Mains Hook-up Cable Considerations and your Charger.


Note : Since we first wrote this page back in 2012 there has been a growing awareness that large battery banks generally can cause multiple issues inside the Motorhome. 

During 2016 we have seen fewer battery banks exceeding 200Ah. However, a Motorhome Habitation battery charger still places a significant load on the Mains 240v EHU that the below is still relevant, especially for those visiting places with only a 8a EHU supply.


With multiple battery banks of up to 440Ah now becoming more common in Motorhomes the Battery Charger is  a significant consumer of the 230v supply current. 

It is therefore important to maximise every amp you take from the site bollard, and you might be surprised what part the Hook-up cable plays. 

Although Caravan/Motorhome EHU cable is usually referred to as a '16amp' cable, the wire itself is actually rated at 25amp, It is the connectors that are rated at 16amp, if it is BS7671 compliant. 

The reason the British Stand for an EHU cable, BS7671, specifies a 25amp cable for extra safety capacity with minimal heating in the cable. 

A EHU Lead made from 16a/1.5mm cores will get significantly hotter than a 25A cable with 2.5mm cores. As the temperature rises so the resistance increases inflicting a bigger voltage drop.

It is not just about safety but usability, as the correct cable gives more available power inside the Motorhome.


We recently purchased a 25metre Caravan mains hook-up Lead that was described as 16amp Caravan/Motorhome cable. The cable was indeed 16amp cable but with undersized 1.5mm cores, when it should have been 25 amp (2.5mm Copper cores) specified by BS7671, so was not up to standard for Motorhomes/Caravans, yet it was advertised as specifically for Motorhomes/Caravans. 

There is a lot of this substandard cable being sold.

 

If the cable is further poorly assembled with loose or poor connections, the impact this will have (apart from it breaching safety regulations) is that the voltage/current drop down a 25metre cable may be significant, meaning that the 230v/16amps power you take from the Site Bollard might be significantly less by the time it gets inside the Motorhome. In other words, the 'extra' imposed by a poor cable might mean that the 16amps being used in the Motorhome results in 18amps being drawn at the Site Bollard which then trips out. 

In order to prevent the Site Bollard from tripping, the power consumed inside the vehicle might need to be reduced to 14amps. Exagerated, but shows the issue.


As the cable heats up, so the power drop down it will increase. Copper has the lowest resistance at lower temperatures and this rises as the cable gets hot.

 

If the charger is a high power 36Amp output unit (about 4 amps in 220v terms) in full flow, subtract it's 4 amps from the 'real' 14amps available, to leave just enough power for the Fridge and  the space heater, with little spare for the other 240v items.

If you happen to be at a site with only 8/10amp available, you might find that the charger takes so much that the site bollard trips out when you switch on a 220v 60w lamp!

If you do have a high capacity charger it might be worthwhile turning it off during the day when you require the amps for other things, then back on at night?

 

To maximise the 'real usable' current in the Vehicle use the shortest, fattest EHU cable you can. If you can get close enough to a site bollard, then use a 10metre lead.

 

The correct cable assembly should use 16A connectors with 25Amp, 2.5mm cable. The higher resistance and greater frailty in a 16amp, 1.5mm cable may cause significantly higher heat build up, hence the point of the regulations. 

The regulations stipulate a MAXIMUM 25 metres length, again to contain voltage drop/heat build up.

We have seen one 30metre 1.5mm cable for sale, which is a fire waiting to happen, that is when everything is new and perfect, imagine the extra risk as it ages?


However, there is another, very significant, factor with 1.5mm cable that we had not really thought about until recently.


Here is a cable in the photo below that, despite the 2.5mm core, was far from perfect. 

It can be seen in the photo that both the Earth and Live cables have suffered in the 'Copper cable clamp' at the connector head. Even though the thicker core should result in a better contact inside the screw down connector with a longer more durable life, the Copper has deteriorated with the Live cable breaking away completely and then burning. 



The second photo below shows how hot the cable has been, the insulation melting and the Copper overheated.



This cable came off a van where everything worked semingly perfectly, except the 230v Water Heater would only get the Water Luke warm.
The charger ran the lights and charged the battery, the Television worked perfectly on 230v and our Volt Meter inside the van showed 232v at the 13amp sockets.
I cut the cable back to sound Copper and fitted a new Blue Socket and the Water Heater worked perfectly.

I can only guess that the contact inside the connector was just good enough to supply everything, except the high current of the Water Heater?

The cable above shows no sign of damage. The sheath clamp was tight on the Orange outer sheath. The marks of which are clear and sharp, which clearly doesn't have the 'ghosting' that is often seen when the sheath is subject to stress and movement, yet the Copper had suffered. I then looked very carefully at the set-up to see if I could identify any weakness.   

When I fitted the new connector I noticed that the 2.5mm Copper Cores were an easy fit into the Brass tube where they get clamped solidly by a screw. So easy were they to insert, that I considered adding extra Copper strands into the tube to get a really good contact.

That made me think about how sloppy the much thinner 1.5mm of Copper strands would be inside what is an International DIN connector, therefore the same Plug used on 1.5mm cable.

Imagine 1.5mm Copper cores (maybe fewer AND thinner strands that 'escape' up the side of the screw to avoid being 'clamped') inside the large diamater Blue Brass connecting clamps where the screw should tighten down onto the Copper strands for good contact?  I wonder how much care the factories who assemble the 1.5mm cable take?

A thinner 1.5mm core cable inside a standard 16a Plug/Socket 2.5mm Brass 'tube' cannot possibly provide as good long term connectivity with minimum voltage drop, can it? From my brief experiments with a 1.5mm cable inside the 16a Blue connector resulted in only around 50% of the Copper strands being properly 'captured' under the screw head. 
That might effectively make the cable equivalent to just 0.75mm, way below the 2.5mm standard.  

With a 2.5mm cable, although not all the 2.5mm strands were 'captured' under the screw, they were packed much more tightly together ensuring almost all were in contact with each other and the Brass tube wall. 



When people quote the Electrical rating figures arguing the case for the safety of 1.5mm 16amp cable, they are assuming that everything to do with the cable is optimum, yet it is unlikley to be. Potentialy just a couple of strands carrying 16amps of power, that may lead to a far higher voltage drop/Overheating in the real World?


If you use undersized 1.5mm cable, we suggest you don't, but at the very least regularly check the security of the Copper cores at each connector. Preferrably folding over the bare Copper cable inside the screw clamp thereby doubling the amount of Copper inside the Brass tube for the screw to bite securely onto, for obvious reasons.




Probably why we have such things as British Standard 7671, 2001 for Caravan/Motorhome Mains Hook-up Cable, which states :

"For connecting the caravan to the Site socket outlet, a connecting cable is required. This cable must be 3 core 2.5mm²PVC/PVC, flexible cable and 25 metres (+/- 2 metres) in length". 

 

The below Caravan Club document covers the regulations very well :