208v to 120/240v

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rugellin_2009

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washington
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Electrician
customer wants 120/240 4 wire receptacle 50 amp for an RV. supply voltage is 120/208v 3 phase. would i take 4 wires, two hots, neutral and ground from panel to an autotransformer or what is the best way about going with this? all autotransformers im seeing dont have a neutral connection. would the neutral just go straight thru autotransformer and into receptacle? thanks in advance
 
If you want 120/240V 3 wire it is usually best to use an isolation transformer.
Using buck-boost autotransformers can be problematic as the neutral point might also get shifted rather than being a balanced center point.
 
infinity said:
I think that you need a buck/boost transformer. This is for slightly over 60 amps.
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If I understand the wiring diagram on that page correctly, and if the turns ratio is 32:208 so that you get 240V L-L, that autotransformer would give you a L-N voltage of 134V, with an L-N-L phase angle of 127 degrees. So probably fine for a 240V 2-wire load, but the L-N voltage is wrong.

Cheers, Wayne
 
wwhitney said:
If I understand the wiring diagram on that page correctly, and if the turns ratio is 32:208 so that you get 240V L-L, that autotransformer would give you a L-N voltage of 134V, with an L-N-L phase angle of 127 degrees. So probably fine for a 240V 2-wire load, but the L-N voltage is wrong.

Cheers, Wayne
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I don't think so. The spec sheet says:
1-Phase Buck/Boost Step-Up Transformer - 208V Primary - 120/240V Secondary - 62.5 Amps.
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Jraef said:
I don't think so. The spec sheet says:
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Yeah, I don't believe it. The wiring diagram is excerpted below. There's no way that diagram produces an L-N-L 180 degree phase angle.

My computation in the earlier post is correct, if the L-L voltage becomes 240V on the output side, then in Cartesian coordinates with N at (0,0), L1 would be say (-120V,-60V) and L2 would be (120V, -60V). Which gives 134V L-N (i.e. sqrt(1202+602)) and an L-N-L phase angle of 127 degrees (i.e. 2*arctan(120/60)).

Cheers, Wayne

WiringDiagram.png
 
Jraef said:
I agree. Most 240V residential stuff is fine with 208V. Heating elements just put out a little less heat, motors just draw a little more current.
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Maybe quite a bit less heat - typically square low reduction.....
 
Very seldom an RV uses 240 appliances or loads. Most loads, A/C’s, RV dryers and such are 120 volt. They do this so they can be used on 30 or even 20 amp 120 volt circuits, providing they have an energy management system. There are manufacturers that make specific automatic buck/boost transformers that raise the voltage a few volts to compensate for RV park voltage drop.
 
jim dungar said:
Note:
These instructions say the neutral point of the buck-boost must be connected to the output only. This is in direct conflict with the NEC requiring the neutral to be connected to ground.
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Yes, I have been interpreting that statement to mean that the output neutral is connected to the input neutral and to nothing on the transformer. Because that is what the NEC requires for any autotransformer, that the output circuit have a grounded conductor connected to the input circuit grounded conductor. And that is the basis of the computations in post #5.

If, instead, they mean that for the boost configuration the wiring diagram is to be as below, that would be an NEC violation as you say, since the new "neutral" point would be 60V to ground. But it would give 120/240V single phase with respect to the new "neutral".

Cheers, Wayne

WiringDiagram2.png
 
1) My understanding is that most if not all RVs are intentionally designed with only 120V loads, so that they work just the same with 120/208 or 120/240 supplies.

2) Agree with the above on not using a 'buck/boost' transformer to derive 120/240 from 120/208. Either it is a very complicated system to correct for phase shift, or (as in the Larson unit described above) it screws up the N-G voltage of the derived 120/240.

3) Give me a pair of transformers, each with a single 50V coil and two 70V coils, and I can give you 120/240 from 120/208, with neutral connected to both primary and secondary, as an autotransformer. This is left as an exercise for the reader :)

-Jonathan
 
rugellin_2009 said:
customer wants 120/240 4 wire receptacle 50 amp for an RV. supply voltage is 120/208v 3 phase. would i take 4 wires, two hots, neutral and ground from panel to an autotransformer or what is the best way about going with this? all autotransformers im seeing dont have a neutral connection. would the neutral just go straight thru autotransformer and into receptacle? thanks in advance
Click to expand...

Another vote for just connecting it directly.

All the loads in RVs are 120 only including an electric dryer, so you can plug into a 120 supply with adapters if that is all you have, and everything will still work. Just not all at the same time.

With 120/208 and a neutral, everything just works and no transformer burning up the customer's power year after year.
 
To add to the other comments on RVs, most, if not all, the ones requiring 50A is because they have two A/C units. However, the A/C units are 120V. The extra amperage is so you can have both units on. No 240V loads on standard RVs.

So, the 208V/120V would be just fine, no transformer needed. There will be no heat loss, or extra time needed for anything that needs heat.
 
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