Upgrade electricity supply to 3 phase

UKworkshop.co.uk

Help Support UKworkshop.co.uk:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Jacob

New Luddism. Awake and resist!
Joined
7 Jul 2010
Messages
29,136
Reaction score
5,580
Location
Derbyshire
Looking to upgrade electricity supply to garage with 3 phase to power two machines about 3HP each, plus lighting and rest of house etc. Getting a quote, what ampage should I ask for?
 
3HP = 2.2kW ish x 2 machines = 4.4kW
I bet you don't really need more than 5kW for any length of time in your workshop, unless you start adding 3kW electric heaters that will stay on for hours at a time.

A big modern house is likely to have a 100A single phase supply. That's enough for about 25kW of demand.
3kW oven + 3kW kettle + 9kW shower + maybe 6kW on an induction hob all at the same time = kids home taking a shower while Xmas dinner is cooking = 21kW
If you had 100A single phase, sending 5kW of that down to the workshop would be incidental.

The 3 phase equivalent of 100A single phase is 60A per phase more or less.
I bet you'd get away with less but if you think you might ever want an electric car charger (7kW for a few hours on end) or an electric heat pump (several kW) it might be nice to have the capacity.

Ask your supplier where the price breaks are and go as big as is economic.

If it's silly money, buy a 7kW digital phase converter for the shed, it's probably cheaper.
 
When I had it installed they gave me no options, I had a 100 amp single phase supply and got a three phase supply with 100 amps per phase using a 25mm CSA cable. This came into the new meter with three 100 amp fuses and then into a junction box where all phases went through an isolator and down to the workshop with one phase taken to the main fuseboard in the house. The DNO was ok that the company fuses protected the cable to the workshop and did not request a fused isolator. The means of earthing was the bigger concern as I had a PME system, had to use extra earth rods at the house and workshop.
 
Thanks for that. I'll get on with asking for a quote. Hadn't thought about EV charger but that could be an asset, as long as the EV craze lasts, which may not be long IMHO!
 
We installed an 18kw three phase supply a year ago (in France). So far enough power for everything, which slightly surprised me! That’s about 25 amps per phase. In France we pay a different standing charge depending on the maximum power, but we’re also on a digital meter, so I can just phone up and have the power rating increased up to 36kw in 3kw increments and the change is made remotely.

A new installation to the meter in the UK will most likely be on 25mm sq cable which can support up to 100 A, which is going to be over 70kw, which is a massive amount of power. Although your total power requirement is likely to be well below that, you will need to consider that your existing single phase house installation is going to skew your requirements somewhat. If your peak house usage is currently (pun intended) 15kw, and let’s say your peak 3 phase requirement is 9kw, your three phase supply will need to provide 18kw per phase, 54kw in total. Even though your total peak power is only 24kw, the house requirement is all on one phase, rather than being spread over three phases.

If you can easily change the power after installation (and the installation will generally support the highest power available) you could just start low and increase the power if needed.
 
Would a couple of VFDs not do the job, for much less money (assuming you only need 3 phase for those two machines)? Not relevant if you want 3 phase for a bunch of other stuff too, granted.
 
so I can just phone up and have the power rating increased up to 36kw in 3kw increments and the change is made remotely.
That means they have installed a larger cable which may never be used, I would think you have the maximum power available but are prevented from using it by the digital meter, what happens if you exceed the 18Kw ? Will it automatically put you on a higher charge for the higher supply or just drop out.
 
There are a number of factors that will determine what power you should ask for. Since the 1 April you ate no longer charged for infrastructure improvements which makes getting 3phase a lot less painful as you used to have to pay for the percentage of the transformer your load took. The cost is now determined on how far you are from the nearest electrical pole, and what if any concessions are needed from land owners for any changes or infrastructure changes.

It’s normally the job of a professional electrician to work out What supply you need. A simplified guide to working out what you need is to add up all the loads that you are ever likely to have running concurrently and then allow 20% extra as a minimum for any future needs. I myself use a 40% factor, this allows 20% growth in demand taking me to 80% of the rated fuses. However, most domestic requirements will never fully load a 100A per fuse supply giving a 72KVA supply. A house normally has a supply of 24KVA with 100A fuse or 19KVA with 80A fuse. If you get a 3 phase supply with 80A fuses you get a supply of 57KVA. Again normally more than adequate for most homes even with an EV fast charger, air source heat pumps and a small workshop.
 
That means they have installed a larger cable which may never be used, I would think you have the maximum power available but are prevented from using it by the digital meter, what happens if you exceed the 18Kw ? Will it automatically put you on a higher charge for the higher supply or just drop out.
The cable from the pole in the road to the meter is 35mm sq aluminium, effectively equivalent to 25mm sq copper so in theory it would support 100 A per phase. For a domestic tariff, the maximum supply is limited to 36 kW for three phase, which is about 50 A per phase. You can go up to 18 kW for a single phase supply, which is more like 80 A, but you'd have to argue quite hard to go beyond 12 kW on single phase.

If we go over our subscribed power for an appreciable time (i.e. it's not instant, but I don't know what the time is) then the digital meter cuts out and we can't reset it for several minutes. If this happens a lot (it did when we were on 9 kW single phase) then eventually EDF phone up to ask if you want to upgrade.

I would be surprised if UK installations less than 10 years old were connected between the network and meter with anything that could carry less than 100 A (e.g. 35 mm2 Al or 25mm2 Cu). Older installations might be 16 mm2 Cu I guess. The cabling on the consumer side of the meter could be anything big enough to support the installation.
 
That concept of having different tarriffs is really good because it rewards users who use less and for the sake of reducing usage can drop you into a lower band, something that should be thought of in the UK.
 
Most continental rural properties are three phase due to distance from the transformer, but in France they do not supply the Earth as a matter of course, this means most properties are TT, the convenience of single phase can not be appreciated until you try to use an electric double oven, manufacturers supply hobs that can use two of the phase's but not ovens? At the point of trying to use more power than your tariff, the supplier installed RCD trips out and plunges you into the dark, you soon learn not to use the washing machine, tumble dryer, immersion heater and cook all at the same time.
 
We had commercial tariffs years ago 1/ which set the the unit price by a maximum demand meter, and 2/ with three very different rates over 24 hours.

We used to have huge annual hairdressing shows/competitions and the sparks, a friend, tried desperately (in vain, he couldn't get his head around it) to persude the boss to hire in a huge generator for the evening as the massive spike when all the driers went on (on top of the other demands from dealing with so many people) that one evening effectively raised the unit charge for the whole year.

The other tariff had three rates something like 10p per unit from 6am - 8pm, 7p per unit from 8pm - 1am and 2p per unit from 1am - 6am. The problem being we were virtually unable to use the 2p electricity because of the noise overnight.
 
Back
Top