I have a Intermatic ML600TW lighting transformer which has internally two 300W 12VAC secondary supplies. I want to feed one 600W lighting circuit. Can I connect them in parallel and assume they will equally share the load? Do I need to add a buffer of some type to each leg to insure any circulating currents are reduced (a choke coil in each winding connection to limit any differences in voltage).

 

It depends. If they are closely matched this is done all the time. Often the windings are bifilar wound which makes for very good matching and any circulating currents would be very low. One way to tell for a particular transformer would be to tie one end of each secondary together and check the voltage difference between the free ends. If it's close to zero it shouldn't be a problem. Or if you have a sensitive clamp on ammeter and paralleling the secondaries doesn't change the primary current, you're home free. Be careful that they are phased correctly or things will get exciting. Worst case, you could ask Intermatic.

Regards
cww

 

This is what Intermatic said:

"You cannot put the 2 outputs together. This is designed to have a maximum of 300 watts on 2 circuits. If the transformers are
wired together, you now have the possibility of 40+ amperes on the cable which will melt it."

I am using 10Gauge stranded copper wire which should be rated to 55Amps which at 12VAC is 660W. 600W at 12VAC should be 50Amps.

 

Rob Parrish... NEC Ampacity for single-conductor #10 AWG, insulation rated for 90 deg C, and installed in free air, is 55 A.

But, for 2-conductor, # 10 AWG, insulation rated for 60 deg C, installed in raceway (interpret enclosed), then Ampacity is only 40 A! For, 40 deg C, Ampacity is reduced to 30A.

Regards, Phil Corso

 

Rob Parrsish... if the load circuits consist of 2 pairs of #10 AWG in the same enclosure, then the Ampacity is reduced to 80% of the values listed earlier!

 

I would use 8 for that current and fuse it for 60 and you must make sure that the wire size is adequate from the connection on out. That's if you choose to ignore their advice. Part of what they are saying might be if one secondary opened up the other would carry excessive current. This is by way of explanation. The problem in this case is that it sounds like this transformer is approved as a "definite purpose" device as part of an "approved lighting system". And somewhere, there is very likely a regulation or rule that low voltage lighting circuits of that type should be limited to 300 watts. This is for conditions of use or for user installation. Or quite possibly to fall under "power limited" circuit classifications for the NFPA or other regulatory body. So not knowing what exactly you are doing, it's probably a legal question rather than a electrical question. If those regs apply to the situation, I'd spring for the extra wire. It may not be any more expensive than the less common heavy wire to work around the limitation. Quite often the legal answer and the physical answer are different. Can't be helped, they mean well.

Regards
cww