Classic cars forum & vehicle restoration.

[Ray White]

Given that the MG TC has a particularly poor reputation for dashboard fires I am attempting to re wire my car to a safe standard and following advice given here and elsewhere I intend to fit fuses and relays where ever needed.

There seems to be a plethora of relays available; some with and others without fuses attached however quite which I should choose is confusing me.

For example, does the fuse rating relate to the relay; to the load of the accessory or to the gauge of wire?

I am confused because there are relays with attached fuses that go from 20 to 40 amps yet it seems there is a requirement for much lower ratings need to protect switches and other things. Does this mean I need two different fuses?

There are relays without attached fuses and again I am unsure which fuses to use.

Any guidelines or advice would be a help.

[ukdave2002]

Ray

Remember that the fuse is there to protect wiring, the relay is there to avoid having heavy loads the dashboard switch's.

So select a relay size that will switch at least twice the load, and ensure that all wiring is protect by an appropriately sized fuse.

I'd go for separate relays rather than ones with built in fuses , as long as you don't add live feeds directly from the battery , you shouldn't have to add any more fuses.

Dave

[Ray White]

Thanks Dave. Can you tell me what would be an easy rule for me use to judge the appropriate size of fuse.? Would it be more or less than the amperage quoted on the accessories?

...and how do you assess what size fuse goes with what gauge wire?

I will stick with "twice the load" for relays. That's a handy rule of thumb for me to use. Thanks.

[ukdave2002]

Hi Ray

Are you planning to have separate fuses for each accessory that the relay switches? The fuse has to be of a higher value than the accessory otherwise it would blow, traditional glass fuses are normally rated at their fusing value, ceramic fuses are rated at the maximum continues current that they can carry (typically half the value they fuse at)

So rule of thumb, if the load is 3A use either a 6A glass fuse or a 4A ceramic (as 3A is right on the limit).

Dave

[Ray White]

[Ray White]

I would appreciate any advice about which relays I should get for the TC.

https://www.ebay.co.uk/itm/High-Quality-12V-30-Amp-5-Pin-Relay-With-Diode-Used-in-Many-Morgan-Cars-Roadster/131561296851?hash=item1ea1aa13d3:g:k9kAAOSwXedZ1kS0

I have found these 5 pin quality relay with a diode that is supposed to protect vulnerable electronics from high current spikes. I thought perhaps this would be good for my ignition system that has an expensive electronic distributor.?

What about the 20/30 amp value? Is this high enough for all the relays.? I will have fuses to protect the main circuits and I plan on using a relay with each fuse.

I wonder if fitting a relay and fuse to each halogen headlamp is sensible ... or over kill. I just thought if one lamp went out there would still be one working O.K.

[ukdave2002]

Hi Ray

the maximum load that you will ever need to switch is around 10A (both headlamp) so those relay will be fine. The diodes across the coil are a good idea
given you have some solid state electronics, just remember the coil will now be polarity sensitive.

I wouldn't go to the length of putting individual relays in for headlamps, its just an added complication.

Dave

[Ray White]