Getting electricity through a bulkhead

[Keeping Up]

I'm replacing some of the boat's main wiring with thicker cables because I'm losing quite a bit of voltage between the batteries and the main distribution box. Unfortunately the existing conduit through the steel bulkhead between the batteries (in the engine bay under the cruiser stern deck) and distribution box (3 metres away inside the rear cabin) will be too small for all the new copper.

 

I was looking at the rear bulkhead and contemplating the effort of drilling a hole big enough for more cable and a conduit of some sort, and trying to work out the best way of coping with the fact that the cable will have to bend through a very tight right-angle after it emerges, when another solution occurred to me.

 

If I pass the new thicker positive lead through the existing conduit, I just have to pass the negative through the bulkhead. Suppose I drill a small hole (say 8mm) and clean the paint from around it, I can then put a bolt through it with a decent ring terminal on each side. The required right-angle bend is achieved without any bending, and the poor conductivity of the bolt is irrelevant because the steel bulkhead represents a very large cross-section for the current to pass through. The bulkhead is at battery negative potential already; in fact I was going to improve the hull-neutral bond anyway so in fact I could simply remove the existing bond and rely on my new cable.

 

It seems all too easy - am I missing something?

[Keith M]

Both BEP and Blues Seas have Bulk head termination studs

 

will this be the only point the negative is terminated to the hull?

 

Keith

[bottle]

 

- am I missing something?

 

I do not think so but I am sure somebody will come up with something .

[Keeping Up]

Yes the negative supply is otherwise well isolated from the hull.

 

What would be the advantage of a termination stud, over a simple through bolt?

[nicknorman]

Are you sure there is no other connection to hull? For example, it is normal for the engine to be connected to battery -ve for starter motor, alternator and various senders. It seems unlikely (though not impossible) that the engine is isolated from the hull - typical earth paths are through control cable outers, the prop shaft, exhaust pipe and maybe the engine bearers.

 

Therefore if you interrupt a high current carrying cable and connect it to hull, there will inevitably be a tendency for some of that current to take a path through the hull, something that is generally to be avoided for corrosion reasons.

[Keeping Up]

Yes there is going to be some fortuitous contact through prop shaft, exhaust pipe etc, I would have thought that the amount of current that would choose to flow via that path would be negligible and insignificant, when compared with the easy option of just 50cm of 35mm cable, but I would welcome any views on that.

 

I might add that the maximum current expected through this route is 70 amps; power to the inverter does not take this route.

[Arthur Brown]

You are very UNlikely to have a system that is isolated from the hull, The engine is usually well bonded to the hull for the purposes of returning the starter and alternator currents. With fuel pipe, exhaust pipe prop shaft and engine mountings it's almost impossible to separate the engine from the hull .

[Keeping Up]

You are very UNlikely to have a system that is isolated from the hull, The engine is usually well bonded to the hull for the purposes of returning the starter and alternator currents. With fuel pipe, exhaust pipe prop shaft and engine mountings it's almost impossible to separate the engine from the hull .

 

The engine is not bonded to the hull, other than through the common bond which it shares with the domestic supply. The starter and alternator currents return through the cables. As I said above however, I agree that there will be a degree of fortuitous connection through the exhaust pipe, prop shaft, etc. But does this matter?

[dor]

It would be normal to have a bolt through a bulkhead to act as an earthing point to which a battery negative would be attached. I don't think it would be too odd to also have a negative return attached to that hull post. The fact that it is on the other side of the bulkhead would seem pretty insignificant.

 

I'm a great believer in minimising negative connections to the hull (radio aerials up the front being one of the most common) but I can't see this doing much damage. It is so close to what would be a normal negative to hull connection I can't see it making much difference. It's not like using the hull for earth return over a significant length of the hull.

 

I can't see anything in the BSS that would stop it wither.

[smileypete]

How many mm² is the new cable?

 

ETA if you really want to put a stud thro the bulkhead, consider using a cable gland as an insulating bush, the inside end is pretty low profile:

 

 

cheers, Pete.

~smpt~

Edited November 19, 2013 by smileypete

[Keeping Up]

How many mm² is the new cable?

 

cheers, Pete.

~smpt~

 

I'm trying to decide between 25 and 35 mm²

 

At the moment it's only (I think) 12mm which gives me significant losses. Increasing to 35 would be best but I am also wondering whether 25 would fit through the hole that I have available. Trouble is, the only way to find out would be to buy some and see!

[by'eck]

How about using a deck gland. You only need one fitted to the most convenient side of the bulkhead. An uninterrupted cable can then be passed through the oversize hole.

 

[smileypete]

Can you just replace the cable and conduit up to the bulkhead, then connect to the existing? Some decent close up pics might help get suggestions.

 

ETA: Another alternative would be leave the existing run of 12mm² then add another run of 12mm² in parallel, to give the equivalent of 24mm². 12mm would get round a tight bend better too.

 

cheers, Pete.

~smpt~

Edited November 19, 2013 by smileypete

[ditchcrawler]

Can you just replace the cable and conduit up to the bulkhead, then connect to the existing? Some decent close up pics might help get suggestions.

 

ETA: Another alternative would be leave the existing run of 12mm² then add another run of 12mm² in parallel, to give the equivalent of 24mm². 12mm would get round a tight bend better too.

 

cheers, Pete.

~smpt~

Didn't we do this in another thread.

[MtB]

Easiest way is probably to buy another boat with good wiring....

 

 

MtB

[smileypete]

Didn't we do this in another thread.

 

Sure did.

 

With an extra run you could also keep them separate and supply different loads at the other end, if you want. Wonder what the volt drop problem is affecting.

 

cheers, Pete.

~smpt~

Easiest way is probably to buy another boat with good wiring....

 

 

MtB

 

It's probably one of the better ones as it is!

 

cheers, Pete.

~smpt~

[Keeping Up]

Yes it's a good boat but at the time Stoke-on-Trent were not good on electrics.

 

Originally it was wired in 8mm cable, from the alternator to the dashboard ammeter (3 metres) and thence down to the batteries (2.5 metres) which gave quite a volt-drop on the positive line. The cables got warm too at 70 amps, so I increased it to 12mm. The negative was 2.5 metres of 8mm as well, which I increased to 12mm. The trouble was, the cable gland was then very full - there was no room for parallelling the existing cables

 

I've rationalised a few things so there aren't so many other cables passing through the gland (an extra connector meant that the two cables for the battery charger disappeared, for example) and one or two items need changing anyway (the battery fuse block has fallen to bits for example), so now I'm trying to improve things. After all, at the moment I think I lose about 750mV in the wiring when charging. The Adverc compensates of course, but it's a lot of wasted power - and presumably the alternator is running hotter too.

 

Then after I've stopped charging the current in the wires is of course a good deal less, probably never peaking more than 20 amps with the lights recently converted to LED and the TV/DVD being more efficient than their older predecessors - but even so I expect I lose 150-200 mV when on load, and that's before I've started losing volts in the cables after the distribution box.

 

I must admit I do like the idea of keeping just some of the existing cables for their short run through the bulkhead, by inserting a couple of judiciously placed connectors

[smileypete]

Sounds like it could do with a shunt ammeter in there somewhere, there are round meters around

 

http://www.ebay.co.uk/itm/Round-Housing-Zero-Setting-AMP-Current-Test-Meter-Ammeter-AC-0-100A-/121196283150

 

 

Then maybe replace the 12mm² between alt and batts with 25mm², the 12mm² could be re-used to beef up the run to the charger and dist board. Maybe double up until the bulkhead and then use the existing.

 

3A per mm² is a reasonable conservative rule of thumb for normal PVC cables, so 70A though 8mm would be a nono.

 

Also when running at 3A per mm² it gives 0.1V drop per meter separation between source and load. As you have equivalent of 4m separation between alt and batts and are running around 6A per mm² the reported drop of 750mV sounds about right, being roughly 4m X 0.2V.

 

If you could up the alt cables to 25mm² that'll give you 0.4V drop and brings you back down to 3A per mm² which is better for a hot engine room and where cables may be bundled together. Anyway I'll stop droning on now

 

cheers, Pete.

~smpt~

Edited November 19, 2013 by smileypete

[Keeping Up]

To round this one off:

 

Yes I'd love to have found and fitted a suitable moving-needle centre-zero shunt-ammeter but I couldn't find one.

 

Taking up the helpful suggestion of putting a junction box either side of the bulkhead to allow a change to thinner cables for just a very short distance, using these junction boxes as commoning points meant that eventually I was able to wire everything in a mix of 35mm and 50mm cables, just reducing to 25mm to pass through the bulkhead. I had the larger cables made up for me by Acorn Engineering who also supplied the 300-amp junction boxes.

 

So now everything is in much much bigger cables, as well as looking much tidier and being more versatile for the future. Stoke on Trent had used 6.5 or 8mm for most things, and I had previously increased it to 12mm plus a couple of sections doubled up, but now only the 30-amp charger has the tiny 12mm connections. Also the fusing was a bit haphazard but now everything is properly fused according to its cable size.

 

I'm very pleased with the end results. Now I just have to update my wiring diagram!

[smileypete]

Glad it's sorted.

 

If the right centre zero meter can't be found, it's possible to run 2 ammeters off one shunt as long as they have the same full scale current and shunt requirement.

 

So you could have a round analogue ammeter on the engine panel to monitor charging, then a digital ammeter elsewhere to monitor loads or charging. OK the engine panel ammeter would hit the zero stop when drawing a load but shouldn't get harmed. Or even have a 2PCO relay that reverses the analogue ammeter connections when the engine is on.

 

Blue seas do rectangular centre zero ammeters - at a price!

 

cheers, Pete.

~smpt~

Edited December 8, 2013 by smileypete

[Timleech]

Glad it's sorted.

 

If the right centre zero meter can't be found, it's possible to run 2 ammeters off one shunt as long as they have the same full scale current and shunt requirement.

 

So you could have a round analogue ammeter on the engine panel to monitor charging, then a digital ammeter elsewhere to monitor loads or charging. OK the engine panel ammeter would hit the zero stop when drawing a load but shouldn't get harmed. Or even have a 2PCO relay that reverses the analogue ammeter connections when the engine is on.

 

Blue seas do rectangular centre zero ammeters - at a price!

 

cheers, Pete.

~smpt~

 

Something like this would do the trick, snag is that the proper shunts for these are much harder to find than the meters, and tend to be expensive if you do manage to find the right one. Might be easier to work out the shunt value needed and find a 'modern' one to suit.

 

Some of the CAV ammeters have the shunt mounted directly on the back of the meter between the terminal studs, with one of those you could remove the shunt from the meter and contrive a mounting for it closer to the action.

I've used the alternator cabling as shunt for mine, with a little resistor at the meter for fine adjustment.

 

Tim

[Keeping Up]

I did find a meter, but the meter plus shunt would have cost me over £100.

 

The work I have done cost around £200, for which I have probably reduced the alternator voltage by 650 to 700 mV but more importantly gained about 150 mV at the distribution box when discharging (plus general improvements in fusing etc). The further reduction in alternator volts by using a shunt meter (probably around 200mV more) would be good but is not essential because of the Adverc, and the small improvement when discharging (only about 25 mV better) just didn't feel worth it.

 

(Edited to correct my Rithmaticks)

Edited December 9, 2013 by Keeping Up

[Keeping Up]

... Now I just have to update my wiring diagram!

 

Done that (small pdf file here).

 

On the whole diagram there are only 4 wires which have not been changed in some way.