MGB & GT Forum

I added a maxi-fuse holder between the starter terminal and the brown wires. I also added a mini-fuse holder on the white wire coming off of the ignition switch.

I used a 60A maxi-fuse and a 25A mini-fuse. I haven't had any nuisance trips but I am a bit concerned that they are too big to adequately protect the wires.

Thoughts?

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T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1978 MGB V8

Since the brown wires didn't have a fuse, I don't see why a 60 amp interrupter should be a problem. If you trip that fuse, you might want to find out why. 60 amps is enough to weld metal.


GMc

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Wait for me at the gate Tuffy, because without you they're never gonna let me in.

SDCH WTCH-X Twin Oaks Tuff Nutt
2008 - 2022

Hi Keith

As you rightly suggest, the fuses are to protect the wiring. If you compare the diameter of the fuse wire to the diameter of the wire you are protecting, you will notice that the fuse is much thinner, which is good. This means that the wire can withstand a higher current than the fuse, for the short time that there is a short. To use a fuse of the current capacity of the load, say an incandescent light bulb, can lead to blowing the fuse, since there bulb has, what is termed, an inrush current, which means that when first turned on the filament is cold and has a low resistance, hence a high current. The filament then quickly heats up and its resistance increases dramatically, giving the rated current. Similar with electric motors. So I suggest that a fuse needs to be of a higher rating than the ultimate load.
I don't think that your fuses are over rated, and should be just fine.

Herb

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Many automakers simply use a fusible link there - essentially a piece of wire thinner than the main power line in the harness.

The brown circuit powers everything else - so the normal current draw in brown may be the sum of all other fused and non-fused (e.g., white) circuit draws. Therefore, you do need a fairly hefty fuse. Your hope is that the fuses will protect the other circuits (e.g., green). My concern is always the non-fused white circuit. Nothing protects white in the stock setup (but your recently added fuse on the brown circuit provides some protection).

I suspect that you could drop down to a 50 amp fuse. My rationale is that the 16ACR alternator (34 amp output) is adequate above 2000 or 2500 RPM to supply all the normal MGB electricals when powered on. It is slightly deficient at idle (when its output drops to around 20 amps) - but not by too much. You might try temporarily replacing the fuse with a high current ammeter. Then, turn on every accessory in the car - headlamps, heater fan, wiper on fast, radio. See what current flows then add 25 to 40% and use that value of a fuse. I'd like to hear back what you measure. I'm betting that the worst case total draw is less than 40 amps. Of course, there are surges when things like heater motors spin up - so you do need at least 25% (and maybe 40%) higher than the steady worst case draw.

I like self resetting circuit breakers but admit that they add slightly more voltage drop than fuses.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

HI Terry,
In those areas where the brown wires are doubled up (like at the starter) is there a concern with one of the pair overheating?
-Clay

Or worse yet, if the small brown wire (16ga.?) shorts out, will the 60A fuse protect it?

I'm going to have to find someone with a multi-meter that can read 50ADC; mine can't.

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T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1978 MGB V8

Sometimes you'll see a thin brown wire paired with a thick brown wire going to the solenoid. The thin wire was voltage sense for battery-sensed alternators (most were "machine" sensed and did not use the thin brown wire - but it remained in the harness anyway even when the alternator didn't use it). Provided the thin brown wire still goes to the voltage sense pin on the alternator (identified by having a small-large-small connector) almost no current would flow through the thin wire. So, place the fuse between the solenoid and the thick wire.

In later years (beginning around 1972 according to the diagram) the harness had two thick wires leading to the solenoid. One was from the alternator and the other led to the electrical system. Here you need to make a choice.

If the fusible link (or fuse)is placed between the solenoid and the terminals of both thick brown wires (that used to connect to the solenoid) then the fuse is handling the net current that flows to/from the battery. However, this will not take into account the fact that the alternator is providing current to the brown circuit that will not be sensed by the fuse. It will still provide some protection and you may be able to use a smaller fuse.

A better scheme might be to leave the thick wire from the alternator directly connected to the solenoid and place the fuse in the wire that leads to the brown circuit throughout the car. This way the fuse detects total current in the brown circuit, whether it is sourced from the battery, the alternator or a combination of the two. It will not protect the alternator but generally if you put a fuse in the alternator circuit and it blows under high current you've probably ruined the alternator anyway. Also, hopefully the wire from the alternator to the solenoid is heavy enough to take full alternator output (it was designed for that). I suppose if the solenoid power terminal shorted to ground that would produce excessive alternator current, but in that case the diodes in the rectifier pack would likely burn out (thus preventing a fire but sacrificing the rectifier pack).

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Maybe. A dead short will often pull a very heavy current that should burn out the fuse before the wire. I.e., a 16 gauge wire can probably handle 60 amps for a second or two. So, if the brown wire shorts to ground then probably the fuse will blow. But this is why I recommended measuring worst case load and then factoring in a 25% or 40% safety factor on the fuse size. The problem comes if the load is (say) 50 amps - in which case the 60 amp fuse won't blow but eventually the wire will.

The MG's wiring is fairly primitive in this regard. There are too many circuits (like white and blue) that aren't fused at all.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

There is no 16 AWG wire used in a MGB wire harness. With electrical wire, size matters, perhaps we best get it right?

The MGB Brown colour wires are sized 28 & 44 strand count, or the lightest wire strand cound Brown colour wire in a MGB is 28 strand.

Fuse appropriately

B

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Life's most persistent and urgent question is, "What are you doing for others?"

While it is true that British and American wire gauges are specified differently, I think the OP was pointing out that the wires he is dealing with are about the same as 16 gauge. Note that even British Wiring shows both the "strand" rating and the American gauge equivalent (https://www.britishwiring.com/PVC-Wires-s/67.htm)

But, I would actually have thought the brown wires coming from the starter were closer to 14 gauge (or maybe even 12 gauge). The exact gauge probably varied over the years. There were changes between (IIRC) 1971 and 1972.

"Fuse appropriately" is difficult in this situation because the fuse on the brown wire has to protect, not only the brown wire, but the unfused circuits that derive from it (which may not be capable of handling as much current as the thicker brown wires). For instance, a short on the blue (headlamp power) circuit will draw current through the brown wire. But the blue wires are, IIRC, thinner than the main brown wire. So, a fuse that protects brown may still allow damage on blue. However you can't simply put a lighter fuse on the brown circuit (appropriate for the blue circuit) because brown splits off into every other circuit in the car and the appropriate blue fuse (in the brown circuit) might blow when other drains are on.

Basically, a single fuse in the brown circuit cannot fully compensate for the lack of fuses elsewhere in the system. That said, a single fuse in the brown circuit is better than nothing at all.

The Brits deliberately chose not to fuse the headlamps. Was this a mistake? Well, their theory was likely that a short on blue (perhaps due to a chafed wire) was likely intermittent and it would be better to have the headlamps flicker than to go off completely (when a fuse blew). Personally, I disagree with that theory, but acknowledge the thinking behind it. So, I placed a self resetting circuit breaker on my blue circuit. That way, an intermittent short on blue will trip the breaker which will leave me without headlamps for a few seconds until the breaker resets. A compromise between a fire and driving in the dark.

I do not understand why there is no fuse on the white circuit. It certainly has the potential to cause a fire. I suppose the thinking was that a failure on white will cause the car to stall, perhaps at a crucial moment. Maybe the lack of a fuse was just a cost saving measure.

BTW - I don't believe the number of "strands" is the proper measure for wire sizes in our cars (although it is commonly used, even on the British Wiring website). For instance, you can have a single thick strand or multiple thinner strands (which are more flexible) that will have similar current carrying capabilities. There is an article about British wire sizes at https://en.wikipedia.org/wiki/Standard_wire_gauge

There is also a converter at https://www.convert-me.com/en/convert/wire_gauge/gaugeAWG/gaugeAWG-to-gaugeSWG.html It shows that an American 16 gauge wire was approximately equivalent to a British 18 gauge wire.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Personally, I have never unwound a stranded wire just to mic the diameter of a single strand, so strand count was never a variable for me in deciding what to use...nor has the wire originally used by MG ever been a determinant in what I use in constructing circuits. MG was building cars and saving money. I deliberately overbuild because the cost difference is negligible at best. If my 5 amp circuit wiring can handle 20 amps, so what? It's the fuse that is supposed to be the weak link.


GMc

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Wait for me at the gate Tuffy, because without you they're never gonna let me in.

SDCH WTCH-X Twin Oaks Tuff Nutt
2008 - 2022

Yes you are correct the fuse is there to protect wires.

The standard of strand count using 0.30mm strand is a British wiring thing, not a MG thing. AWG is an US of A thing.
While strand cound is not 'important' to you it is or should be to many that wish to use OE type bullets. I know this is not your case, you mention this often.

B

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Life's most persistent and urgent question is, "What are you doing for others?"

I think the point we're trying to make is that very few of us have access to true British SWG wire. In fact, it appears that the British standard used on our cars is now obsolete and has been replaced by a newer standard (with different convention for specifying gauge). So, whatever wire we use will likely be an approximation of what was there originally. I agree with Greg that it is difficult to figure out what size SWG wire was used originally on a particular circuit - even if one could it isn't likely to be available at the local Princess Auto or Harbor Freight. So, we are forced to use what is available and will choose based on the correct gauge for the application.

In my instance, I moved all of the brown connections off the solenoid and up to an enclosed terminal block. This allowed me to conveniently add the self-resetting circuit breaker to the blue circuit (which derives power from brown). I needed a new wire from the solenoid to the circuit block. Much like Greg, I looked at the old wires and choose a slightly thicker American gauge wire (IIRC it was either 8 or 10 gauge). Whatever I used, it was heavier than the original.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

forget it

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Life's most persistent and urgent question is, "What are you doing for others?"



Edited 1 time(s). Last edit at 2024-04-27 07:16 AM by riley1489.