TR2 & TR3 Forum

Been told awhile back that i should put in an inline fuse. Guess now is the time.
Suggestion as to where and what size? If i recall was told most important place would be a fuse in line with the lights. Yes?

All suggestions welcome,,

Always something

If you are worried about headlights, I would recommend an auto-resetting circuit breaker. That way you won't be stuck out after dark with no spare fuse and no lights. They sell breakers at any parts store.

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John
Arlington, TX

'55 TR2

You drive them in the dark??

There's no need for a fuse with filament bulbs. If the filament sees too much current it will simply burn brighter until it eventually burns out which then opens the circuit of that bulb.

The 12V DC bulbs are just a long wire in a sealed glass with a gas (halogen) or a noble gas such as argon or xenon or other gas.

The purpose for a fuse is to protect a device in a circuit if a short occurs. The bulb is essentially a fuse.

Roy

Attached is a wiring diagram for the TR3 showing an inline fuse between the light switch and the panel light switch. It is true that incandescent bulbs will blow if too much current hits them so they act as a fuse. However, there is no bulb to act as a fuse between the light switch and and the panel light switch. A fuse in that line is a good idea. Give yourself some extra length of wire so you can access the fuse easily without pulling the panel. Also attached is a picture of that in line fuse, bypassed with a washer by the PO, and the end result. That burned wire also burned through one of the white ignition wires. The TR3 was dead until I found the burned wires. Simple change, add some piece of mind.
Charlie D

Thanks for the info gentlemen,,,

Always something

Thanks Charlie for the information.
I happen to work on this right now myself. Would you know what size fuse should be used between the light switch and the panel light switch?

If you wire the online fuse as shown on your TR3a diagram where it idms in line with the panel light bulbs and stop/tail and license bulbs here is how to determine the fuse rating. The necessary fuse rating depends on the wattage ratings of the bulbs you use.if the bulbs have the wattage printed on the bulb you can use this formula to calculate the amperage of each bulb.

Amps = watts divided by volts

There are 6 panel bulbs. Those small bulbs are up to about 11 or 12 watts. A full battery charge is about 12.6 to 12.8 volts.

Each panel bulb then draws about 11 watts divided by 12.8 volts = 0.86 Amos

6 bulbs x 0.86 Amos = about 5.2 amps.

A stop/tail light like a # 1156 is about 27 watts.

Each draws about 27 watts divided by 12.8 = about 2.1 amps.

There are 5 of those so they draw 5 x 2.1 Amos = about 25.5 amps.

So all the bulbs draw about 30.7 amps.

So you could start with trying a 30 amp AGC/SFE rated fuse. AGC/SFE fuse ratings are the value at which the fuse should be able to pass the rated value for 4 hours without burning out. Their minimum voltage rating is 32 volts. A 30 amp 32 volt rated fuse would blow with about 960 watts. All your bulbs are running about 6 x 11 plus 5 x 27 = about 201 watts.

Fuses can blow due to an instantaneous over current or a long term heating at a lower current (a thermal trip).

If the 30 amp fuse blows to frequently go up to the next rating. If it doesn't blow frequently you could try going down a rating until you reach a rating that blows too frequently. Then go back up one rating.

A Lucas fuse rating number will be twice the amperage running in the circuit. So if you use a Lucas fuse it should have a rating number of 50 or 60 amps to be equivalent to a 30 amp AGC/SFE fuse. The 30 amp AGC/SFE fuse will blow if the instantaneous current substantially exceeds the 30 amp rating number.

Note that because fuses burn because of the temperature they reach due to the current flowing, a fuse in a hot location will blow sooner than one in a cold location.

Roy

Thank you Roy, good background information to spec the fuse.

Would anyone know what size (amperage) Triumph specified for the inline fuse?

Sorry, I do not know what fuse size was recommended originally. Here's a link to a website that goes over how to determine the size fuse you should use. Remember it is the wire that burns so the size of the wire will determine the maximum fuse value. A 30 amp fuse will protect 10 gauge wire, 14 gauge wire will burn before the fuse blows.
https://www.oznium.com/blog/how-to-determine-the-fuse-wire-size-for-your-project/
Charlie D

Actually the wire sizing is based on its resistance which causes thermal heating which leads to either the insulation overheating or if the current is high enough the wire will overheat to the point of melting.

But in a 12 volt DC circuit these are not a problem for 14 gauge or 16 gauge wire. if there is a fuse the fuse will blow long before the wire burns the insulation.

14 gauge wire in a 12 volt circuit can easily carry 30 amps.

You will see that most wiring in your car is 16 or 18 gauge wiring or in some instances even 22 gauge.

Your battery cables are large because they need to carry 50 or even 75 to 100 amps draw when the starter is activated without having a significant voltage drop.

You will note that the Lucas fuses at the fuse box are 50 amp. The equivalent AGC/SFE rating would be 25 or 30 amp.

The numbers I gave in my earlier calculation are based on guesses for the bulb wattages (except a # 1156 bulb is about 27 Watts and in a 12.8 volt DC circuit it draws about 2.1 amps.

The fuse contains a fine gauge wire. It will blow open instantaneously long long before any of your wiring is endangered.

Roy



Edited 2 time(s). Last edit at 2021-04-09 09:57 PM by 60TR3-Red.

I am fairly sure that the fuses I have found in this location were 35 amp. If you look on these forums they have lots if info on conversion between what British rated and our SAE.
Charley

Just to close the loop on this, I found reference to the size of the inline fuse here:

https://www.britishcarforum.com/community/threads/smoky-story-and-tr3-console-knobs.9862/

This matches what Charley said above (35A UK, 20A US).

A 20 amp US (AGC/SFE) fuse might work but suggest you see if the wattage ratings are ink marked on your bulbs. There probably is no marking on the panel bulbs but the TRF e might be.

The wattages should be used to estimate the current draw for the total of panel bulbs, stop, tail and license bulbs. As I indicated before with the math, the 5 stop, tail, license bulbs will draw about 5 x 2.1 amps = 10.6 Amos. The 6 panel bulbs probably are less than 1 amp each. So total amper6 of about 14 - 16 Amps and a 20 amp fuse (US) should work fine.

You might even get by with a 15 amp but since all that's needed is for something to open the circuit a 20 amp should be fine.

Roy

Sorry to resurrect an old thread, but I could use some clarification. I see how adding a fuse between the lighting switch and panel rheostat would work as all the red wires run from the rheostat on later cars. On my 57 and earlier cars, all of the red wires run directly from the light switch with the exception of the panel lights. Where should I add a fuse?

Thanks!

Steve

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68 TR250
57 TR3
Both regularly driven
49 Triumph 2000 Roadster project