12V xformers in parallel?

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jjhoward

jjhoward

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Northern NJ
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Owner TJ Electric
Can I tie the outputs of 2 magnetic xformers together?
I have multiple 300 watt transfomers for a landscape lighting install but one of the runs is long and has over 300 watts of lights (308).
Before I re-wire things, I keep thinking about tying another xfomer on the one that is over loaded.
Both transfomers would be sourced by the same 120V feed.
Is this a bad idea??

Thanks.
 
jjhoward said:
Can I tie the outputs of 2 magnetic xformers together?
I have multiple 300 watt transfomers for a landscape lighting install but one of the runs is long and has over 300 watts of lights (308).
Before I re-wire things, I keep thinking about tying another xfomer on the one that is over loaded.
Both transfomers would be sourced by the same 120V feed.
Is this a bad idea??

Thanks.
Click to expand...


Check out 411.6
 
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It should work, as long as the transformers are identical, and both the secondary and primary are permanenetly wired together so they are always on as a pair, and they are always have the same relative phase.

Can you not just relocate one lamp to get the load under 300W?

Over 300W on a long run is going to need some heavy copper to keep the voltage drop manageable :)
 
Don't do it! The peaks and valley's of the sine wave from each transformer will never line up good enough. You'll get a voltage pretty close to what you're after, but one will be bucking the other a little bit, and they'll both run pretty hot.
 
Yes, I think partitioning the load might be in order here.
There are 3-50 watt lamps fairly close to each other. I might just put them on another xfomer. Yes, the 12 awg wire isn't enough for the length and the load.

Sitckboy, is that requiement in 411.6 talking about the 120V supply being a 20 amp max circuit? I am feeding these transformers with a 15 amp 120V branch circuit.
 
Hi Marc,
Why will sine waves not be the same?
Both xformers are the same model. Will the manufactoring tolerences result in different phase shifts??
 
jjhoward said:
Hi Marc,
Why will sine waves not be the same?
Both xformers are the same model. Will the manufactoring tolerences result in different phase shifts??
Click to expand...
Beats me. I just know what I can measure. I first ran into this trying to parallel 24 volt hvac transformers. The scopemeter made it all pretty clear. The one was definately out of sync with the other. I'm not smart enough to know why.
 
Ok, enough said. If you seen it, no need for me to hook them up and then wait for them to get hot just so I can say...Gee, Marc has seen this too!
Thanks.
 
jjhoward said:
Sitckboy, is that requiement in 411.6 talking about the 120V supply being a 20 amp max circuit? I am feeding these transformers with a 15 amp 120V branch circuit.
Click to expand...

Yeah, was suppose to say 411.2, they limit the output to 25 amps...
 
Bad idea.

Assuming that you are successful, you will have a source of 50 Amps at 12 Volts. Your wire is probably already sized for 25 Amps and would have to be protected at 25 Amps.

The extra current through the wire will give you much more voltage drop and will overheat the wires.

You are far better off keeping them separate and running separate low voltage circuits from each of the transformers.

Maybe you could find some high-efficiency bulbs so you could get more light from the same amps and not have to upgrade the wiring.
 
Yes Stickboy and Bob you are correct, thank you.
Duh...parallelling the xformers would be allowing a 50 amp load.

Does section 411 prohibit any 12V transformers larger than 300 watts?
Or is it just a defintion for that section??
 
mdshunk said:
Beats me. I just know what I can measure. I first ran into this trying to parallel 24 volt hvac transformers. The scopemeter made it all pretty clear. The one was definately out of sync with the other. I'm not smart enough to know why.
Click to expand...

Reverse the polarity of one transformers primary leads.

_______________________________________________

But I agree with the others, don't do it.

It is a NEC violation for this application.
 
jjhoward said:
Does section 411 prohibit any 12V transformers larger than 300 watts?
Or is it just a defintion for that section??
Click to expand...

300 watts is the max, you can buy a 600 watt transformer, but it will have two secondaries on it.... 300 watts a piece...
 
LarryFine said:
Now there's a scary suggestion to make to anyone who might try it without checking first. :rolleyes:
Click to expand...

So if it wasn't checked in the first place why is it more dangerous to change it after the fact?

Roger
 
I should point out that I was replying to Marc's example of HVAC control transformer so I assumed 40 VA 24 volt secondary not much 'danger' in just swapping leads other then maybe releasing some toxic smoke. :smile:

I would not recommend 'just swapping leads' on a 75 KVA 480 volt transformer. :smile:
 
iwire said:
You lost me here, in Marcs example they where already out of sync with each other.
Click to expand...
roger said:
So if it wasn't checked in the first place why is it more dangerous to change it after the fact?
Click to expand...
Marc made it sound like they were "slightly" out of synch, not 50 amps' worth. That's hard to miss.
 
LarryFine said:
Marc made it sound like they were "slightly" out of synch, not 50 amps' worth. That's hard to miss.
Click to expand...
It's been maybe 10 years since I observed it, but the one was shifted maybe 30-40 degrees compared to the other.
 
mdshunk said:
Beats me. I just know what I can measure. I first ran into this trying to parallel 24 volt hvac transformers. The scopemeter made it all pretty clear. The one was definately out of sync with the other. I'm not smart enough to know why.
Click to expand...

If the transformers are identical in size there should be no TECHNICAL problem UNLESS you have the primary connections reversed, in other words where the 'hot' is connected to L1 on one of the transformer and to L2 on the other and the 'neutral' is reversed the same way. The other way this could happen if the transformers are connected to two different phases of a 120/208V three phase or 120/240V two phase system.
 
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