Installing a VSR - cable and fuse size

[FairweatherDave]

So I am working on my shopping list for 12volt planet, and my head is spinning a bit. I have my wiring diagram courtesy of Paul Rainbow in this thread
Wiring diagram - circuits added for new stuff to my 1-2 both boat electrics - opinion/advice appreciated post #19
The VSR I am looking at is this Durite Voltage Sensitive Split Charge Relay - 12V 140A
I phoned 12volt planet who suggested 16mm2 cable rated for 80 amps to the VSR from the batteries which I presume is correct. Paul's diagram indicates both reds have fuses but I am curious what rating they should be. You chose the fuse to protect the cable ...so I am confused. That rule would imply a fuse of less than 80 amps, but the spec for the VSR says 140 amps. I'd be grateful if someone can explain things a bit I do have a basic grasp and I'm halfway through the 12volt Bible and I'm trying to understand, but the forum can really help!

 

[FairweatherDave]

I think the answer is somewhere in the difference between maximum continuous current and maximum surge current, but still not sure what that means for fuse choice. I got that thought from these spec on what I think is a similar VSR
https://cdn.shopify.com/s/files/1/0053/7840/8482/files/Voltage_Sensitive_Relay.pdf?2289

 

[William_H]

Your VSR is not capable of jump starting engine so max current for the VSR wiring will relate to alternator max capability and possibly some surge when VSR makes contact to connect a well charged engine battery to a well discharged service battery. The 140 amp rating of the VSR simply says it will melt at any greater current. If you have eg 100AH service battery max charge current might come in around 15 amps max. Some resistance (light weight )in the wiring might be an advantage in limiting surge current and will not significantly increase time to recharge service battery. I would suggest 80amp cable more than adequate if not over kill. Fuse of course should relate ie less than max current capability of the cable. A fuse at each battery connection. If you want correct protection from over current in cable. The danger being current from either battery shorting to negative so over heating cable. In practical terms if cable is short and clear of anything combustible fuses could be omitted. ol'will

 

[VicS]

So I am working on my shopping list for 12volt planet, and my head is spinning a bit. I have my wiring diagram courtesy of Paul Rainbow in this thread
Wiring diagram - circuits added for new stuff to my 1-2 both boat electrics - opinion/advice appreciated post #19
The VSR I am looking at is this Durite Voltage Sensitive Split Charge Relay - 12V 140A
I phoned 12volt planet who suggested 16mm2 cable rated for 80 amps to the VSR from the batteries which I presume is correct. Paul's diagram indicates both reds have fuses but I am curious what rating they should be. You chose the fuse to protect the cable ...so I am confused. That rule would imply a fuse of less than 80 amps, but the spec for the VSR says 140 amps. I'd be grateful if someone can explain things a bit I do have a basic grasp and I'm halfway through the 12volt Bible and I'm trying to understand, but the forum can really help!

The fuses must be no greater than the max safe current rating of the cable AND no greater than the max current rating of the VSR. ie no greater than whichever is the lower

Therefore with 80 amp cable and a 140 amp VSR they must be no greater than 80 amps

12 volt planet recommend a fuse rating to be about 70 to 80% of the cables rating so in practice for 80 amp cable a 60 amp fuse would be appropriate

 

[FairweatherDave]

Thanks ol'will and VicS. As usual really appreciate both your replies.

 

[VicS]

deleted .. duplicate post

 

[Alan S]

As William says in post 3, I don't see the need for fuses if the cable runs are short.
After all these cables are just connecting 2 battery positives. You don't put fuses in the cables connecting multiple positives in a parallel battery bank.

 

[VicS]

As William says in post 3, I don't see the need for fuses if the cable runs are short.
After all these cables are just connecting 2 battery positives. You don't put fuses in the cables connecting multiple positives in a parallel battery bank.

There should be a fuse to protect the VSR

 

[Alan S]

VSR is rated at 140 Amps. Extremely unlikely to be exceeded.

 

[VicS]

BTW 16mm² cable is rated at 110 amps

 

[PaulRainbow]

So I am working on my shopping list for 12volt planet, and my head is spinning a bit. I have my wiring diagram courtesy of Paul Rainbow in this thread
Wiring diagram - circuits added for new stuff to my 1-2 both boat electrics - opinion/advice appreciated post #19
The VSR I am looking at is this Durite Voltage Sensitive Split Charge Relay - 12V 140A
I phoned 12volt planet who suggested 16mm2 cable rated for 80 amps to the VSR from the batteries which I presume is correct. Paul's diagram indicates both reds have fuses but I am curious what rating they should be. You chose the fuse to protect the cable ...so I am confused. That rule would imply a fuse of less than 80 amps, but the spec for the VSR says 140 amps. I'd be grateful if someone can explain things a bit I do have a basic grasp and I'm halfway through the 12volt Bible and I'm trying to understand, but the forum can really help!

I'd fit a Victron VSR, such as, Victron Energy Cyrix-ct 12/24V 120A Intelligent Battery Combiner - CYR010120011R

Cable size and fuse would depend on max charge rates from any charging device, usually the alternator. Let's say you have a 60a alternator, i'd use 10mm cable and a 70a fuse. Assuming nothing else can charge above 60a and no solar panels.

You don't need to fuse both cables if one or more are suitably installed/protected and cannot short, but you do need at least one to protect the VSR from overload.

 

[FairweatherDave]

Thanks Paul

Reading the spec of the Victron it says.....
Parallel connection in case of emergency (Start Assist)
The Cyrix can also be engaged with a push button (Cyrix remains engaged during 30 seconds) or a switch to connect batteries in parallel manually.
This is especially useful in case of emergency when the starter battery is discharged or damaged.

If that is something different to the emergency combiner switch in your wiring diagram then I'm guessing I don't need the emergency combiner switch. However I would then be concerned that the cable to the Victron VSR should be more on the starter battery leads scale?

 

[PaulRainbow]

Thanks Paul

Reading the spec of the Victron it says.....
Parallel connection in case of emergency (Start Assist)
The Cyrix can also be engaged with a push button (Cyrix remains engaged during 30 seconds) or a switch to connect batteries in parallel manually.
This is especially useful in case of emergency when the starter battery is discharged or damaged.

If that is something different to the emergency combiner switch in your wiring diagram then I'm guessing I don't need the emergency combiner switch. However I would then be concerned that the cable to the Victron VSR should be more on the starter battery leads scale?

The switch in my diagram is the better solution Dave. It's wired with starter sized cables and can be used to start the engine, or the engine battery can be totally isolated and the domestics used for everything, or vice-versa,

 

[FairweatherDave]

Fantastic Paul. That makes good sense. Been measuring up for the cables today and hopefully ordering tomorrow - I'll probably pose another basic question in the morning, probably on busbars but need to do more reading up first. Learned a lot today tracing various wires.....going to places I have tried to avoid

 

[dukeofted]

The switch in my diagram is the better solution Dave. It's wired with starter sized cables and can be used to start the engine, or the engine battery can be totally isolated and the domestics used for everything, or vice-versa,

I have my boat wired up with the three cutoff switches and I'm using the Victron Cyrix split charger. I haven't bothered wiring up the manual combine on the split charger as I prefer a mechanical switch for that level of load, and as Paul says you can power any circuit with any combination of batteries using the three switches. So much better than the old 1-2-both switch. The Victron switch is worth the extra cost over the cheap split chargers, we had a cheap one but it failed on in the permanently connected state. You can hear the relay clicking but it doesn't change the electrical connection, possibly fused closed. The Victron is smarter and has temperature sensors and can disconnect if you overload it.

 

[FairweatherDave]

Order placed . Thanks all. Really looking forward to getting stuck in. Was uncertain about busbars but got excellent phone advice from 12volt planet......very reassured I have the right stuff.