BSS - Domestic Battery Isolator Cable Size

[Withywindle]

We’ve had our boat for  almost three years and our first BSS is coming up later this year. 
 

I’ve gone through the BSS requirements (which are truly wonderful bedtime reading) and I’m as confident as I can be of everything apart from one thing - the domestic battery isolator cabling. This is not 25mm, I would estimate it at 16mm so on the face of it I probably should get it upgraded. All other battery cabling is at least 25mm and the interconnects between the batteries themselves is even thicker so no worries there. 
 

As for the boat itself, it’s a 2003 Colecraft with a Beta 38 and unusually it has a Dometic travel power generator which Beta offered as an option at that time.  As for high drain appliances we have a washing machine which we’ve never yet used, a 240v fridge and the usual immersion heater for use when plugged in on the Marina. There was originally a tumble drier on board too (no I’m not joking), we had never used that either, but I finally got rid of it this week as I needed the space for more important things!
 

Reading the BSS it looks like I should be upgrading the domestic isolator cable to 25mm but on the other hand the boat has sailed through every previous BSS so if was an issue why would that be? Furthermore the previous owner was a Marine Engineer who was very meticulous about maintenance and kept everything on the boat pretty much 100% so I’d be surprised if he wouldn’t have upgraded the cabling if he thought it was necessary. 
 

Am I missing something?

[Tony Brooks]

Are you reading the regs as outside diameter and not conductor cross-sectional area? 25 sq mm CCSA would have an OD of about 16mm

[Tracy D'arth]

As you have a 240v AC fridge that infers that you have an inverter as well as the travel power. The cable to this from the cabin batteries definitely needs to be much more than 25mm.

Volt drop is the deciding factor not current carrying capacity according to the cable tables.

I would consider that 16mm2 is adequate for 12v cabin feeds from batteries to  distribution via the isolator switch providing the volt drop is less than 3% under normal loading.

[Withywindle]

36 minutes ago, Tracy D'arth said:

As you have a 240v AC fridge that infers that you have an inverter as well as the travel power. The cable to this from the cabin batteries definitely needs to be much more than 25mm.

Volt drop is the deciding factor not current carrying capacity according to the cable tables.

I would consider that 16mm2 is adequate for 12v cabin feeds from batteries to  distribution via the isolator switch providing the volt drop is less than 3% under normal loading.

Yes I do have an inverter but I’m 90% sure it’s just the isolator cabling that is undersized. I will double check, but that’s my understanding. It was actually mentioned in our survey before we bought the boat and the surveyor thought everything else was ok, but I will have a look to be sure. 
 

Update: I think the penny has just dropped thanks to your reply @Tracy D'arth - I reckon I’m ok. 

 

There’s a caveat in the regulations which I’ve highlighted. It obviously just applies to 12v appliances not the 240v supply as that comes via the inverter/travepower. As all of my 12v equipment is low power as described, I’m ok.
 

I just need to confirm that the supply to the inverter is big enough - I’m sure it will be. 
 

 

Edited March 29 by Withywindle

[Alan de Enfield]

27 minutes ago, Withywindle said:

Yes I do have an inverter but I’m 90% sure it’s just the isolator cabling that is undersized

 

Does the supply for the inverter come directly from the batteries or via the isolator ?

(With the isolator 'off' is your inverter still live ?)

[Withywindle]

9 minutes ago, Alan de Enfield said:

 

Does the supply for the inverter come directly from the batteries or via the isolator ?

(With the isolator 'off' is your inverter still live ?)

I’ll check that too thanks Alan. 

[David Mack]

51 minutes ago, Tony Brooks said:

Are you reading the regs as outside diameter and not conductor cross-sectional area? 25 sq mm CCSA would have an OD of about 16mm

https://www.autoelectricsupplies.co.uk/p/battery-cable-flexible gives the outside diameter for different cross sectional areas of typical battery cables. This is only one particular supplier and other brands may vary, but it will give you an idea of the conductor cross sectional area for a particular outside diameter.

[Withywindle]

Ok I’ve checked my cable sizes as per your list @David Mack (thanks for that btw). The domestic battery isolator cabling is at least 10mm sq and probably 16mm sq, the starter isolator is 25mm sq and the cable to the inverter is 50mm sq and probably 60mm sq - the size was in between on the table. The thickness of the insulation clearly has a bearing and obviously varies for different brands of cable. 
 

@Alan de Enfield The inverter continues to work with the domestic isolator off as does my Webasto incidentally, so both are connected direct to the domestic battery bank not via the isolator. 
 

So the only equipment connected via the domestic isolator as I see it  is - my lighting, water pump, shower pump and the radio, pretty much the items specifically mentioned in the BSS guidelines as ok, minus the fridge which is on the 240v circuit. On that basis I reckon I’m fine. Hopefully the examiner will reach the same conclusion! 

[Tony Brooks]

I do wish people would make it clear what they are talking about, cable cross-sectional area or conductor cross-sectional area.

[Withywindle]

13 minutes ago, Tony Brooks said:

I do wish people would make it clear what they are talking about, cable cross-sectional area or conductor cross-sectional area.

Sorry about that Tony, I’ll try to be clearer next time. ☺️

[Tony Brooks]

1 hour ago, Withywindle said:

Sorry about that Tony, I’ll try to be clearer next time. ☺️

 

No need to be sorry, confusing CCSA and total cable cross-sectional area, and more commonly cable diameter, is a very common mistake, hence the content of my first reply. I think that unless the majority of posters make it clear which they are talking about, new posters are likely to perpetuate the confusion.

Edited March 30 by Tony Brooks

[Tacet]

Do you mean cable diameter or conductor diameter?

[Tony Brooks]

13 minutes ago, Tacet said:

Do you mean cable diameter or conductor diameter?

 

Don't know who you addressed that to, but this is my point. I think the OP gave us the OD of the insulation in his first post and did not even know about conductor cross-sectional area. As far as regs and currant/volt drop calculations are concerned, it is always CCSA that matters. Unfortunately, even the catalogues refer to mm rather than sq mm of the conductor cross-sectional area.

[Tracy D'arth]

31 minutes ago, Tony Brooks said:

 

Don't know who you addressed that to, but this is my point. I think the OP gave us the OD of the insulation in his first post and did not even know about conductor cross-sectional area. As far as regs and currant/volt drop calculations are concerned, it is always CCSA that matters. Unfortunately, even the catalogues refer to mm rather than sq mm of the conductor cross-sectional area.

That is because in the electricians' world cable is only referred to by the conductor cross sectional area as so many mm.

No account of insulation thickness as a size  is ever considered. As long as the type of insulation and its voltage rating is correct the insulation has no importance.

[Withywindle]

2 minutes ago, Tracy D'arth said:

That is because in the electricians' world cable is only referred to by the conductor cross sectional area as so many mm.

No account of insulation thickness as a size  is ever considered. As long as the type of insulation and its voltage rating is correct the insulation has no importance.

You are correct Tracy, I meant cross sectional area but seem to have started world war three. 🙄😂

[Tracy D'arth]

4 minutes ago, Withywindle said:

You are correct Tracy, I meant cross sectional area but seem to have started world war three. 🙄😂

Oh no you haven't!  Wait till they get going on the RCD regs and BSS!

The electricians know that you refer to mm and not MM2 ( can't do superscripts ) so they are not confused but any argument on here passes for discussion, or vice versa.  Don't worry about it, sit back and watch the feathers fly.

[Alan de Enfield]

12 minutes ago, Tracy D'arth said:

No account of insulation thickness as a size  is ever considered. As long as the type of insulation and its voltage rating is correct the insulation has no importance.

 

Insulation thickness is a function of voltage rating and will vary between 12v, 240v and 500/600 volt.

As a wire (CSA ) increases in size the insulation thickness will also increase -  a typical 0.6mm thickness on 240v 1mm2 would be a bit daft on 95mm2.

[Tracy D'arth]

2 minutes ago, Alan de Enfield said:

 

Insulation thickness is a function of voltage rating and will vary between 12v, 240v and 500/600 volt.

As a wire (CSA ) increases in size the insulation thickness will also increase -  a typical 0.6mm thickness on 240v 1mm2 would be a bit daft on 95mm2.

I think that is what I said.

[ditchcrawler]

8 hours ago, Withywindle said:

@Alan de Enfield The inverter continues to work with the domestic isolator off as does my Webasto incidentally, so both are connected direct to the domestic battery bank not via the isolator. 

 

I hope they go via a fuse near the batteries

[Withywindle]

1 minute ago, ditchcrawler said:

I hope they go via a fuse near the batteries

They do. 

[Tacet]

3 hours ago, Tracy D'arth said:

That is because in the electricians' world cable is only referred to by the conductor cross sectional area as so many mm.

No account of insulation thickness as a size  is ever considered. As long as the type of insulation and its voltage rating is correct the insulation has no importance.

Is there no-one left that recalls 7/029, 3/029 and 7/049?

[Paul C]

24 minutes ago, Tacet said:

Is there no-one left that recalls 7/029, 3/029 and 7/049?

 

Don't remember it but is that where the number of strands; and the size of each strand, is specified? So you could identify different (for example) 4mm2 cables, one with many fine strands which will be very flexible; and one with fewer thicker strands, not quite so flexible but probably cheaper.

[Tacet]

2 minutes ago, Paul C said:

 

Don't remember it but is that where the number of strands; and the size of each strand, is specified? So you could identify different (for example) 4mm2 cables, one with many fine strands which will be very flexible; and one with fewer thicker strands, not quite so flexible but probably cheaper.

Yes.  Its regular use predates the single core cable; 7/029 is broadly equivalent to 2.5mm2.

 

[MtB]

34 minutes ago, Tacet said:

Is there no-one left that recalls 7/029, 3/029 and 7/049?

 

Me! I remember wire being specified by number of strands and their diameter. Still see it occasionally, I think on American sites.

 

Hmmm maybe that isn't it....

 

[ditchcrawler]

1 hour ago, Tacet said:

Is there no-one left that recalls 7/029, 3/029 and 7/049?

7/044