How many amps does a static caravan need?

[epsilonGreedy]

A mains electric meter installation is scheduled for next Monday and then later in the day my own electrician will fit a 2 x RCD CU in the meter box and run 22 meters of armoured cable to the caravan.

 

He was not impressed with the residual 3 meters of mains cable dangling from the underbelly of the caravan which connects into a small 6 x RCD CU in a bedroom cupboard, he reckoned it could handle only 16 to 20 amps. The suggestion is to run 6.5mm2 cable good for 40amps all the way from the meter box into the caravan.

 

The caravan looks very well insulated under the floor and has double glazing plus gas central heating. I reckon the electrical crunch point would arrive should the c/h boiler fail in the middle of a cold spell and we resort to a few 2kw heaters and then switch on the kettle.

 

The price for the job is just £160 for time and materials which I am ok with but 40amps seems ott. I will be digging the trenches.

Edited October 11, 2018 by epsilonGreedy

[joe90]

It depends on what you are going run in the van, electric shower?, electric heaters?, electric kettle?, tot them all up (plus lighting) and gauge what your needs are.

[ProDave]

I have mine connected on 6mm SWA on a 40A mcb (no RCD at the source in my meter box) that feeds the small CU in the caravan. 

 

Remember you don't connect a caravan to a PME earth, so I cut the armour off the cable under the 'van insulated it and just routed the inner cable up into the 'van and connected it's own earth rod.

[Jeremy Harris]

On 11/10/2018 at 10:39, epsilonGreedy said:

A mains electric meter installation is scheduled for next Monday and then later in the day my own electrician will fit a 2 x RCD CU in the meter box and run 22 meters of armoured cable to the caravan.

 

He was not impressed with the residual 3 meters of mains cable dangling from the underbelly of the caravan which connects into a small 6 x RCD CU in a bedroom cupboard, he reckoned it could handle only 16 to 20 amps. The suggestion is to run 6.5mm2 cable good for 40amps all the way from the meter box into the caravan.

 

The caravan looks very well insulated under the floor and has double glazing plus gas central heating. I reckon the electrical crunch point would arrive should the c/h boiler fail in the middle of a cold spell and we resort to a few 2kw heaters and then switch on the kettle.

 

The price for the job is just £160 for time and materials which I am ok with but 40amps seems ott. I will be digging the trenches.

 

 

No such thing at 6.5mm² cable, as far as I'm aware.  You can easily calculate the cable size needed for a given load (remembering to apply the diversity rules in BS7671) from this handy cable voltage drop calculator: https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

 

Select the cable type (most probably two core SWA, as you'll need to TT the caravan installation) and also that it will be buried.  Select an ambient of 10 deg C, as that's about the maximum soil temperature, then just stick in the 22m length and your estimated load and it will give you the required cable size and maximum voltage drop.

 

As a guide, a 10 kW load at 22m would need 6mm² two core SWA, and I'd guess that 10 kW (allowing for diversity) should be more than enough for a caravan supply.

 

Edited to say that I cross posted with @ProDave

[epsilonGreedy]

Follow up:

1. Job completed successfully yesterday in 2 hours.
2. The main surprise was the size of the 40amp armoured cable, this stuff is huge and difficult to lay precisely in trenches.
3. The electrician gave me the reel of 22m before starting the job so I could uncoil it into the trench route before he started. If any twists get left in it is like wrestling with an angry python.
4. The electrician noted that a PMU? earthing connection back to the substation was missing within the builder's supply meter box on legs, so as per @ProDave's advice he put in a local TT earth, apparently this has to be a regulation minimum distance from the meter box.
5. I had ordered a smart meter which caused congestion within the meter box.
6. I got a mild ticking off from the electrician for crossing the 40amp caravan cable underground with the inbound 100amp utility supply cable, he said one sinking scaffolding leg could sever and connect the cables eliminating the protection provided by the main fuse box.

[Jeremy Harris]

If this is a caravan supply then the TT earth is supposed to be at the caravan end, not the meter box end, to comply with the regs, I believe.  This also means fitting the RCD at the caravan end, too.

 

You aren't supposed to connect a caravan to a PME/TN-C-S supply and use that earthing scheme, neither is a temporary building supply allowed to be earth protected this way, it also has to be TT.

[ProDave]

1 hour ago, epsilonGreedy said:

1. I had ordered a smart meter which caused congestion within the meter box.

Why?  What benefit do you believe it will give you?

 

1 hour ago, epsilonGreedy said:

 6. I got a mild ticking off from the electrician for crossing the 40amp caravan cable underground with the inbound 100amp utility supply cable, he said one sinking scaffolding leg could sever and connect the cables eliminating the protection provided by the main fuse box.

 

Highly unlikely. More likely to short the supply cable making a very big bang.

[epsilonGreedy]

1 minute ago, ProDave said:

Highly unlikely. More likely to short the supply cable making a very big bang.

 

 

He raised an interesting point about sub station trip out behavior and something I will remember because he reinforced the point with an anecdote about the death of a local agricultural worker.

 

Apparently when the sub station trips a supply it then makes 3 re-connection attempts at 10 second intervals. The poor farm worker in question drove into a supply cable, there was a big bang at which point he jumped off his machine but then seeing no further electrical dramatics he attempted to remount his machine just at the point of a 10 second retry and his body created a lethal earth.

 

The retry logic is there to provide automatic re-connection following a lighting strike though you probably know that. Surely he was making a reasonable point about maintaining a physical separation between an unfused mains supply and cabling downstream of the fused meter box?

[epsilonGreedy]

26 minutes ago, ProDave said:

Why?  What benefit do you believe it will give you?

 

 

No visiting meter readers knocking on the door asking where I have hidden the meter cabinet.

 

Satisfy a mild curiosity about consumption figures. Folks here like to publish snapshots of their PV generation/consumption graphs and presumably I will get access to a web site maintained by the supplier with such graphs.

 

Finally I got the impression during the sign-up process I was going to get a smart meter unless I specifically objected. Is there a downside?

[Jeremy Harris]

Interesting anecdote re: the agricultural worker, but flawed because there will normally  be a fuse in the distribution line from the sub-station, not any form of automatic reconnection system.  The explosion from a short at a few hundred amps on an LV cable is mighty impressive though, and more than enough to kill.  The local LV fuses are typically around 500 A, with a maximum value of 800 A.  If there's a major cable fault, then that fuse is supposed to blow to protect the local network cables.  It's debatable whether it actually does, though, as there are a few tales around of concentric incomers burning back when shorted, without the distribution fuse blowing.

 

We had a pole fire outside our old house a few years ago, when an old pitch-filled pot joint failed and set the pole alight like a burning Christmas tree, dripping burning pitch.  The power didn't shut off, as the thing kept making periodic flashes as a new short developed and then burned out, so it seems to be true that the local LV distribution network fuses can fail to rupture under some fault conditions.  When the fire brigade arrived they had to just sit and wait until the power was turned off.  All told the pole was burning and banging for around an hour, then the power stayed off for another three or four hours whilst the cables were replaced and new joints made.

[epsilonGreedy]

17 hours ago, JSHarris said:

The local LV fuses are typically around 500 A, with a maximum value of 800 A.  If there's a major cable fault, then that fuse is supposed to blow to protect the local network cables.  It's debatable whether it actually does, though, as there are a few tales around of concentric incomers burning back when shorted, without the distribution fuse blowing.

 

 

Does this not lend credibility to the account of a lighting surge protector killing the farm worker during its 10 second re connection cycle?

[Jeremy Harris]

16 minutes ago, epsilonGreedy said:

 

Does this not lend credibility to the account of a lighting surge protector killing the farm worker during its 10 second re connection cycle?

 

There isn't a reconnection cycle, for safety reasons, and it's not connected with lightning surge arrestors, what they do, in effect, is short an over-voltage spike to ground. 

 

For safety reasons, a local LV distribution point is fused, or sometimes protected by a resettable overload breaker.  The idea is to protect the MV/HV side of the local distribution transformer, by trying to ensure that the LV side is disconnected in the case of an overload.  Trying to reconnect when there is an overload condition jeopardises more of the network.  MV and HV fuses are harder to change, and are usually either ejecting bayonets or explosive fuses, and both require a higher degree of skill to change, plus they need the MV/HV side power to be disconnected, which causes disruption over a wider area, hence the reason that the LV side is designed to disconnect first and stay disconnected until reset.