Jump to content

BSS - exactly where to you need 25mm2 cable?


Mike Adams

Featured Posts

I have been ready a number of posts where boats have failed the BSS because of the size of the battery cables. When I rewired the boat I used the correct size cable for the current demands of the circuits. So for the starter motor circuit I used 50mm2 from the starter battery to the starter motor via the isolating switch and for the negative return from the engine. For the domestic battery I used 75A cable to cope with the maximum output from the alternator and to wire in the split charge relay and for the wiring from the auxiliary battery to the distribution board. So what counts as a battery interconnect? Obviously for direct battery to battery connection but what about that through the split charge relay? The relay I have used would not accommodate 25mm2 cable anyway. Or does the 25mm2 cable just need to go to the isolating switch? I can't find anything in the BSS that clarifies this.

Link to comment
Share on other sites

Any connection between batteries counts as a battery interconnect and should therefore be a minimum of 25mm2. That includes the VSR, because  the VSR interconnects the two batteries (house and start). 
 

 

8 minutes ago, Mike Adams said:

The relay I have used would not accommodate 25mm2 cable anyway.

I’m sure that a suitable crimp lug could be found.  What terminals does the relay have?

Edited by WotEver
Clarity
Link to comment
Share on other sites

If the relay has a maximum current of 70A what is the point of thicker cable? The only time it could take a higher current would be if the relay stuck closed and the starter motor draws current from the house battery in which case the 100A main fuse on the house battery would blow. Should the BSS not read as a direct interconnect?

Link to comment
Share on other sites

I used 25mm2 cable and 100A fuses when I fitted a VSR. The VSR is rated at 140A.  I can't charge at anything like this, but I suppose the starter motor could draw a fair current from the domestic batteries via the VSR, until the voltage drops enough for the VSR to drop out.

Link to comment
Share on other sites

31 minutes ago, Mike Adams said:

If the relay has a maximum current of 70A what is the point of thicker cable?

The fact that it's the rules? Careful what you wish for, or they'll add "Is any VSR rated for at least 100A" next...

Link to comment
Share on other sites

The VSR drops out at 12.6v. I would assume that when the engine is off it will have dropped out but is that the case? Otherwise it defeats the object of not discharging the starter battery. Anyway the battery terminal voltage will drop significantly as soon as you press the starter so the relay will drop out immediately.

Link to comment
Share on other sites

5 minutes ago, Mike Adams said:

The VSR drops out at 12.6v. I would assume that when the engine is off it will have dropped out but is that the case? Otherwise it defeats the object of not discharging the starter battery. Anyway the battery terminal voltage will drop significantly as soon as you press the starter so the relay will drop out immediately.

With solar fitted, the voltage is higher during the day and the relay is in (assuming I don't have some enormous domestic demand running). I assume it drops out as soon as I push the starter button, but I haven't checked.

Link to comment
Share on other sites

If you have a sizeable solar array and large controller then you need 25mm from solar controller to batteries. We used it from our 96 A midnite classic to the batteries as we have an oversized array so could in theory have 96A output . Interestingly midnite being American recommends 4 gauge wire which equates to 25mm as closest metric but actually it was a tad too big for the terminal blocks so we had to take some strands out to make it fit , imperial to metric not quite working it seems . 

 

Actually at weekend we did see almost 90 amps from the midnite 1200 watts from the array for a short time so glad we didn’t undersized the cable . 

 

 

Edited by RufusR
Link to comment
Share on other sites

38 minutes ago, RufusR said:

If you have a sizeable solar array and large controller then you need 25mm from solar controller to batteries. We used it from our 96 A midnite classic to the batteries as we have an oversized array so could in theory have 96A output . Interestingly midnite being American recommends 4 gauge wire which equates to 25mm as closest metric but actually it was a tad too big for the terminal blocks so we had to take some strands out to make it fit , imperial to metric not quite working it seems . 

 

Actually at weekend we did see almost 90 amps from the midnite 1200 watts from the array for a short time so glad we didn’t undersized the cable . 

 

 

 

Rather than remove strands, you should used appropriately sized reducing lugs where the terminations aren't big enough to accept the cable.

 

https://www.cablecraft.co.uk/copper-round-pin-terminals-95mm?gclid=CjwKCAjw1ej5BRBhEiwAfHyh1CyAR1pgHx9gkaasCP1IG232a0gANFIF4OotlSbDfsNBXdnKytckhhoCuVIQAvD_BwE

  • Greenie 1
Link to comment
Share on other sites

Now I have another issue. I have two 110AH domestic batteries at the bow, where the engine is, and two 125Ah batteries at the stern(the only place I could fit them). Each has its own isolating switch and fuse and they connect to a loop supply around the boat with fuse boxes at various points. So am I going to have to upgrade the 10mm2 wiring to 25mm2 to comply with the next BSS? Seems bonkers to me.

Link to comment
Share on other sites

11 minutes ago, Mike Adams said:

Now I have another issue. I have two 110AH domestic batteries at the bow, where the engine is, and two 125Ah batteries at the stern(the only place I could fit them). Each has its own isolating switch and fuse and they connect to a loop supply around the boat with fuse boxes at various points. So am I going to have to upgrade the 10mm2 wiring to 25mm2 to comply with the next BSS? Seems bonkers to me.

If you get a BSS examiner that reads the rules, then yes, it would seem that you will need to.

 

It has been the requirement since 2013 (or earlier)

 

3.2.2 Are battery cables of a sufficient current-carrying capacity? R

 

Check the size of the following cables by comparing them against a typical sample cable.

· battery to master switch;

· battery or master switch to starter solenoid;

· battery to battery;

· engine return to battery or master switch;

· battery to bow thruster motor;

· battery to anchor winch motor;

· battery to inverter system (over 1000w size). 

The battery cables prescribed in the check must be approximately 25mm2

 

Edit to add :

 

In 2002 the requirements were :

 

Standard 3.2

Electrical Cables Cables shall be of adequate current carrying capacity and of suitable construction and grade.

They shall be insulated and/or sheathed so as to be impervious to attack by fuel or water.

They shall be adequately supported or run in adequately supported suitable conduits.

3.2.1 Verify that following cables are of minimum size:

Battery to master switch 25mm2

Battery to starter motor 25mm2

Battery/Battery 25mm2

Other cables between battery and fuse/distribution box suitable current carrying capacity for the installation.

 

 

Edited by Alan de Enfield
Link to comment
Share on other sites

Fuses and MCBs are rated to protect the cable down stream. As long as the cable size matches or exceeds the fuse/mcb rating there is no problem. Other than volt drop of course.

Battery connecting cables should now be 25mm2 but if there is protection to such circuits as bow charging, split charge relay/VSR then I would say not, but what do I know, I predate all the regulations !??

Edited by Tracy D'arth
Link to comment
Share on other sites

4 minutes ago, Tracy D'arth said:

As long as the cable size matches or exceeds the fuse/mcb rating there is no problem.

There is a problem - whatever your own thoughts may be, and however much you 'squeem and squeem', the problem will not go away.

 

If you have 10mm cable (and suitable fuses) it will still fail the BSS examination where a minimum of 25mm is required.

Link to comment
Share on other sites

So if each battery connected to the ring has its own fuse then there is no direct battery to battery connection. I think the idea is based around battery banks say 2 x12v in series for 24v to cover the interconnect wiring which is sensible.

Link to comment
Share on other sites

4 minutes ago, Mike Adams said:

So if each battery connected to the ring has its own fuse then there is no direct battery to battery connection. I think the idea is based around battery banks say 2 x12v in series for 24v to cover the interconnect wiring which is sensible.

And six (or 4,5,3, whatever) batteries in parallel.  
 

So how do your stern batteries get charged?

Link to comment
Share on other sites

Very slowly due the slight volt drop down the boat! They are basically a reserve set only used if the two main ones run down say if we are moored up at an event for several days. They are normally out of circuit but I can switch them in if required to top them up or I connect an intelligent charger to the inverter  when they are out of circuit. May seems strange but the inverter and charger combined only cost about £40.

Link to comment
Share on other sites

3 minutes ago, RAP said:

On a related topic, if a 3kw inverter requires 95mm2 cables connected to the 12v battery bank (positive via the battery isolator) should the battery interconnect cables also be 95mm2?

Ideally, yes. 25mm2 is a minimum, not a fixed size. 

8 minutes ago, Mike Adams said:

They are normally out of circuit but I can switch them in if required...

Well if a sharp-eyed inspector notes that then yes, the cables by which they are connected to the fore bank needs to be 25mm2

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.