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BSS battery cable size, 25m2 ?


mascip

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Hi,

 

the BSS says that cables between a battery and

- another battery, or
- an inverter over 1000W, or
- a master switch,
- etc,
should be "sufficiently" big: "must be approximately 25mm2".
To me, "approximately" means "as long as it's really safe". With two leisure batteries of 110Ah in parallel, is it safe to have a 16mm2 cable between them? I have seen 16mm2 cables on several of my friends boats. I am wondering whether I should propose to change their cables to 25mm2.
Also, if a cable to the master switch is 16mm2 and has a 50A fuse, that should be safe and hence pass BSS, right?
PS: Woops, I cannot change the title anymore, it should be "25mm2". Mistyped.
Edited by mascip
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This is probably a daft question but there you go. The wires linking my batteries don't look very thick, but I don't know how to measure the thickness. 25mm, what would the diameter of the wire, including casing be?

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As mentioned by Seadog above the easy/correct answer to this is do the calcs for current draw, out of that you will know what cable size is needed to carry the current. Go up to the nearest larger cable size and as far as the cable is concerned its safe.

 

Don't forget you may have to go up another size if say you have a very long cable run to minimise voltage drop. again this can be calculated

Edited by martyn 1
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This is probably a daft question but there you go. The wires linking my batteries don't look very thick, but I don't know how to measure the thickness. 25mm, what would the diameter of the wire, including casing be?

The "casing" varies according to the make and type of cable, so your question cannot be answered. However you should be able to measure the diameter of the actual copper bit and convert it into area using pi x ((diameter/2) squared).

Edited by nicknorman
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Are you speaking about cable to the starter motor?

 

I'm speaking about cable between batteries

The battery cables. These can be passing high currents for long periods, unlike starter cables which are on duty for only seconds and are also likely to be very short in length. Good advice in Martyn's post above.

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Nick is correct that the sheathing thickness will of course affect this, at least in a small way.

 

However for typical cables, a 16mm2 cable at just over 8mm diameter over the sheathing will probably be identifiable versus a 25mm2 at more like 10mm.

 

Some typical sizings here

 

This has always seemed to me a slightly odd requirement to make mandatory for all boats. Even 16mm2 is good for over 100amps, and many boats neither have alternators with anything like that capability, nor a total combined loading of everything on board (even should it all be turned on at once) that comes even close.

 

I'll admit quite a few of the domestic bank connects on our last boat appeared to be less than 25mm2 - this was never queried by three different BSS inspectors, even if "sub optimal".

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How do you calculate the current that might pass between two batteries? Do you consider the battery as a generator and a resistance, and use this for calculations? Or is there a simpler way?

If they are already wired you could measure it with a DC clamp-meter with sufficient rating (expensive if it's over 200V), but calculating would be better really.

 

So calculating the maximal load if everything was on, and checking the alternator output. That should tell you what is safe.

 

Maybe that 25mm2 cable is required, in case you would change to a bigger alternator for example?

Edited by mascip
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How do you calculate the current that might pass between two batteries? Do you consider the battery as a generator and a resistance, and use this for calculations? Or is there a simpler way?

 

If they are already wired you could measure it with a DC clamp-meter with sufficient rating (expensive if it's over 200V), but calculating would be better really.

 

smiley_offtopic.gif I assume you mean "if it is over 200A"?

 

There are readily available clamp meters with a 400A DC scale that only cost around £30.

 

Even measuring a starter motor current starting the engine has not exceeded that on any of the boat diesels I've had so far, and unless people are running massive inverters, it seems unlikely you would see 400A flowing through domestic bank wiring.

 

Quick sanity check.....

 

400Amps at 12 Volts is 4,800 watts. Even a 3,000 watt inverter of very poor efficiency will not draw that.

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£30 for a 400A DC clamp meter? I have just bought a 200A one for £80, I couldn't find any cheaper.

 

Many clamp meter which are sold as "DC AC" do DC voltage readings, but only AC current reading, not DC current.

Edited by mascip
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£30 for a 400A DC clamp meter? I have just bought a 200A one for £80, I couldn't find any cheaper.

 

Many clamp meter which are sold as "DC AC" do DC voltage readings, but only AC current reading, not DC current.

 

Well Maplin are currently £40, but regularly have this Unitrend meter discounted to £30

 

The same thing can be had for well under £30 from many e-Bay sources, (although some involve dispatch from the Far East).

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Nick is correct that the sheathing thickness will of course affect this, at least in a small way.

 

Depends on what you mean by a "small way" of course, but it should be born in mind that due to the R squared term, an extra few mm on a large diameter cable is "worth" a lot more csa than the same extra few mm on a small diameter cable.

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