3- 300 watt tranies in parallel

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Dennis Alwon

Dennis Alwon

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Other than art. 411 prohibiting more than 25 amps on the secondary side, is it possible to wire three 300 watt tranies in parallel in order to boost the wattage on a run. Let's say the wire size is not a problem and you want one run with about 750 watts of lights.

Will this work?

What are or can be the problems?
 
I seem to remember that the percent impedance of each transformer has to be a close numeric match to the others. Otherwise, the secondaries will circulate currents in themselves, creating heat and wasting energy.

One finds this as a spec for larger transformers, but I have never thought to look for it for these little 300 VA units.
 
Forgetting the code issues, if the transformers where of like design and impedance you could get 900 watts from them. But man would you need some big copper to handle 75 amps at 12 volts without having serious voltage drop issues.
 
al hildenbrand said:
I seem to remember that the percent impedance of each transformer has to be a close numeric match to the others. Otherwise, the secondaries will circulate currents in themselves, creating heat and wasting energy.

One finds this as a spec for larger transformers, but I have never thought to look for it for these little 300 VA units.
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Thanks Al

Well a friend of mine says they had a 900 watt trany for low voltage lighting (12v). A 900 watt trany consists of 3-300 watt tranys under the same roof.

He said that when he got there he notice that there were three leads coming off each trany all tied together to the wiring for the lights. Apparently it has been working like that for years.

So if the tranies were the same ones (brand size etc) would you expect the impedance to be the same in each or could it vary?
 
iwire said:
Forgetting the code issues, if the transformers where of like design and impedance you could get 900 watts from them. But man would you need some big copper to handle 75 amps at 12 volts without having serious voltage drop issues.
Click to expand...

The real life situation actually had 2- runs but I bet they were any bigger than 10 or 12. I will have to have him check.
 
Thank you both, Al and Bob. I knew there could be problems but somehow it work in this install. They got lucky, perhaps. My friend changed it and fixed the 411 violation and wired the tranies properly.
 
al hildenbrand said:
I seem to remember that the percent impedance of each transformer has to be a close numeric match to the others. Otherwise, the secondaries will circulate currents in themselves, creating heat and wasting energy.

One finds this as a spec for larger transformers, but I have never thought to look for it for these little 300 VA units.
Click to expand...

You are thinking of a three phase application and for that what you say is true.

When installing transformers in parallel on the same power supply AND connecting their secondary sides, the load will be shared in proportion to their impedances. So minute differences - within manufacturing tolerance - could cause one transformer to carry more than the other two and vice-versa. So if you do this; check the primary currents and see if you're not overloading them.

I do not think it is wrong from the technical standpoint, but see potenital for safety, inability to protect the transformers properly, etc.
 
One option is to separate the load into three circuits.

I've seen single transformers with multiple secondaries.
 
The original install worked, as stated, but it was not NEC compliant and the violation was corrected. Unfortunately there was only 2 circuits going out to the lights and we all know 3 would be necessary for a 900 watt trany if the loads were higher than 600 watts.
 
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