Mains Hook-up Cable Considerations and your Charger.
Note : Since we first wrote this page back in 2012 there has been an awareness that large battery banks generally 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 is so that the voltage drop over 25 metres is negligable 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 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 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.
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 30Amp output unit (about 4 amps in 240v 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 240v 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 in a 16amp, 1.5mm cable may cause significantly higher heat build up, hence the point of the regulations.
The regulations stipulated 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.
British Standard 7671, 2001 for Caravan/Motorhome Mains Hook-up Cable states :
"For connecting the caravan to the Site socket outlet, a connecting cable is required. This cable must be 3 core 2.5mm², PVC/PVC, flexible cable and 25 metres (+/- 2 metres) in length".
The below Caravan Club document covers the regulations very well here :
Caravan mains electrical Installations BS7671.pdf
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Type : pdf