Cable size requirements for battery system

[Russdl]

Our house, when built, was wired for a battery system, plus it had cabling installed for an emergency power supply (both of these cables sit redundant).  We're at the stage now where we are seriously considering a battery and I'm looking at the Givenergy All In One battery system. The installation documentation for the All in One states "...minimum cable size requirements for the All in One are 6mm2 minimum"

 

The existing unused battery cable is 4mm2 and the existing unused cable for the EPS is 2.5mm2. Running a new 6mm2 cable to the position needed would be virtually impossible so I asked the installer the stupid question,  does 4mm2 + 2.5mm2 = 6.5mm2, in other words can I combine the two existing cables and use them as a parallel supply? To my surprise he didn't laugh at me but instead said he'd check with the manufacturer.

 

This is the manufacturers response - a bit of a non answer

 

"The wiring would have to be as per UK electrical Regs in terms of wiring and connections".

 

The installer is going to check what they actually mean, but can someone help me here, is it lunacy to even suggest such a thing or is it safe/acceptable to parallel the supply with certain protections in place? Any guidance gratefully received.

[ProDave]

Putting two different size cables in parallel may not work as expected.

 

My question to the battery manufacturer (it might be in the specifications somewhere) is what does the maximum current carrying capacity of the cable need to be?

 

Depending how the 4mm cable is installed, i.e is it in contact with insulation, encased by insulation in free air etc, could be anything between 22A and 37A 

[Russdl]

Thanks for the reply @ProDave I’ll see if I can find the max current in the specs.
 

Both the 4mm and 2.5mm cable will, on the whole, be in free air though pushing through a rock wool batt here and there I suspect (can’t remember precisely, and I don’t have sufficient photos). 

[JohnMo]

The 6mm2 goes between battery and and the gateway.

 

Max current is 32A

 

All in One Installation Manual.pdf

[JohnMo]

Datasheet

 

datasheet---all-in-one--june-2023- (1).pdf

[Russdl]

@JohnMo, thanks. Any idea if the max current of 32A has any bearing on my original question re paralleling the 4mm² with the 2.5mm²? 

 

[JohnMo]

10 minutes ago, Russdl said:

@JohnMo, thanks. Any idea if the max current of 32A has any bearing on my original question re paralleling the 4mm² with the 2.5mm²? 

 

Sorry no. How far away is the battery from the gateway?

[Russdl]

It will be from the loft to ground floor, probably around 12-13m as the cable flies. 

 

A bit of Googling looks like the worst case for 4mm (conduit in wall) is max 26 amps and for the 2.5mm (conduit in wall) is max 20 amps. @ProDave has the worst case for the 4mm as 22 amps so with a similar reduction to the 2.5mm to, say 16 amps, I still have a combined 38 amps. 

 

My cables are not conduit in wall so will be better than those figures, amperage wise it would look like I'm home and dry?

 

The cables will be similar though not identical in length, what other issues are there likely to be? 

 

 

[MikeSharp01]

6 hours ago, Russdl said:

4mm as 22 amps so with a similar reduction to the 2.5mm to, say 16 amps, I still have a combined 38 amps

You perhaps need to work this out as the current in two resistors in parallel to see the full effects of doing this you don't want one cable carrying more than its capacity when the other has not reached its maximum. 

[Russdl]

1 minute ago, MikeSharp01 said:

You perhaps need to work this out as the current in two resistors in parallel to see the full effects of doing this you don't want one cable carrying more than its capacity when the other has not reached its maximum.

Trouble is I don’t have a clue how to do that. Any pointers?

[Adrian Walker]

38 minutes ago, Russdl said:

Trouble is I don’t have a clue how to do that. Any pointers?

Quick answer (for resistors is parallel)  1/Rt=1/R1 + 1/R2, longer answers is available later

 

 

Edited November 7, 2023 by Adrian Walker

[Russdl]

5 minutes ago, Adrian Walker said:

longer answers is available later

Thank **** for that 🤣

[ProDave]

For just 2 resistors it simplifies to Rt = (R1 X R2) / (R1+R2)

 

When I get time I will look up the ohms per metre of the 2 cables and try an example.

 

I don't think you actually need the parallel resistance.  All you need is the ohms per metre of the 2 cables and then work out the current through each cable when in parallel at a combined total of 32A.  You are looking to see if they share the current in proportion to their max current carrying capacity or of one gets overloaded.

 

 

[RobLe]

I suggest each cable has its own appropriate mcb rather than paralleling both ends - perhaps a 16A and 24A mcb, although the sparky should be choosing and fitting this sort of stuff.  Nb there shouldn’t be an RCD involved unless it’s the expensive dc capable type.

[JohnMo]

 

You can limit the watts going between the battery and the gateway, your install can set hard stops also, which the end user cannot change later. Discharge and charge power are all setting that can be changed. If this simplified the install.

 

 

[Alan Ambrose]

The two in parallel should be fine - say 47A in free air. I think the resistance arguments, while theoretically correct, will be irrelevant as the cables with be roughly the same length. You probably can't measure the length of each cable exactly as they're buried, but if you have the right equipment, you could measure the small resistance. I still think the calcs will get you v. close to the theory for 6.5mm^2. However, they'll be some who find that 'it doesn't meet regs' (although be hard-pushed to say why) and some will rightly point out that 'it isn't to manufacturer's spec'. So your decision and your responsibility unless you get a spark to sign off and use his/her PI.

[Russdl]

Thanks everyone for the feedback, much of which was over my head.
 

I don’t plan on doing anything myself but will use the provided information to hopefully prod the sparky in the direction I wish to travel but ultimately I guess he’ll decide if he’s happy with it or not. 

[Adrian Walker]

3 hours ago, Alan Ambrose said:

The two in parallel should be fine - say 47A in free air. I think the resistance arguments, while theoretically correct, will be irrelevant as the cables with be roughly the same length. You probably can't measure the length of each cable exactly as they're buried, but if you have the right equipment, you could measure the small resistance. I still think the calcs will get you v. close to the theory for 6.5mm^2. However, they'll be some who find that 'it doesn't meet regs' (although be hard-pushed to say why) and some will rightly point out that 'it isn't to manufacturer's spec'. So your decision and your responsibility unless you get a spark to sign off and use his/her PI.

Assuming it's at 230V, 47A = 9.7kW!

[Russdl]

1 hour ago, Adrian Walker said:

Assuming it's at 230V, 47A = 9.7kW!

As an indication of how far I am from understanding the finer points of this, I don’t know if:

 

“…= 9.7kW!”

 

is a good thing or a bad thing - but it’s clearly a thing. 

[Adrian Walker]

9 minutes ago, Russdl said:

As an indication of how far I am from understanding the finer points of this, I don’t know if:

 

“…= 9.7kW!”

 

is a good thing or a bad thing - but it’s clearly a thing. 

If your PV panels are 350W each, that would mean ~28 panels, which I don't think you have?

 

Maybe you're planning to send DC down the cables to your inverter/battery system, hence the large current requirement. 

[JohnMo]

1 minute ago, Adrian Walker said:

If your PV panels are 350W each, that would mean ~28 panels, which I don't think you have?

 

Maybe you're planning to send DC down the cables to your inverter/battery system, hence the large current requirement. 

The cable between battery and Inverter (all one integrated unit) and gateway is AC, nothing to do with PV or DC. PV ties in to the gateway again in AC.  The battery/inverter kicks out up to around 7kW. It takes a charge of 6kW.

[Adrian Walker]

5 minutes ago, JohnMo said:

The cable between battery and Inverter (all one integrated unit) and gateway is AC, nothing to do with PV or DC. PV ties in to the gateway again in AC.  The battery/inverter kicks out up to around 7kW. It takes a charge of 6kW.

Thanks for the clarification, I don't know the Givenergy All In One battery system, but it's now on my radar. So 6kW is less than 9.7kW with a nice margin

Edited November 7, 2023 by Adrian Walker

[Russdl]

The installer/sparky got the following response from Givenergy:

 

”Unfortunately, GE will only give approval for a singular 6mm cable to the AiO Battery.”

 

I suspect that means no warranty etc if it’s not installed as approved, I also suspect the sparky won’t install it unless it’s iaw the manufacturers now very specific guidance which is fair enough I suppose. I’ll run the points raised in this thread past him but it looks like it’s time to reassess. 
 

Thanks for all the input (despite me only understanding half of it!)

[JohnMo]

Obvious question, can you move the battery into the area as the gateway?

[Russdl]

1 minute ago, JohnMo said:

can you move the battery into the area as the gateway?

Sadly not, not enough room. The only option identified so far is outside the back door which is pretty close to everything and there is a duct in place. 
 

But I don’t want it outside, I want it in the loft. It’s a tough life isn’t it 🤣
 

There are of course numerous solutions but the AIO appealed the most. C’est la vie, I’ll work on it.