gauge of feed wire

[Fred Page]

I have an ancient transformer which powers a 6 v 18W bulb in a scientific instrument. The mains feed wire to the transformer is missing, how do I determine the required gauge of new wire? Is the gauge determined by the input or output charges?

 

[Chrispy]

The minimum size you need will be determined by the fuse you use in the plug top, ie it must be able to carry maximum current of the fuse.

 

[Spectric]

The total power in the primary side of a transformer will equal the total power in the secondary side of the transformer, ie primary volts out times current drawn will equal the mains primary side volts in times current drawn. So your secondary is 18/6 = 3 amps. Your primary will be less than 100mA so using a 1 or 1.5mm cable and just fuse at 3 amps.

 

[Fred Page]

The total power in the primary side of a transformer will equal the total power in the secondary side of the transformer, ie primary volts out times current drawn will equal the mains primary side volts in times current drawn. So your secondary is 18/6 = 3 amps. Your primary will be less than 100mA so using a 1 or 1.5mm cable and just fuse at 3 amps.

Thanks for that theory Roy, much obliged.