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Bmc rewire


Deano1988
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Hi all 

 

I have a bmc 1800 engine in my narrowboat and the wiring is a mess i keep having problems like connector blocks corroding anand looking to rewire it. 

 

I have found the wiring diagram that came with the boat i was hoping that somebody could help with the wire sizing and should any cables be a certain size?

 

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Please read the electrical notes on my website tb-training.co.uk before going much further. It explains how to size cables. The cable size depends upon current carried, cable length and the degree of acceptable volt drop for a particular piece of equipment. Hopefully the notes will help you with more than just cable size.

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What @Tony Brooks said. However, any battery to master switches and starter, or battery to battery via split charge relay and the related negative wires to the engine block and the boat hull ground need to be 25mm^2 at a minimum for the boat safety scheme. The alternator to battery wire will need to be thick. At least 10mm^2. The cable to the ignition switch and from there to the glow plugs will need to be at least 4mm^2 as these take a lot of current. Ignition switch to starter solonoid, maybe 2.5mm^2. Much of the rest 1 to 2.5mm^2 is adequate as this is mostly small currents for signals to instruments. I wired up a friends BMC 1.5 a few years ago and it hasn't failed yet!

Your diagram shows no fuses. You'll need one for the instrumentation at least.

Jen

Edited by Jen-in-Wellies
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@Tony Brooks I have had a quick read of the info on tb training it looks like it will be really helpful im going to have a proper look through before I do it. Struggling to understand what amps would be drawn from each item and going from 1 item to another. 

 

Going off what you say on your website and what @Jen-in-Wellies oversized cables is better especially as I want to tie them together so they look neat would 4mm be acceptable for everything except the larger items and just use 25mm for them. 

 

Also @Jen-in-Wellies talks about fuses i have seen on your website about the no fuses should be used on ignition switches. So should I be just be using fuses on engine volt meter,charge lights,temp gauges etc 

 

There is a main fuse next to isolator switches at the moment I was going to keep these (100a)

 

 

Thanks to you both

 

 

 

 

 

 

 

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1 minute ago, Deano1988 said:

@Tony Brooks I have had a quick read of the info on tb training it looks like it will be really helpful im going to have a proper look through before I do it. Struggling to understand what amps would be drawn from each item and going from 1 item to another. 

 

Going off what you say on your website and what @Jen-in-Wellies oversized cables is better especially as I want to tie them together so they look neat would 4mm be acceptable for everything except the larger items and just use 25mm for them. 

 

Also @Jen-in-Wellies talks about fuses i have seen on your website about the no fuses should be used on ignition switches. So should I be just be using fuses on engine volt meter,charge lights,temp gauges etc 

 

There is a main fuse next to isolator switches at the moment I was going to keep these (100a)

 

 

Thanks to you both

 

 

I think you need to read those notes. There are a number of opinions over fusing the ignition. The maximum load will be the glow plugs plus the instruments plus the starter solenoid so if you fuse the switch input the fuse will have to be large enough to carry the total load but then some of the cables will not be protected by that fuse so that means if you want fuse protection on the individual circuits each needs its own fuse specified to protect  the cables. In my view it's a question of deciding the likelihood of a short circuit on cables. As far as the instruments and warning lights go the chances of a short between ignition switch and instruments/warning lamps is small  and the  actual instruments or bulb will protect the rest of the cable to the senders.

 

On a petrol engine fusing the ignition feed could result in an instant engine shut down that could be dangerous whereas continuing to run with burning wring might allow the boat to get to safety.

 

Once again cable size is dependant upon current and LENGTH so any attempt to specify cable size without both those factors can not be accurate. The current may depend upon how you wire certain things such as the glow plugs. If you use a relay close to the engine then the cable from the switch can be thinner than if you wired direct from the switch.

 

On a rear engine, rear instrument panel boat 2 sq mm CCSA cable is likely to be fine for all the instruments and warning lights. The feed to the ignition switch will need to be heavier as will the glow plugs unless a relay is used. I would also use a slightly larger one for the starter solenoid control.

 

The instruments and warning lamps may be considered to draw 1 amp or less each. The glow plugs on a 1.8 at least 50 amps but many marinisers seem to use underspecified cables without too many problems.

 

25sq mm CCSA cable will be exceptionally difficult to use for the feed to the ignition switch, glow plugs and starter solenoid because its far too thick and stiff for easy installation and termination.

 

 

 

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1 hour ago, Deano1988 said:

Also @Jen-in-Wellies talks about fuses i have seen on your website about the no fuses should be used on ignition switches. So should I be just be using fuses on engine volt meter,charge lights,temp gauges etc 

Again, what Tony said. I personally would have an appropriate fuse to protect the wiring to the instrumentation. If the instrument fuse blows, you'll lose all the information from the instruments, but the engine will keep on running and you can still start it. Better than the instrument panel wiring catching fire!

Another reason for not using over sized cable is cost. Copper is expensive.

Jen

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