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240v running to 110-125v usa socket?

Postby jim the plumb » Thu Oct 14, 2010 7:14 am

Hey Guys,

I've been building a garage and have run in the lighting circuit and ring main. The garage has been designed with my car in mind which has an engine coolant heater from the states which is 110-125v and has a USA 3-pin plug on the end.

At the moment I have a table mounted transformer converting 240 to 110 volts. I plug the US cable into this which then powers the coolant heater and it works great apart from taking up space on the worktop.

What I want to do is get rid of the 110 transformer and run a new cable from the consumer unit, terminating in a 110v US plug socket.

Obviously at some point in the cable I have to change the voltage from 240v to 110v.

How can I do this? I need a minimum of 110-125v 2000w.

Can I use a yellow 110v transformer box along the way? I would prefer something more subtle that can be placed in the timber studwork either by the consumer unit or the US socket.

Can anyone help?

jim the plumb
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Postby ericmark » Thu Oct 14, 2010 2:20 pm

To reduce voltage there are some problems.
1) The USA use a single phase 110vac to neutral and we use a split phase 55-0-55 supply so with our system a twin pole breaker or fuses on both lines are required.
2) The yellow brick is very inefficient you only have to touch one and see how hot it is to realise that and in the main do not comply with current regulations due to the way automatic disconnection in fault conditions is achieved.
3) There are three basic ways to reduce voltage:-
a) Use a resistor.
b) Use an isolation transformer.
c) Use an auto transformer.
Often 110v equipment is in fact two 110v units that can be used in parallel for 110 and series for 220v the first is to find out if the unit can be converted to 220-240v.
Isolation transformers for continuous use are about twice the cost of a brick type and as a result likely you could buy a 230v heater for less than price of transformer.
The autotransformer is not much cheaper still looking at for 2Kw around the £180 mark.
Without a box I have seen new at around £80 which is a little better but I think a little information as to what is meant by isolation and auto transformers.

With an isolation transformer like the yellow brick any RCD protection before the transformer is non effective and if items requiring an earth are used then a second RCD will be required. With double insulated items then as with shaver socket there is no requirement for RCD as long as only one item is used.

With an auto transformer the supply RCD will still work and it loses less power than isolation type but if it loses it's neutral it can allow input voltage at output so it is not as safe. It only has one winding not two.

Normally with USA equipment one would use an autotransformer however this does not change the frequency so motors run slow and built in transformers can run hot.

The yellow bricks at £63 for a 3KVA are cheap and I would like personally to see them banned as they cause no end of fires the problem is normally only the input is fused so for a line earth fault 13A in times 230 divided by 55 gives 54A before the supply fuse will rupture. This needs 10mm cable to take current and often only 1.5mm is used. So if there is a direct short at end of a 110v extension lead it can set things on fire before it blows the fuse.

Yes 55v or in USA 110v does give less of a kick than 230v but if you look a deaths caused be electrical generated fires compared with electrocution you will find fire is a much greater risk and I would look hard at using a 230 volt heater rather than a botch up with a USA 110v one.
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