My son and I are converting an old horse box into a catering trailer. It is planned that we use a generator to run two fridges, one freezer and some LED lights. Maybe a couple of vintage type 240v lamps also. The wiring I have decided is this. If here is something wrong would somebody please advise? Also, is better to create a ring circuit rather than just one wire down one side of the trailer and one down the other. Generator, 2.8 - 3 Kw output. 13 amp plug to external, waterproof socket on the trailer. Cable from the socket to a consumer unit with two 16 amp RCB's. One RCB to power 1x bottle fridge,(180w) + 1x double bottle fridge(230w). The second RCB would power one fridge (180w) and 1x freezer (150w), LED strip light 5mtrs (50w). Then maybe 2-3 60w vintage lamps. Cable will be 2.5mm twin and earth in heat proof flexible cable. Do you consider this okay. Should I earth the generator through a ground spike. Any help would be appreciated.
Mobile or transportable units have their own set of regulations. It was section 717 but my copy of BS7671 is 2008 so a bit old. You must have RCD protection or if using an IT supply an insulation monitoring device, cable types are specified, socket outlets IP44 or better, there are diagrams showing the examples.
There are four broad types of refrigeration units, Peltier uses a DC supply normally 12 volt but have seem built in power supplies so will work on 230 Vac, the absorption can also run on 12 volt or 230 volt depending on element and does not really care if AC or DC and often can also run on gas, but in both cases not very efficient.
The heat pump used in most domestic fridges uses a single phase AC motor which is very susceptible to volt drop and although running may only draw 60W the start current can be 2000W so any generator needs to be able to supply that peak power, or likely it will burn out the overload built into the motor which is designed to operate if switched back on before the pressure in pipes has dropped, also they are very sensitive to volt drop.
There are a couple of variations on this, specials with DC motors designed for use on board boats, and the inverter drive which uses a three phase motor with a soft start inverter built in, both will have a much lower starting amps, with my inverter freezer it uses a max of 110W and that is when the frost free heater cuts in.
With a freezer using frost free means there is a fan circulating the air when motor is running, this in turn means an even temperature in the freezer compartment with retail food one has to test and keep records of the temperature so in real terms either a chest or frost free upright to have non frost free with an upright design you could not ensure all the freezer was cold enough. Also need to consider how often the door is opened.
Noise is also a problem with generators so using an inverter generator will reduce both cost and noise as it ticks over when no load, but it also takes time to come up to speed when load applied so with single phase motor of refrigeration devices this could cause a problem, so likely you will need inverter drive refrigeration devices.
With max power of 3 kW and size of horse box no need for a final ring circuit. As to earthing it would depend where the generator is or if likely to use external power supply. However every thing will need bonding. I would tend to use an earth spike not for the electrics, but to stop any static build up.
What you are doing is a specialist job, what I would suggest is to pretend your going to buy one ready built, and go around looking at them and what is included, clearly in a paved car park you can’t knock in an earth spike, however it is likely standing on four metal steady legs which would give you some earth connection. On a grass field you would likely use plastic weight distributors on the bottom of legs, so then may need an earth rod.
I have noted the problems where generators are used keeping food cold while towing the unit to site, distance will alter how much of a problem, but any out door event you must expect local authority food inspectors to be around, so although with my caravan I can get fridge cold before I leave and then tow to site and take a chance that food has warmed up, that is not an option with a catering trailer. So have a look how others do it, and copy. I have many times thought of using a standard domestic fridge in a caravan, but I don’t know how they would take the vibration of towing, and those designed for boats are so expensive.
Thanks for your reply Eric. Various ideas there to work on. Sorry I called it a catering trailer, I could not think of another term, until now. It is actually a mobile Bar, so food, for one thing, will not be a problem. There will be one caravan type wash bowl, so is this the only thing requiring bonding as I presume the fridges will be earthed through the cable. What about the lights, they will be earthed through the cable also I would have thought. Will metal corner steadies suffice for earthing the trailer, obviously not when on wooden blocks?
bonding and earthing are not the same. If the power supply is built into the trailer then only thing an earth stake will do is stop static build up, and really there is no need your corner steadies will do the job, although likely would not comply with regulations.
However take that power supply out of the trailer and put it on the ground and things start to change, again likely as long as earth on generator and trailer connected together unlikely to cause a problem.
I have used 100's of generators supplying site huts with no problem using simple 3 pin commando plugs, with the frame of generator connected to frame of tea hut just through the 2.5 mm cable and plug.
In the main any problem and the RCD trips before anyone gets a shock, you would be very unlucky to have a problem, however if you were unlucky then first question is who wired it.
The Emma Shaw case showed us what happens when it all goes wrong, it is one of those where there by grace of god goes I.
The point made was the guy who tested the wiring was not qualified and the guy who allowed him to do the work knew he was not qualified. Now all the guy had to do was plug in a tester and write down what it said, likely OL but he knew it should not say OL so asked what the reading should have been and wrote that down instead. Result was fault was not found until it killed some one.
So if I wire the trailer and I have insurance and some one is killed, then insurance pays out and I may get a fine or prison if they can show it was my fault, but really would need to have done something daft.
But you with no qualifications do exactly the same job, and first thing is why did you do it, it was all caused because you tried to save money and did not employ an electrician. Straight away it's your fault. Plus likely insurance will not pay out, and you have to pay for rest of your life.
Some times it's not getting it right, be being able to blame some one else. I can stand up in court as say in my professional opinion I did not think it needed an earth. And they would have to find some one with a degree better than mine to say I got it wrong. With you they could say black is white and you could not argue.
My father-in-law told me bulbs do not give out light, what they do is suck in the dark, and once full they fail, that's why bulbs go black when they blow and candle wicks are black, they are dark suckers.
OK we know it is wrong, but how could you prove it?
Thanks again Eric. I am not just doing the electrics to save money, more convenience where the trailer is being converted and painted. My plan was to do the wiring as correctly as possible, hence asking the questions, then ask a qualified electrician, who is known to my son, to give it a final check to make sure it is safe. This site is called DIY Doctor and that is the reason I am asking. I was a qualified Motor Mechanic, but how many people work on their own cars and ask a Mechanic neighbour if they are doing the job right? Or even worse, do not ask a Mechanic and the brakes they have been working on fail and kill somebody. I wanted to know whether the items I was using were correct and whether to create a ring circuit or just run spurs from the consumer unit. Also, the earth matter, does it need a separate earth to and from the chassis, or does it suffice going back to the generator. Thanks again for you time.
Personally I would bond everything but not bother with an earth rod.
The big question is fridge/freezer and generator that's the problem, rest is reasonably straight forward.
So any refrigeration unit will normally have a compressor and as with any compressor it needs to start with no load, to do that the thermostat is designed so there is at least 3 minutes between switching off and switching back on again, but in case it is manually switched off and on there is an overload, so if it tries to restart before the pressure has dropped this will trip and it will take at least 3 minutes before it cools down enough for the unit to try starting again.
This is all well and good in a house with mains supply, however to start a motor needs up to 20 times the current it needs to run, so my chest freezer needs 64W to run but around 2000W to start.
Now a sudden load of 2kW will likely slow down the generator and cause a voltage dip, if that voltage dip is too much then the motor will be slow to start and the pressure can start to build up before it has got running, in which case the overload will trip, if it never starts you will clearly know there is a problem but if it starts most of the time and only fails every so often it can put a strain on the overload and then the overload fails.
So what you need is a generator with a huge flywheel and a very fast governor, 2 cylinder Lister for example. However they are rather noisy beasts and you need to talk to customers, so what you want is a generator which you can hardly hear running, the inverter generators are very good they only rev up when needed so would seem ideal, but the question is will they rev up fast enough for the refrigeration motors not to be damaged, and this I can't answer.
If there is going to be a problem with the standard refrigeration unit then the other method is to use a different type of refrigeration unit which does not have a massive start load, and you have three types.
1) Inverter controlled fridge or freezers work in two completely different ways, some never switch off but vary motor speed, others do switch off but have a soft start, the motor is normally three phase the inverter has single phase in and three phase out and it gives the motor exactly what it needs, there is no overload to burn out and the start load is less than 100 watt so will work fine off an inverter generator, however it's a new idea and as to if you can find one the size you want is another question. Also expensive. There is also a version with a DC motor designed for boats, normally 12 or 24 volt again expensive.
2) There is the gas absorption refrigeration system used in caravans it is a simple heater so no sudden high load but not as efficient will also run on 12 volt and gas in most cases, and likely your best option.
3) Peltier refrigerators normally small and need DC power so normally 12 or 24 volt also not very efficient but is not effected by being upright I have a small one which will take 4 cans of beer and it uses nearly the same power as one which will take weeks shopping. Used in the main for cool boxes.
So the selection of generator and refrigeration plant goes hand in hand, a standard fridge is so affected by voltage near every one states do not use an extension lead. The caravan fridge can set you back £700 but then the generator is smaller so you save on that, in fact you could fit 12 volt lights and use a gas bottle to run fridge and not have a generator.
Thanks again Eric. I have heard various amounts of increase power needed to start fridges etc, from 2-3 times to 10 times and now yours at 20. I am flummoxed to know where to start. I had worked on 10 times and planned for the big fridge to start first, so about 2300 watts, then let that settles back to it's 230w then the 180w and the the two 150's. I came to the conclusion that a genny that puts out 3.5kw. Do you think maybe a larger genny might work. I had thought about caravan fridges, but unfortunately they are very small and would not hold much stock, plus around £800 each for something getting near the size required. Then all the extra ventilation for the fumes if run on gas, so it is a lot easier to go with mains, just getting the things sorted. Could I just ask? You said install a radial, rather than a ring, I thought they were both the same just a different name. Please would you advise me of the difference. Cheers.
Thanks again Eric. I have heard various amounts of increase power needed to start fridges etc, from 2-3 times to 10 times and now yours at 20. I am flummoxed to know where to start. I had worked on 10 times and planned for the big fridge to start first, so about 2300 watts, then let that settles back to it's 230w then the 180w and the the two 150's. I came to the conclusion that a genny that puts out 3.5kw. Do you think maybe a larger genny might work. I had thought about caravan fridges, but unfortunately they are very small and would not hold much stock, plus around £800 each for something getting near the size required. Then all the extra ventilation for the fumes if run on gas, so it is a lot easier to go with mains, just getting the things sorted. Could I just ask? You said install a radial, rather than a ring, I thought they were both the same just a different name. Please would you advise me of the difference. Cheers. PS. Just had a look. So, a radial is just what I thought was a big spur. If I run one double and one single fridge from one 16amp MCB and one single fridge and one small chest freezer of the other 16amp MCB, that is better than trying to run a ring main round the van.
The British ring final system uses 20 amp cable running around in a circle to a 32 amp MCB so each socket has two cables which share the power drawn. It allows one to use 106 meters of cable so you can wire a house using less copper and it was invented during the war to reduce the copper required for the massive house rebuild which would follow the war.
Since your trailer is so small there is no way you will need over 30 meters of cable and neither are you going to use over 20 amp so no need for ring system.
I have an energy meter which I have used to test fridges and freezers, my chest freezer uses an average of 14 watt, run it uses 64 watt and it recorded a peak of 230 watt. However others have recorded 82 watt run and 2259 watt maximum that one was an old Hoover and a reasonable new Samsumg RL60 Fridge Freezer run 48 watt and peak 129 watt, the 129 was the auto defrost and the start load was around 60 watt, this fridge freezer uses an inverter drive which is why the start load is so low.
However until you actually measure you really have no idea on what the start load will be, there is no one formula fits all, and rotating mass of the generator does matter, this is the problem the whole county has with wind farms, if you change the load on a steam turbine from 200 MW to 400 MW the shaft was slow down a bit before the governor opens but not that much, but with a wind farm there is not tons of rotating mass so the volts will drop before any regulator can take up the slack.
You have the same problem, a large 7.5 kVA Lister engine powered generator will not have time to slow down with a load of 2 kW before the load drops to 70 watt, but use a small petrol generator 2 kVA with inverter technology so speed adjusts according to load, and a sudden 2 kW load will cause the voltage to dip by a huge amount as the generator tries to speed up and you will get one of two outcomes.
1) The generator speeds up before the gas pressure in the freezer builds up and the unit works OK.
2) The generator does not speed up in time and the overload trips out first so the freezer fails to start.
Now the two can swap depending what is running before it tries to start, and it is often worse when nothing running then when running light load, With a 2.5 kVA Lister single cylinder generator on a farm, if we started the sharping disk then tried to use the shears they would work, but if just the shears the generator would auto close down.
Unfortunately only way to find out what will and will not work is to try it. There is no formula you can use, however if using inverter drive fridges and freezers then likely if you allow 150W each it will work even with an inverter generator so generator around £200 and 700W but the fridge and freezers would cost a lot more, mine is discontinued but looking at around £700 mark. Using a cheap non frost free and non inverter drive the fridge freezer drops to £200 but the generator will not only cost more, but will be very heavy, as can't use inverter generator as likely will not speed up fast enough. So the generator will weigh three times the weight of a inverter model. Although not that much more expensive. But 70 kg is a lot of weight.
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