240V to 120V step down transformer

Hello, I have a three part question that I need help on please;

  1. A 3200 sq ft house that will use a three phase power. In Africa where this house is, each leg (A, B, C & neutral) coming from the meter is 250V. Three phase is mostly used. If I want to use 120V appliances and electronics, what size transformer would be needed? Considering everything for an average home in the USA today.

  1. Can only one leg be stepped down so that a few outlets in the Kitchen, living and MasterBed rooms uses 120V? If so, what kind and size transformer would be needed.

  2. What size load center would be required, 100A or 200A?


Reply to
Kissi Asiedu
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240 V. to 120 V. step-down transformers are readily available. You can get an inexpensive dry type that mounts on the wall in various ratings. It all depends on the load you wish to run. The fuse or circuit breaker protecting your transformer needs to be sized to fit the circuit and the rating of the transformer.

Question -- If your standard line voltage is 250 and you attempt to install North American style circuit breakers and outlets at 120 V., will you not be in violation of your local electrical codes? ... or does that not matter.

In the USA, it used to be that 100 A panels (split-wired for 120/240 center-tap neutral) was standard. Now 200A panels are more or less the minimum due to increased appliance use.

Now you don't mention it, but your real issue may be frequency. If your electricity is supplied at 50 Hz (and it sounds like it is where you live) and you want to use North American style appliances, some, if not the majority may require 60 Hz. To do a frequency conversion gets more expensive with an increase in current drawn. Instead of a transformer, you might want to look into a 60 Hz inverter system and power your inverters with DC rectified from your 250 V. mains (or solar power!).

Basically, for electric heaters, frequency does not matter. For some electric motors, pumps, blowers, clocks and some electronic equipment, it does matter. Some users of this newsgroups have noted strange behavior when they wanted to run a dishwasher at 60 Hz instead of the factory specified 50 Hz. You need to investigate this for everything you are going to connect at the lower voltage .

Also, you will probably want to consult with a local electrician about grounding issues for your safety.


Reply to


Sorry I am not familiar with Africa so I could totally be off base on this suggestion. Most 240-250v 3 phase wye systems I am familiar with would produce 120v to ground and neutral. A 240-250V delta system you could use 2 of the 3 legs as 120v to ground/neutral. The other leg would be what we call a wild/high leg.

I can not size the transformer until the load is known.

Not knowing what your voltage to ground/neutral is makes it impossible to guess at what you really need. I suggest you talk to the local utility and get a clearer picture of what they will provide.

Not knowing the load makes it impossible to guess at the needed supply ampacity.

Reply to

You are right, the frequency is 50 HZ over there, however, there will not be a code violation when converting to the USA type. As far as the load, I am looking at running 200 Amp. I have bought a little transformers that steps down from 240V to 110V but of course that was 1000 watt transformer to be used for my stereo system. It worked just fine even on microwaves with

110V/60 HZ. I was hoping there will be similar type to work on the whole house.
Reply to
Kissi Asiedu

Why bother?- you really aren't gaining anything by going to 120V appliances. If you have some- then check which ones will work properly -i.e. toasters, kettles and other "heating" elements. Most electronics are dual voltage. Anything with motors may not be worth the effort. I'm sure that their equivalents are available for 240-250V, 50Hz. For example, Fisher and Paekel (sp?) of New Zealand first produced dishwashers and other appliances for the 240V, 50Hz systems before they broke into the North American market. Believe me, a 240V 3Kw kettle is a lot faster than a 120V 1.5Kw kettle. Check out the economics before doing anything.

Reply to
Don Kelly

If you consider the type of 240 to 110 volt transformers similar to the ones used for power tools on a building site - This will give you an idea of the type of mobile system that can be experimented with. They are normally

*required* as a safety measure - by health and safety

The questions you must first ask are as follows.

What are your power requirements.

i.e. your stereo system probably will not be operating at max (all the time?!)

Load Name Power Voltage frequency (power factor) =========================================== Lighting 1000[W] 110[V] * 1 Motors 1 1500[W] 110 [V] 50/60** 0.8 Motors 2 Heating . . .Load 5 ============================================

  • for Lighting and heating, load frequency is unimportant for *resistive* type systems (bar heaters and incadescent lighting)
** AC Motors run faster at higher frequencies..as they are essentially transformers the can have higher losses - some equipment manufacturers allow either to be used - important to note that overheating of motors can occur at wrong frequency



This will *after a few easy calculations* give you an idea of the transformer size you require - you can then think of spare capacity required.

Remember that transformers are designed to operate at a certain frquency (written on its nameplate) and losses can be considerably greater outside this range. As the other poster mentioned things like washing machines *might* be designed for use at a particular frequency - perhaps over or underspeeding of motors cause the machine to vibrate. (transmits more vibration to frame at say 50hz than 60hz)

This is the first step grasshopper......:-)

You might then think of cable sizes, fuse ratings, connector selection for sockets, fuse board selection,future requirements

Hope it helps


Reply to

The standard for 3 phase 50hz systems is 230 volts line to neutral and

400 volts phase to phase WYE connected with the neutral grounded at the building entry and otherwise as the supply authority thinks fit.

Funny delta and corner grounded systems are peculiarly North American 60 hertz aberrations. A wild/high leg is aptly named.

The OP would do well to source his equipment locally and doe as the others do.

Reply to
John G

Why would you want to do this ?

Why not stick with the local system. What about when an appliance packs up? its gonna be cheaper to get a local voltage replacement, than having to import one.

sQuick ..

Reply to

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