Sterling ProCombi S - Battery Wire Size?

[Richard10002]

I have a new Sterling Pro Combi Sine Wave Inverter Charger (12v / 2500w). It replaces a 25A Mobitronic charger, so I need to upgrade the wiring from the battery bank, which is about 4m from the unit, (as the wire runs). The instructions say to use 90mm2 wire with a 400A fuse, but I am having trouble understanding why so big?

The charger has a max of 70A, so the existing 20mm2 wire would happily accept this current, albeit a 35mm2 wire would reduce the voltage drop. The inverter at 2500W will have a max of around 210A plus something for inefficiency, say 250A?? So why couldn’t I use 50mm2 cable fused at say, 300A, or 70mm2 cable fused at 400A, (or even 35mm2 cable fused at say 200A)?

There is a surge rating of 7200va, but this would suggest up to 600A, so Sterling are specifying a 400A fuse to protect the wire, rather than the wire being specified for the surge.

Could it all be to do with voltage drops? If so, is this more for the charger side of things, or the inverter, or both?

The answer might be - "Just do what the manual tells you!!" - and I dont have a problem with that. I'm just trying to understand why?

Many Thanks,

Richard

 

[VicS]

I have a new Sterling Pro Combi Sine Wave Inverter Charger (12v / 2500w). It replaces a 25A Mobitronic charger, so I need to upgrade the wiring from the battery bank, which is about 4m from the unit, (as the wire runs). The instructions say to use 90mm2 wire with a 400A fuse, but I am having trouble understanding why so big?

The charger has a max of 70A, so the existing 20mm2 wire would happily accept this current, albeit a 35mm2 wire would reduce the voltage drop. The inverter at 2500W will have a max of around 210A plus something for inefficiency, say 250A?? So why couldn’t I use 50mm2 cable fused at say, 300A, or 70mm2 cable fused at 400A, (or even 35mm2 cable fused at say 200A)?

There is a surge rating of 7200va, but this would suggest up to 600A, so Sterling are specifying a 400A fuse to protect the wire, rather than the wire being specified for the surge.

Could it all be to do with voltage drops? If so, is this more for the charger side of things, or the inverter, or both?

The answer might be - "Just do what the manual tells you!!" - and I dont have a problem with that. I'm just trying to understand why?

Many Thanks,

Richard

To do with volts drop .

Taking your figure of 4m distance... (making the total of + and - cables 8m) using an acceptable volts drop of 3% and your suggested 250 amps for the current I calculate that you need cables of 94mm². You can safely call that 90mm²

3% volts drop = 0.36 v

0.36 v at 250A = 0.00144ohms

If the resistance of 8m is 0.00144 ohms the resistance of 1000m is 0.18ohms

From tables, AWG 3/0 , 85mm², has resistance of 0.2 ohms/1000m

85mm² x 0.2/ 0.18 = 94.4 mm²​

 

[yachtbits]

90mm² cable is to basically reduce the volt drop in the cables when on load.

All of the little volt drops add up, and it is easy to trigger low voltage alarms/cutout's if the system isn't the best it can be.

With a large inverter, you also need to consider the drop in terminal voltage at the battery bank. Depending on the size and type of batteries, you may find your battery bank voltage drops significantly when you "turn the kettle on".

We have installed quite a few of these Sterling combi's (and Victron units). We always try to keep cable lengths to a bare minimum. 4 mtrs is quite a long run, so don't compromise on cable size.

You've invested a significant chunk into a decent inverter, close your eye's, ignore the cost, and buy the 90mm² cable or you may find it doesn't perform how you want.

Fuses work by heating up and melting. This takes time. Therefore it is possible for a fuse to cope with more than its rating for a short period of time (seconds). This allows for inrush currents in motors and other inductive or capacitive loads. This is why a 400A fuse can provide for 600A surge rating.

Kevin

 

[Richard10002]

Thanks for the info.... Voltage drop it is then.

I've bitten the bullet and ordered some 95mm2 wire and some 95-8 terminal ring lugs.

 

[yachtbits]

if you've ordered crimp on lugs, don't try and crimp them with a vice or mole grips or any of those other daft idea's, including blow torch and solder. (seen all of them before)

Find an electrician, with a proper crimper and get them fitted properly, otherwise, guess what, Volt Drops!!!!

 

[mitiempo]

if you've ordered crimp on lugs, don't try and crimp them with a vice or mole grips or any of those other daft idea's, including blow torch and solder. (seen all of them before)

Find an electrician, with a proper crimper and get them fitted properly, otherwise, guess what, Volt Drops!!!!

I agree 100%

 

[fuss]

When I bought my boat the 2000w 24v inverter was installed with 25ml cables. You can imagine the problems that caused. Cables got very hot and inverter used to switch off with some sort of thermal check. I was told that the inverter was faulty. I replaced the cables with 70mm. 6 yrs later, inverter still works fine.

 

[Richard10002]

if you've ordered crimp on lugs, don't try and crimp them with a vice or mole grips or any of those other daft idea's, including blow torch and solder. (seen all of them before)

Find an electrician, with a proper crimper and get them fitted properly, otherwise, guess what, Volt Drops!!!!

Just prepared my first lug for crimping, as per Nigels' instructions, and find my hydraulic crumpets won't take 95mm2 lugs - bugger!

How much can I expect to pay for 6 lugs to be crimped onto the wire ....... Assuming the wire is stripped back, and the lugs are on the wire, so they only need crimping, no preparation.

I ask because I can buy a hydraulic crimper for about £60 which will do 10mm2 up to 120mm2 crimps, so if it's £8 - £10 per crimp, it will make sense to buy the crimpers.

 

[Richard10002]

Just prepared my first lug for crimping, as per Nigels' instructions, and find my hydraulic crumpets won't take 95mm2 lugs - bugger!

How much can I expect to pay for 6 lugs to be crimped onto the wire ....... Assuming the wire is stripped back, and the lugs are on the wire, so they only need crimping, no preparation.

I ask because I can buy a hydraulic crimper for about £60 which will do 10mm2 up to 120mm2 crimps, so if it's £8 - £10 per crimp, it will make sense to buy the crimpers.

Found a local old fashioned auto electrical shop. They tell me they can crimp my lugs for about 50p per crimp. Looks like a result!