Unless there is some special reason to need an isolated supply, then an auto-transformer will be fine and will be much cheaper and smaller.


But do you need a transformer at all ? 230 volt heat lamps will work on 208 volt supplies, could higher wattage or more numerous lamps be used in order to compensate for the reduced output.


Or obtain 208 volt heat lamps these are available from the USA, which might perhaps be the location or destination of the equipment. 208 volts is very rare in the UK, more an American thing.

300 watts 230V  is ~ nearly 1.5 amps per lamp. * 6 is 9A. call it more like 10A at 208V.


Now compare the price, the size, and the weight,  of a 208V transformer with a 10A 230V secondary,  versus a  230V to 24 V 10A secondary, and you will see why an auto-transformer arrangement is vastly preferable when the amount to be added or taken off is a small percentage (in this case only about 10%) of the total.  The only thing I'd add, if by 2 phase you mean neither side is neutral, then the switching for changing the lamps needs to be double pole. And you may need  a bit of inrush limiting inrush limiter - an examples  to make the lamps last longer and reduce the risk of blowing out the supply on switch on - halogens are about 1/10 of the resistance when cold they are when hot - so your  300 watt lamp,  around 190-200 ohms or so when hot is more like 18-20 ohms for the first 10th of a second, and 6 of them will be quite a thud, as your 10A on load is 100A inrush. Do realize though that those simple limiters work by warming up from some modest no. of ohms to a much lower resistance state when hot, so need to be in a ceramic choc block, and keep the body away from  PVC wire.

Many thanks for the replies.

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