I have 12 x 12V 10W lamps in series would I be correct in saying I need a 230-12V 120W transformer to suit?
Are you sure that they are in series? If they are truly in series, then you need 144v. What is the application? Did you wire them yourself, or did they come that way?
You need a transformer that has at least 12 amperes output at 12V to make it lasts longer life. Obtain a 15A one even better. Never use the calculate value. Always give some more extra current for spare to make the transformer lasts longer.
12 12V lamps in series need 144V to operate. Running this light string at a higher voltage will draw more current and burn out your bulbs. It is true that 12 x 12V 10 W lamps will draw 120 W plus the dissipation of the transformer so you never, never want to run a transformer at full strength for any period of time because it will heat up and fail. A good rule of thumb is to size your transformer at twice its load so you will need a 240W transformer with 144V output for this application.
If the lamps are in parallel, a 230 to 12 volt transformer rated 120 watts or more is what you need. If the bulbs are in series, adding 7 more 12 volt bulbs 19 total is perhaps the best fix or you need a 230 volt to 144 volt transformer, rated 120 watts. Both transformers are likely special order items expensive. You should set your 120 watt transformer on a thick metal plate with a fan blowing on it, to reduce the probability of early failure. Better use a 200 watt, or more transformer so it will run cool. Alternately you can get an 86 volt transformer rated about 100 watts, and wire it as an auto transformer. 230 volts - 86 volt 144 volts also likely a special order item, but possibly, about 80 volts center taped, or about 90 volts center taped is stocked by someone you won't make a connection to the center tap. 90 volts will give you about 11.7 volts per bulb slightly dim, but the bulbs will have long life. 80 volts will give you about 12.5 volts brighter, whiter light, but shorter life for the bulbs. Another way to lose about 86 volts is a 150 watt incandescent bulb designed for about 120 volts. This bulb will give a rather dim light with a hint of orange, but it will extend the life of your 12 volt bulbs even if they get 13 volts each, by reducing the starting surge. A 10 ohm, 10 watt resistor can be added in series to lose 8 or 9 additional volts for either the auto transformer or 150 watt light bulb circuit. The 150 watt bulb will likely last more than 2000 hours, with the help of the 10 watt resistor. An incadescent bulb rated about 250 watts at 230 volts will possibly be a good substitute for the 150 watt bulb, but it will give dim red to orange light. Neil
If 12 x 12V 10W lamps in parallel then it correct in saying that you need a 230-12V 120W transformer to suit
