[MPTESE]

Hello,

I have a 125/250v - 50amp twistlock receptacle in a 3phase building (so, 208v).

Currently, when needed, I split the phases/hots to attain two lines of 120v, each with 50amp potential.

If I installed a knock down transformer (208->120) from the receptacle, would I be able to have a single run at 120v with 100amp potential?

 

[brric]

No....

 

[MPTESE]

No....

Are you saying that if I introduce a step-down transformer after the receptacle, I would only have 50amps to work with on the "out" (Secondary) ?

 

[surferdude2]

You need to realize that an appropriate transformer for the application will weigh in excess of 150 pounds and be pretty expensive (roughly $1200). Transformers are pretty efficient devices so you could come near your goal, probably get 95% to 98% of what you want, depending on circuit wiring losses.

Imagine if you will what the secondary winding would look like on a transformer that could deliver 100 amps.

 

[MPTESE]

You need to realize that an appropriate transformer for the application will weigh in excess of 150 pounds and be pretty expensive. Transformers are pretty efficient devices so you could come near your goal, probably get 95% to 98% of what you want, depending on circuit wiring losses.

Imagine if you will what the secondary windings would look like on a transformer that could deliver 100 amps.

Thanks for the reply.

Yes sir - They can indeed be pretty heavy and expensive. I've seen a 60AMP transformer used portably by attaching it to a dolly.. Perhaps read here for similar application..
http://www.screenlightandgrip.com/html/Transformer_Benefits.html

So - An appropriate transformer for this scenario (I suppose a 15kva) can indeed deliver close to the 100AMPS I'm looking for?

If so, then logically, I'm just having a bit of trouble wrapping my head around the why? Is it because the AMPS is dictated by the wattage and voltage as variables, and not the other way around? I suppose I'm still reinforcing my way of thinking to the basics..

Also - From a safety perspective, does everything that's being powered after the transformer need to be breakered independently from the panel? Or - Will the load make its way back to the original Two-Pole 50AMP breaker.

 

[joed]

It is because of the wattage. Watts in equal watts out, ignoring transformer losses.
Watts equals volts times amps. So if you half the volts on the output you can double the amps. Watts stay the same.
In your case you are not quite halving the volts. 120/208 = .58. In theory you could go from 50 amp in to 50/.58 or 86 amps out. Then there are transformer efficiency losses to make it even lower.

 

[MPTESE]

It is because of the wattage. Watts in equal watts out, ignoring transformer losses.
Watts equals volts times amps. So if you half the volts on the output you can double the amps. Watts stay the same.
In your case you are not quite halving the volts. 120/208 = .58. In theory you could go from 50 amp in to 50/.58 or 86 amps out. Then there are transformer efficiency losses to make it even lower.

I understand now why it's actually 86 and not close to 100.

But, what do you mean by "watts in" . I always think of watts by whatever lighting/device is introduced downstream.

Is the potential amperage dictated by the difference within the transformer (primary/secondary difference) or is it dictated by other power source factors such as the panel? If dictated by the transformer, what If there was another smaller step-up transformer introduced to make 208->240v. If the voltage was then 240 entering the larger transformer, wouldn't that result in 100AMP potential minus the efficiency loss?

 

[RAL238]

The power into the transformer will equal the power out, plus any losses. The ratio of the voltage in vs the voltage out will be determined by the ratio of the windings of the transformer. Similarly, the amperage in vs the amperage out will also be determined by that ratio, but in an inverse fashion.

How much power is drawn will be determined by the loads you connect. If the load requires 100A at 120V, it will try and draw that. Whether the transformer is capable of delivering 100A without burning up is a different story, and depends on the construction of the transformer (e.g. the wire gauge used internally).

So the panel has to be able to deliver the power required (e.g. 50A at 240V), the transformer has to be able to handle that level of power based on its specifications, and the wiring on both sides has to be sized for the amperage it will carry.

If you had a 208V to 240V transformer connected to a 240V to 120V transformer, and everything was powered by a 208V 50A circuit, that would still not give you 120V at 100A. It would still be 86A, minus the losses of both transformers. You'd be worse off than before due to double losses.

 

[MPTESE]

If you had a 208V to 240V transformer connected to a 240V to 120V transformer, and everything was powered by a 208V 50A circuit, that would still not give you 120V at 100A. It would still be 86A, minus the losses of both transformers. You'd be worse off than before due to double losses.

Thank you,

And thanks to everyone else also for your patience and answering my questions.