Query on Materials for Estimate for Electrical Work


Postby Lewis94 » Thu Jul 07, 2016 2:35 pm

Good Afternoon,

I wonder if anyone could give me an idea on a scenario I have with an estimate to carry out some electrical works.

I have had 3 qualified electricians provide me with an estimate to supply an electric supply to my garage from my house. The garage is 10m away and there is already a 50mm smooth bore duct running from the house to the garage for a separate car charging unit. The electric board have previously installed "tails" to the meter box and installed a small consumer unit which has one space left which I have been told I can use for the garage. I would like to install 2no. double plug sockets and one single light. Equipment wise nothing more than a drill, jet hose, saw etc.

All 3 have said I need a garage consumer unit, plug sockets etc in the form of materials, however the connection from the house unit to the new garage unit I have been told by two people that I need SWA 3 core 4mm and one SWA 2.5mm as this is all I will need. As for the internal wiring to the plugs and sockets I have been told 2.5mm twin and earth cable, 10mm T/E cable and 6mm T/E cable.

I am quite happy to pay for the work as electrics I do not touch but wondered if anyone could suggest what would be the best option.

Kind Regards
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Postby ericmark » Fri Jul 08, 2016 12:24 pm

If one assumes only one item at a time is used in the garage then the maximum power required is 13A you can't get a 13A MCB so it would be 16A supply from consumer unit. I am working on that.

So cable size 2.5 mm sq can take 20A so it ample current wise, but there is also volt drop and earth loop impedance to consider. So 25 meters is the normal limit for volt drop with a 16A supply, that's cable length not distance to garage. So if more than 25 meters of cable is required then it needs to be heavier.

As to garage consumer unit, not required a simple switched fused connection unit (FCU) with a 3A fuse to reduce power to lights used as the light switch is all that is required. Even that not really required you are allowed 16A feed to lamps, but better with 3A then any fault is unlikely to cause bulbs to weld into holders.

Having said that if I was doing the job I would use 6 mm sq cable and because it is thicker to get two wires into the first socket or FCU in garage would be near impossible, my dad used a cooker switch inside the garage, a cooker connection unit would do the same job, it allowed you to reduce from 6 mm sq to 2.5 mm sq.

Technically if you reduce the current carrying capacity of the cable you should have an overload at that point. But since the MCB or RCBO in the consumer unit is only 16A no real need.

The requirement of RCD protection could change things, by time you fit an RCD maybe just as easy and cheap to fit a garage consumer unit. Depends if already fitted or if a RCBO will fit in the consumer unit.
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Postby kbrownie » Sat Jul 09, 2016 8:35 am

I assume that the cable supplying the car charger from house to garage unit is not sufficient to add the extra load to?
We need to establish what you currently have with regards to size of MCB at serving the cable to the garage, the size and type of cable running to the garage, before correct information can be given and if the supply cabls is RCD protected or the garage unit is?

If you where able to use the existing supply to garage charger (I am assuming this is a 32A electric car charger)
You would need at least a 10mm SWA cable in place to deal with the charger, socket and lighting load.
If a new cable was required in the duct, then to power two double sockets and a lighting circuit, I would install a 6mm SWA cable and fit a second garage consumer unit.
But saying that there are plenty of variation that can be used, depending on your particular used. It could be that a all that is needed is 1.5mm SWA connected to 13A fused connection at the garage.
But I think Eric has touched on this in the previous post, you may as well future proof the installation and have a higher sized cable installed to deal with any additional load required in the future!
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Postby ericmark » Sat Jul 09, 2016 10:27 am

I was assuming a car charger was to equalise cells in a standard car battery, I was assuming around 1 amp load. I had not considered it being for an electric car. The chargers for an electric car is very different to top up chargers for a normal car battery.
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Postby Lewis94 » Sun Jul 10, 2016 12:22 pm

Thank you for coming back to me.

Having looked at what I need it will be one double socket and one light, but not a fluorescent tube type.

I have tried to attach an image of the consumer unit that is installed for my electric car charging unit. The fitter left a spare connection behind the blanking plate and did say it should be good for a secondary connection. My car charges from 3am in the morning and lasts no longer than 6am so the chance of me being in the garage using a socket or light at this time would be rare.

The cable coming from this unit to my car charger is 6mm armoured cable.

Thank you in advance for your assistance
Attachments
image.jpeg
Car charger consumer unit
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Postby ericmark » Wed Jul 13, 2016 3:58 pm

A C type MCB needs 10 times the rated current for the magnetic part of the trip to work, normally we use a B type which is up to 5 times rated value, this means the resistance of the cable (with AC called impedance) has to be low enough for a C type at 20A to operate so 200A must be able to flow so 230/200 = 1.15 ohms well we have to allow for volt drop so actually 1.1 ohms.

The cable has a resistance both to the house and from house to garage so it is a case of doing some sums so likely supply to house 0.35 ohms impedance so just 0.74 ohms for the cable and that includes the cable all the way to the car.

As a result it is near impossible for anyone remote from the site to work out what cable is required. I find the use of a C or D type MCB suspect. In the old days before the switched mode chargers there was quite an inrush as the charger was switched on, but today I would have expected a B16 MCB would have been ample, but I have not read the info which comes with the charger.

I would not think you need a second supply for the garage, in fact one would need to know the earthing system before you could consider a second supply. In the early days of electric car charging they were considered the same as a caravan. However latter special rules for electric car charging points came out. As a result one needs to first work out the earthing system before commenting on cable sizes. One also needs the loop impedance readings or the prospective short circuit current. Without that information one could easy give the wrong advice.
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Postby kbrownie » Thu Jul 14, 2016 9:06 am

What size fuse it at the distribution/supply end o the 6mm cable?
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Postby Lewis94 » Thu Jul 14, 2016 9:47 am

You will have to excuse my terminology, the consumer unit I posted is linked into the main power connection board via some 'tails' that was installed by the electric company. This as I am told feeds into the consumer unit and has the C20 breaker added for the output supply to the electric car charging unit. The supply from the C20 breaker then goes via a 6mm cable into a Polar Electric charger unit which I cannot/would not access.

Next to the C20 breaker the installed left a 'tail' or whatever it is called to allow the installation of another breaker which he said would easily accommodate a power supply for a plug and light. This is currently hidden by a covering plate.

I hope this answers your question but apologies if it doesn't
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Postby kbrownie » Thu Jul 14, 2016 3:50 pm

At the main consumer unit in the house, where the 6mm cable is connected wht size and type of breaker/fuse protects this cable.
You have already told us what the breaker in the garage is!

Do you know how many cores the 6mm SWA cable has? As I don't understand what extra cable/tails the installer could have fitted to the garage unit that would allow for socket and light to be fitted.

Some internal pictures of the garage could help and a picture of the distribution at the house.
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