact will have a very low resistance, which is good, but possibly significantly different, hence not good for equal current sharing.
But if you meant connecting both relay operating coils in parallel to the new ignition bus, then distributing the various loads reasonably equally between the two relay contacts (but not paralleled) you will be fine.
Hope that all makes sense.
Bruce.
Bruce Mayo

Thanks Chris and Bruce,
so Bruce, just to clarify, are you suggesting that I should operate the two relays separately?

Or in other words, would it be better for each relay to feed its own separate bus-bar rather both for both to feed the same one?

Cheers
Colyn
Colyn Firth

Colyn
Connect both 85’s together. Connect both 86’s together. Then both relays will operate together from the ignition switch.
Connect both 30’s together to get the 12v supply from the battery to each relay contact, BUT, connect each 87 to a separate bus bar and then connect the various load circuits to each bus bar in some rough split of potential current loading.
A diagram would have helped, sorry! And, you may wish to think about some fusing somewhere in all that.
Bruce
Bruce Mayo

Thanks Bruce, much appreciated.

I will get hold of another bus-bar to split the output, just to be on the safe side.

Each of my extra electrical modifications have their own in-line fuses (I have even put a fuse into the lighting circuit), but on my to-do list is a plan to fit a proper fuse box to cover each of the circuits.

Cheers
Colyn
Colyn Firth

Yep -seperate bar for each relay output--
If both bars are joined then there would be the issue of if one relay died the other would take the full load and fizz---
seperately at least half your outputs continue if one fails
Inputs to the relays can be joined
William Revit

This thread was discussed between 08/01/2020 and 09/01/2020

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