DC Wiring help

[electricpuha7]

Im looking for some advice on some dc wiring i have in place. Currently i have some monitoring equipment that wont run when connected to a run of wire about 40m long (1.5mm diameter) it is connected to a 12v 100ah battery and what i think is happening is the voltage drop over that distance is preventing it from running.

The device powers on but the pump wont run and kicks in a power failure error (draws about 6A current to start pump). I have tried the model down in equipment and it works fine (draws about 5A current to start pump)

Im just wondering how much to scale up wire diameter when i rewire it or if this is even the most likely issue?

I was planning on putting in 4mm wire but concerned this may be overkill

 

[Lucien Nunes]

12V DC equipment will typically tolerate a 10% voltage drop. You mention 6A load, I assume that's the total.
12 " 10% / 6 = 0.2 ohms maximum cable resistance.
80 metres of conductor, 1 sq. mm cable resistance 0.019 ohms per metre.
0.019 * 80 / 0.2 = 7.6 sq. mm cable minimum.
So your 4 sq. mm is not overkill, in theory it's not even sufficient. It will work but the unit might throw battery voltage errors prematurely as the battery runs down. What is the equipment?

With 12V equipment, sizing of cables over a metre or two in length is often governed by voltage drop, not current rating. For any given load power in watts, the current is approximately 20 times higher on 12V than on mains and the permissible voltage drop 20 times lower. Therefore a cable 400 times larger is needed for the same percentage drop per watt of load.

 

[telectrix]

6mm at least. even that will give you a 1.6V drop. working at 12V is far more critical than at 240V.

 

[James]

Is it possible to move the battery closer to the pump?

 

[pc1966]

If there is a huge start to running current ratio, and thicker wire or shorter route/placement is not possible, you could have a 2nd smaller battery near the pump for the initial surge, but it will need protection against fault current itself. You might just get away with a big "super capacitor" but it might not be any cheaper.

 

[DPG]

Is it possible to move the battery closer to the pump?

Always easy to overlook the simpler options!

 

[electricpuha7]

Always easy to overlook the simpler options!

Unfortunately this is close to a river that rises and falls rapidly, the pump is waterproof but the battery and solar setup.. not so much

 

[pc1966]

In that case probably your cheapest option is simple thicker cables. For outdoor stuff you would normally use either SWA (steel wire armour) cable if it needs to be robust against physical damage, buried in the ground, etc, or a UV resistant flexible cable such as H07RN-F which is far easier to wrangle and terminate with plastic IP68 glands, etc.
Here is an example of 2-core 6mm rubber cable:

H07RN-F Rubber Cable - 2 Core 6mm2 / HO7RN-F-2C-6

Buy H07RN-F Rubber Cable - 2 Core 6mm2 / HO7RN-F-2C-6 | Order By 1pm For Same Day Shipping | Free Delivery Over £150 *Conditions Apply |

www.cse-distributors.co.uk

Generally the 2-core versions is more difficult to find, as for mains use you are looking at 3-core for practically all single phase applications (L, N and E) or 4 and 5 core for three-phase. If you need thicker conductors you might find it easier to get 4-core 6mm and parallel pairs to get 12mm equivalent.

 

[electricpuha7]

12V DC equipment will typically tolerate a 10% voltage drop. You mention 6A load, I assume that's the total.
12 " 10% / 6 = 0.2 ohms maximum cable resistance.
80 metres of conductor, 1 sq. mm cable resistance 0.019 ohms per metre.
0.019 * 80 / 0.2 = 7.6 sq. mm cable minimum.
So your 4 sq. mm is not overkill, in theory it's not even sufficient. It will work but the unit might throw battery voltage errors prematurely as the battery runs down. What is the equipment?

With 12V equipment, sizing of cables over a metre or two in length is often governed by voltage drop, not current rating. For any given load power in watts, the current is approximately 20 times higher on 12V than on mains and the permissible voltage drop 20 times lower. Therefore a cable 400 times larger is needed for the same percentage drop per watt of load.

It is an ISCO samp

In that case probably your cheapest option is simple thicker cables. For outdoor stuff you would normally use either SWA (steel wire armour) cable if it needs to be robust against physical damage, buried in the ground, etc, or a UV resistant flexible cable such as H07RN-F which is far easier to wrangle and terminate with plastic IP68 glands, etc.
Here is an example of 2-core 6mm rubber cable:

H07RN-F Rubber Cable - 2 Core 6mm2 / HO7RN-F-2C-6

Buy H07RN-F Rubber Cable - 2 Core 6mm2 / HO7RN-F-2C-6 | Order By 1pm For Same Day Shipping | Free Delivery Over £150 *Conditions Apply |

www.cse-distributors.co.uk

Generally the 2-core versions is more difficult to find, as for mains use you are looking at 3-core for practically all single phase applications (L, N and E) or 4 and 5 core for three-phase. If you need thicker conductors you might find it easier to get 4-core 6mm and parallel pairs to get 12mm equivalent.

Thanks for that, I need to get my hands on some 4 core cable as it requires 2 for the power and 2 for data out and pulses from relay in. I already have conduit buried so hopefully I can just hook onto the cable in there and pull the thicker wire through

 

[electricpuha7]

If there is a huge start to running current ratio, and thicker wire or shorter route/placement is not possible, you could have a 2nd smaller battery near the pump for the initial surge, but it will need protection against fault current itself. You might just get away with a big "super capacitor" but it might not be any cheaper.

I tried the second smaller battery at the unit itself and it allows the pump to run but the automatic functions don't work (its activated via relay when the water level reaches a set height)

 

[pc1966]

I tried the second smaller battery at the unit itself and it allows the pump to run but the automatic functions don't work (its activated via relay when the water level reaches a set height)

Were you able to measure the pump voltage during that sort of test?

 

[pc1966]

Thanks for that, I need to get my hands on some 4 core cable as it requires 2 for the power and 2 for data out and pulses from relay in. I already have conduit buried so hopefully I can just hook onto the cable in there and pull the thicker wire through

You wont need thick conductors for the "data" side, so if you have to use a single big cable then look at 2.5mm 7-core as then you can have 2 for "data" and the remaining 5 can be a 2-parallel for '+12V' and 3-parallel set for '0V' to keep the DC power impedance down.

For example:

H07RN-F Rubber Cable - 7 Core 2.5mm2 / HO7RN-F-7C-2.5

Buy H07RN-F Rubber Cable - 7 Core 2.5mm2 / HO7RN-F-7C-2.5 | Order By 1pm For Same Day Shipping | Free Delivery Over £250 *Conditions Apply |

www.cse-distributors.co.uk

 

[pc1966]

It is quite a chunky cable though, but hopefully you can use a 25mm hole-size cable gland to grip and seal it.

 

[pc1966]

If the cable is in duct and protected from direct sunlight, etc, you could use something a bit lighter such as 12-core 1.5mm 'YY' control cable:

YY Cable 12 Core 1.5mm2 / YY-12C-1.5

Buy YY Cable 12 Core 1.5mm2 / YY-12C-1.5 | Order By 1pm For Same Day Shipping |

www.cse-distributors.co.uk

Again, the idea is 2 cores for data and remaining 10 cores as 2 * 5 sets giving the equivalent of 7.5mm conductor sizes for power.

 

[pc1966]

Also available as 'SY' with a steel braid for some protection (nothing like SWA though):

Superlec 12CORE 1.5MM SY PVC CONTROL CABLE - BS EN 50525-2-11 CLASS 5

OVERVIEWSY cables are used as an interconnecting cable between fixed and mobile equipment in conveyors, assembly lines, production lines and machine tool manufacture where the galvanised steel wire braid armour gives excellent mechanical protection.

www.superlecdirect.com