Durite 12v 140a cable sizing

[Randomcrumb]

Hi all, new here and learning about electricity as I go along. I'm looking to install one of these (unless anyone advises me strong against)
https://www.durite.co.uk/itm/55875/Voltage-Sensitive-Relay/12V-Voltage-Sensitive-Relay-140A/072733
Only thing is, in looking at the cable sizes need for my run of 10m (5 each way) and it seems unfeasable. Am I doing it wrong or do people just accept voltage drop (or other losses?) by undersizing cables?

 

[Jim]

Hi there, you've not been ignored, your post has only just gone live, no one has been able to see it til now. This is because all new members posts with links within are moderated in our fight against spammers. You should get some response now. Thanks.

 

[WillH]

Hello and welcome. Don't know enough about your problem but someone will be along soon.

 

[pappajohn]

3% volt drop is acceptable which means 95mm2 cable at 140a.

If you were to fit an inverter this may be an average size cable so not unusual.

Sounds very much like arc welder cable.

If you're using it as a split charge relay you may be better with a 60a B2B charger.
This fools the alternator into putting out its full potential rather than trickle charging as even though the relay and cable can handle 140 amps the alternator may only be putting out 4 or 5 amps anyway due to the engine battery being fully charged. .

 

[andyman]

3% volt drop is acceptable which means 95mm2 cable at 140a.

If you were to fit an inverter this may be an average size cable so not unusual.

Sounds very much like arc welder cable.

If you're using it as a split charge relay you may be better with a 60a B2B charger.
This fools the alternator into putting out its full potential rather than trickle charging as even though the relay and cable can handle 140 amps the alternator may only be putting out 4 or 5 amps anyway due to the engine battery being fully charged. .

Are you sure i always thought 25mm was rated at around 150 amps.
Andy

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[Deleted member 57092]

Have a look at Sterling power products website
There is a cable size calculator on there.

 

[tonyidle]

It's a 140A relay. Cable size needs to reflect the current it will be passing rather than the rating of any switching components. Using a simple split charge device I doubt you'll see more than 20A under any circumstances. Cable size is determined initially by its ability to carry the current then by the need to minimise volt drop. If you base your calculations on 30A you'll still have approaching 50% safety margin.

 

[pappajohn]

Are you sure i always thought 25mm was rated at around 150 amps.
Andy

Just checked a second cable size calculator...

12v 140a 5mtr run.... 102mm2

 

[pappajohn]

It's a 140A relay. Cable size needs to reflect the current it will be passing rather than the rating of any switching components. Using a simple split charge device I doubt you'll see more than 20A under any circumstances. Cable size is determined initially by its ability to carry the current then by the need to minimise volt drop. If you base your calculations on 30A you'll still have approaching 50% safety margin.

Agreed, but the op was asking the cable size for a 140a relay.
It's only an assumption it's for a split charge.
It could well be for an inverter running from the engine battery.

 

[Lenny HB]

For a start I wouldn't bother with a voltage sensing relay, more trouble than they are worth and not suitable if you fit solar panels.
I assume the cable is for the split charge relay, I'm interested where you are going to get the 140 amps from.
In practice a split charge will give a max of 20- 30amps for the first 20 min or so then drop down to 10 - 15a providing you have decent size cables, 16 mm sq will do, 25 mm sq a bit better but no essential.

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[Landy Andy]

The cable/wire needs to be matched to the power it’s handling not the power of the relay.

You won’t get the cables that are mentioned above in the relay you’ve listed. There are only small terminals.

I’ve run two of the above listed relays in Land Rovers (let me know if you want a second hand one), and they are great and have proved to be reliable, but only have, from memory, M5 terminal on the back. I can find exact details if required as they are just in box somewhere in my garage.

Andy.

 

[mitzimad]

as someone else has said voltage sensing relays can be a pain especially when using solar on my self buiild i used a normal dumb 100 amp relay but used a smart com relay to trigger it which saves having to find an engine running signalyou can also switch the fridge 12v with the smartcom i dont know how to do a wiring diagram on a computer but its out there somewhere as i found it

 

[mitzimad]

i will explain how it was explained to me.A voltage sensing relay compares voltage on either side to decide when to make or break on a sunny day with a solar panel the voltage is higher on the leisure battery side and the vsr breaks the circuit A smart com only sense the voltage from the vehicle battery side so keeps the dumb relay made which means you get charging from both sources im sure there will be better qualified people than me who disagree but it worked for us

 

[tonyidle]

Agreed, but the op was asking the cable size for a 140a relay.
It's only an assumption it's for a split charge.
It could well be for an inverter running from the engine battery.

I clicked on his link - it's a voltage-sensitive split-charge relay and of little use for an inverter which would normally be switched on or off via its (low current) control circuitry.

 

[LukElfin]

i will explain how it was explained to me.A voltage sensing relay compares voltage on either side to decide when to make or break on a sunny day with a solar panel the voltage is higher on the leisure battery side and the vsr breaks the circuit A smart com only sense the voltage from the vehicle battery side so keeps the dumb relay made which means you get charging from both sources im sure there will be better qualified people than me who disagree but it worked for us

Can anyone explain this in a little more detail. I have been running a Durite VSR from starter to leisure batteries alongside solar to leisure batteries with no issues for 18months or so but, am concerned the vsr is constantly trying to balance the loads and now seem to be having starter battery issues. Have decoupled for now whilst trying to problem solve.

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[Lenny HB]

VSR's are not suitable for use if you have your solar charging the starter battery as well as the leisure batteries. If the starter battery is being charged by solar when the voltage rises high enough to switch the VSR the leisure & starter batteries will be in parallel.

 

[LukElfin]

Thanksnyou for your response and apologies if I am missing some comprehension here but, what issue would there be in the batteries running in parallel?

 

[LukElfin]

Presumably due to the hugh power output that the starter battery needs to start the van??

 

[autorouter]

Presumably due to the hugh power output that the starter battery needs to start the van??

Two issues really, that's one of them. The other is that it's very easy to run a leisure battery flat, by leaving something switched on when you go out for the day, for example. You don't want to end up unable to start the engine, especially in the middle of nowhere with a flat phone battery too.