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Fuse recommendations for circuit modifications

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Yes I’m back with more electrical questions!  (RUN AWAY!)

As I’m neck deep wiring in the hazard switch at the moment (literally), I started thinking about adding fuses. Triumph always seemed a bit “shy” about adding fuses to their stock circuits, so with my additions over the years I thought it would be a good time to review circuit protection.

Here’s list of the electrical changes/additions I’ve made, so please give me your fusing suggestions:

- “Facet” brand electric fuel pump - connected to the white wire off the ignition switch 12v IGN terminal. Protected by a 5 amp inline fuse, per installation instructions 

- Relays/activation - connected to the ACC position of the ignition switch. (I made this change so I can’t accidentally leave the lights on when used a DRLs)

- Relays/power (one each) for left/right headlights/taillights - When the relays are activated, they connect to the ignition switch 12v terminal. Each relay has a 30 amp fuse.

- Hazard switch - connected to the ignition switch 12v terminal, with an inline 30 amp fuse. Regular directional indicators use the same stock 12v supply as the Voltage Stabilizer (green wire).

- Horn - The PO had installed an alternator and removed the stock voltage regulator, so the horn is connected to the ignition switch 12v terminal. There is a stock inline 35 amp fuse.

- TR6 wiper motor- this utilizes the same switched 12v feed (with stock 25 amp fuse) as the stock motor.

I think that’s all of them. Any fuse changes/addition you’d recommend?

The only thing that gives me pause is, the horn, the two headlight/taillight relays, and the hazard switch, all get power from the 12V terminal of the ignition switch. However I can’t think of a likely scenario where all three would be pulling power at the same time.



Edited by Tr4aJim
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Firstly Jim, fuses are intended to protect the wiring, so should always be rated to blow at a current lower than the rating of the wire.  

I know you are looking for numbers but it really isn't as simple as that, as to give them means having to know the current drawn, the type of fuse  to be used and the rating of the wiring.

All modern blade fuses and US-style glass ones are marked for the highest current they can carry continuously without blowing.  I.e. a fuse marked at 10A will carry 10A all day and will only blow if that is exceeded.  The time taken to blow depends on the overload and a rule of thumb is that twice the rated current is needed to make them blow quickly, so that 10A fuse will need 20A to blow in a second or two. 

The original Lucas glass fuses used in TRs look similar to modern glass ones but have two ratings on them e.g. " 12.5A continuous 30A blow ".    The rating given by Triumph is the 'blow' number.  That means that where the book says 30A  - meaning the Lucas blow rating - but an unwitting owner has used a modern fuse marked 30A, the circuit is now badly protected as the modern fuse will not blow until that current has been exceeded by some margin possibly twice that intended by Triumph.

So the answer to your question is - The fuse you use for each circuit should be capable of carrying the normal load current of the circuit being protected plus some margin to allow for surges, but must be rated lower than the wire rating. 

e.g a circuit which normally draws 3A and fed by wire rated at 12.5A should have a fuse which can carry 3A continuously but blow quickly before the current much exceeds 12.5A.          A modern 5A fuse will do that as it will blow quickly at around 10A so protecting the wire from overheating under fault conditions. It also gives a bit of margin over the normal load current to allow for minor surges which can happen when switching loads on and off. 

OK - enough theory now practicalities:

 I realise you may not know what currents are actually drawn nor the rating of original wiring - just consider that a '4A originally came with a dynamo capable of supplying 22A flat out, which was expected to drive everything electrical on the car.  It follows that since Triumph were penny-pinching, no wiring on the car with the exception of the starter cable and the battery charging circuit, is going to be rated above 25A so no modern fuse used on the standard wiring should really need to be bigger than 12A.

Also consider that most faults on the car are not going to be minor overloads - they are usually short circuits to earth so the currents can be big. 

Typical currents drawn:-

60W headlights draw around 5A each so 10A total.   (that circuit is not fused as standard)

Sidelights (tail and front markers) are 5W each so with number plate lamp etc say 25W total = 2A ish

Indicators 4 x 21W =  7A for 4-way flash

TR6 wiper motor  takes 2A on high speed

Horns - 3A each, so 6A total but can be more depending on adjustment. 

Cabin Heater fan motor  circa 3A

I think you can see that the fuses in most of what you describe above are way too big.  If those 30A fuses are modern ones they will need a 60A fault current to blow quickly. 




Edited by RobH
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Unless I have misunderstood Jim's description, he has an awful lot of high current items being fed through the ignition switch, and that is undesirable.

Better to use the ignition swich to activate relays to do the heavy current jobs.

Ian Cornish

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Hi Jim,

 to echo Ian's comments

All these extra circuits MUST go through their own relay. The relay will take 100mA or so whereas the item may take 5, 10, 20 amps (including the horn).



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Thanks guys!
Rob, that explanation of fuse “theory” is very helpful!

I can see now that I have to resize (lower) the fuse ratings I have installed. However I’m not sure I understand Ian and Roger’s comments regarding adding relays.

I have attached a diagram I made a while ago after I installed the headlight relays. I haven’t had a chance to update it yet, so I just drew in the hazard switch. Besides the relays I have for the headlights, where else should I install relays?

BTW, originally the PO had wired the horn directly to the battery when he installed the alternator, and removed the stock regulator. I moved it to the ignition switch. Could the horn be moved back to the battery as before?



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Yes the horn ought to go to the battery - really via its own fuse.  It certainly shouldn't come THROUGH the ignition switch though connecting it at the feed TO the switch is OK. 

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Rob, ok I will move the horn off the ignition switch. Would plugging it into the connector that joins the alternator and ammeter be ok, or is a direct connection to the battery better? Oh, and the stock horn inline fuse (35) is in place.


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