Cable size

[mikewroe]

Looking to buy a second hand Sterling 30 amp B2B charger. Reading the data sheet it suggests using 6mm cable up to a 8m run.
This looks a bit light?
Renogy by comparison show a larger cable section, even bigger on the input side.
I know big is best but cost and easy of wiring also factor in.
I was hoping to use 10mm or less as I can't crimp fittings to larger cables.

 

[Okta]

I find this cable size calculator very helpful.

Cable Size Calculator - Cable sizing and selection | 12 Volt Planet

Cable Size Calculator - Read our guide to selecting cable of the correct type and size for your automotive, marine or leisure wiring application. Auto and Marine Electrical Components from 12 Volt Planet

www.12voltplanet.co.uk

 

[wildebus]

Looking to buy a second hand Sterling 30 amp B2B charger. Reading the data sheet it suggests using 6mm cable up to a 8m run.

Are you sure it is saying 6mm upto a 8M run? Got a link to the data sheet?
6mm cable is ok for a shortish connection, but for a typical B2B installation with a 30A, I would have though 10mm2 would be minimum for a short run where the Starter and Leisure batteries are within maybe 2M, 16mm2 for maybe 3-6M and 25mm2 for longer. If you had a 8M run I would probably go 25mm2
FWIW, I tend to use 16mm2 cable for B2Bs.

This looks a bit light?
Renogy by comparison show a larger cable section, even bigger on the input side.
I know big is best but cost and easy of wiring also factor in.
I was hoping to use 10mm or less as I can't crimp fittings to larger cables.

Main question is how long is the run (including twists and turns) between the two batteries?

 

[wildebus]

Looked at Manual ... 6mm2 for upto 8M, 10mm2 for over 8M and upto 10M.
How low can you go!

 

[mikewroe]

Thanks everyone. Using the calculator suggested it appears 16mm will be OK for a 6.3m run, that is keeping under the 4% voltage drop. That's a far cry from the OEM suggestion.
16mm it will be, just got to find the cable at a sensible price and a friendly crimper

 

[St3v3]

Thanks everyone. Using the calculator suggested it appears 16mm will be OK for a 6.3m run, that is keeping under the 4% voltage drop. That's a far cry from the OEM suggestion.
16mm it will be, just got to find the cable at a sensible price and a friendly crimper

Are you going to the hereford meet?

 

[wildebus]

Thanks everyone. Using the calculator suggested it appears 16mm will be OK for a 6.3m run, that is keeping under the 4% voltage drop. That's a far cry from the OEM suggestion.
16mm it will be, just got to find the cable at a sensible price and a friendly crimper

Remember to add fuse protection. need a holder by the starter battery connection and one on the output of the B2B.
Use Midi-Holders, not spade fuses (not that you could get standard fuse holdes that would work with 16mm2 cable )

 

[mikewroe]

Are you going to the hereford meet?

Thanks for the suggestion, meets aren't are thing.

 

[St3v3]

Thanks for the suggestion, meets aren't are thing.

Shame, we could have sorted this out no problem

 

[witzend]

spec Dom could manage that or

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[Mr and Mrs Tupcox]

I've used 10mm which is rated a 70amp at 5 mtrs ..with 40amp breakers both ends next to eg and by lb battery. In 20mm flexible pipe under van clipped ever foot.so is that wrong.?

 

[wildebus]

I've used 10mm which is rated a 70amp at 5 mtrs ..with 40amp breakers both ends next to eg and by lb battery. In 20mm flexible pipe under van clipped ever foot.so is that wrong.?

two aspects to cable sizing ....
1) ampacity. This is the amount of current the cable can safely carry.
2) voltage drop. how much voltage is lost from one end to the other for a given current.

Note that for point 1 the quoted current assumes certain things, such as it is in free air (so heat dissipation is maximized). If buried under a floor in conduit, the ampacity will be reduced.
For point 2, remember that a percentage loss quoted is actually a lot more in use than it may sound as a raw value. As an example, a full battery is say 12.7V and a battery at half-charge is 12V. The difference between the two is only 6%. So when you use a voltage drop calculator and it tells you the voltage drop is 4% for example, that is actually a fairly large number (as it is the percentage drop between 12V and ZERO V).
In the case of a 5M run of 10mm2 cable on a 30A B2B, you are dropping around 0.55V. That means the charger needs to work harder to output the same current. It might well mean the charger will not even be able to output the quoted current due to a low input voltage.

Point 1 is important for safety; Point 2 is important for efficiency. (if you specify your cable size to meet point 2, point 1 should generally be met automatically)

 

[merl]

two aspects to cable sizing ....
1) ampacity. This is the amount of current the cable can safely carry.
2) voltage drop. how much voltage is lost from one end to the other for a given current.

Note that for point 1 the quoted current assumes certain things, such as it is in free air (so heat dissipation is maximized). If buried under a floor in conduit, the ampacity will be reduced.
For point 2, remember that a percentage loss quoted is actually a lot more in use than it may sound as a raw value. As an example, a full battery is say 12.7V and a battery at half-charge is 12V. The difference between the two is only 6%. So when you use a voltage drop calculator and it tells you the voltage drop is 4% for example, that is actually a fairly large number (as it is the percentage drop between 12V and ZERO V).
In the case of a 5M run of 10mm2 cable on a 30A B2B, you are dropping around 0.55V. That means the charger needs to work harder to output the same current. It might well mean the charger will not even be able to output the quoted current due to a low input voltage.

Point 1 is important for safety; Point 2 is important for efficiency. (if you specify your cable size to meet point 2, point 1 should generally be met automatically)

All really good points David, furthermore I'd add a couple of additional considerations:
1, When you see a B2B rated at 30A it's natural to assume that the unit will deliver a 30A charge current into the battery, this is true of Victron and Renogy for sure (I've measured and confirmed both) but Sterling's quoted amperage (in this case 30A) is on the INPUT side. This means that ANY voltage drop on the INPUT wiring will result in a reduction of output current into the battery.
2, Correct charging requires very accurate monitoring and control over the battery voltage, the battery voltage is being monitored at the output of the charger and so ANY voltage drop on the output cables between B2B and battery will result in inaccurate and inefficient charging. You shouldn't try and compensate for this voltage drop by increasing the voltage settings on the charger, this is because the error is entirely dependant an current and the current isn't constant, therefore the error isn't fixed and stable. Again use the 12V planet calculator and set the distance and cable size between B2B and battery, I'd suggest a maximum error of 0.1V between the two.

 

[wildebus]

All really good points David, furthermore I'd add a couple of additional considerations:
1, When you see a B2B rated at 30A it's natural to assume that the unit will deliver a 30A charge current into the battery, this is true of Victron and Renogy for sure (I've measured and confirmed both) but Sterling's quoted amperage (in this case 30A) is on the INPUT side. This means that ANY voltage drop on the INPUT wiring will result in a reduction of output current into the battery.

Just a comment on the Victron .... The latest Orion-XS 50A B2B has settable current maximums for both INPUT and for OUTPUT.
And the maximum setting for both is 50A, so for that charger, to get the full 50A into the Leisure Battery (ignoring efficiency losses), the Input voltage MUST be greater than the Output voltage.
Victron have typically (even traditionally) run their chargers to have a higher capability than quoted - so the old 30A Orion might have output 33A, but they have moved away from that and tightened up the control so 50A means 50A now.

2, Correct charging requires very accurate monitoring and control over the battery voltage, the battery voltage is being monitored at the output of the charger and so ANY voltage drop on the output cables between B2B and battery will result in inaccurate and inefficient charging. You shouldn't try and compensate for this voltage drop by increasing the voltage settings on the charger, this is because the error is entirely dependant an current and the current isn't constant, therefore the error isn't fixed and stable. Again use the 12V planet calculator and set the distance and cable size between B2B and battery, I'd suggest a maximum error of 0.1V between the two.

Indeed, so ideally the B2B should be close to the Battery being charged.
Not a fan on the 12V Planet calculator in that the choice goes 'green' a little prematurely (IMO).
0.1V drop is a tight number to aim at though .
And as well as voltage drops in cable, there are connectors and fuses to consider (which is why their "under 4%, good to go" is too loose (again, in my opinion anyway)).

 

[merl]

Surprising how much voltage drop there is across a fuse. From memory 200A fuse (and it's connectors etc) drops around the same as 4 metres of 16mm cable!

 

[mikewroe]

Thanks everyone, still so much still to learn about 12v electrikery.
I have now bought the 2nd hand Sterling 30amp B2B. One question I never considered is where to mount it, but from Wildebus's comment it will be as close to the leisure batteries as possible.
I have sold the old 3 way fridge and replaced with 12v compressor (new supply cables fitted) so the old 12v supply is redundant.
I am hoping now to simply remove the split charge relay which I hope to find in the o/s B post area, as I understand leaving it in can back feed a rouge measurement. Does any one know if the electric step is also wired through that relay, must keep that working for safety reasons.

 

[wildebus]

Thanks everyone, still so much still to learn about 12v electrikery.
I have now bought the 2nd hand Sterling 30amp B2B. One question I never considered is where to mount it, but from Wildebus's comment it will be as close to the leisure batteries as possible.
I have sold the old 3 way fridge and replaced with 12v compressor (new supply cables fitted) so the old 12v supply is redundant.
I am hoping now to simply remove the split charge relay which I hope to find in the o/s B post area, as I understand leaving it in can back feed a rouge measurement.

Does any one know if the electric step is also wired through that relay, must keep that working for safety reasons.

What motorhome do you have? the answer may well be dependant on the model.
I think the step, the fridge and the oem split charge are all independant of each other.

 

[mikewroe]

Thanks, it's a 2008 Boxer with an Eldis Sunseeker conversion. BCA power supply unit fitted.

 

[wildebus]

Thanks, it's a 2008 Boxer with an Eldis Sunseeker conversion. BCA power supply unit fitted.

personally not very familiar with the BCA units so can't really advise other than to say maybe just trial and error to check unless someone has any knowledge on this one?

 

[merl]

One question I never considered is where to mount it, but from Wildebus's comment it will be as close to the leisure batteries as possible.

If you're going to use 16mm cable on the output then closer than 1.5M between the two will give you that 0.1v drop or less.
If you've not purchased your cable yet then this may interest you

20mm² Flexible Battery Cable 135 Amp Black & Red - Automotive & Marine | eBay

Conductor Cross Section : 20mm². Not sure what you need?. Give one of our trained sales advisors a call and they'll be glad to help you out. Product Information. ALM Electrical Solutions is the UK's premier supplier of auto electrical parts and products.

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I used some recently and it's nice stuff. 20mm so slightly heavier/better but cheaper than 16mm probably because of it's slightly oddball size.