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Installed hanging ceiling light (with 30 x 10w bulbs) with a 300w transformer on top built into ceiling connection.
I put this on a dimmer switch which blew transformer & could not get another trans the same.
Got external transformer with same specs & changed light switch.

But 6amp circuit breaker the light is on will trip on 5th or 6th time the light is switched on, have changed circuit breaker. can you please advise.

The transformer is designed to take this load it is on it's limit, so i'd say there was an overload on your circuit what else is connected to this circuit and do have all the lights on at same time.
KB

[quote="robber61"]Installed hanging ceiling light (with 30 x 10w bulbs) with a 300w transformer on top built into ceiling connection.
I put this on a dimmer switch which blew transformer & could not get another trans the same.
Got external transformer with same specs & changed light switch.

But 6amp circuit breaker the light is on will trip on 5th or 6th time the light is switched on, have changed circuit breaker. can you please advise.[/quote]

Try changing the circuit breaker to a type c or d. The problem you have is the inductive load of the transformer on its start up. Possibly you may have to up the breaker to a 10 amp if this is possible too.

Add on to sheffield_sparky's comment about changing mcb rating or type, Before doing so it is essential to measure earth loop impedance of furthest light on circuit to ensure you don't exceed trip time of mcb.
eg 6A type B can be up to 6.14 ohms but a 10A type D would only allow a max of 0.92 ohms. If you can't check this DON'T CHANGE MCB!
Lots of leckies who should know better simply change type 2/B mcb for type C to prevent unwanted trips and circuit later fails on a periodic inspection (for a sale etc.)
regards SPARX