Separate Transformers

[arrail.thomas]

I had a question about if two transformers can be parallel. And what should the overloads look like once everthing is connected.

Theory I: There are 2- 200 amp feeds from source 1 & 2. The wire size for source 1& 2 are rated for 200amp.

SOURCE 1 & 2 are then connected at source 3 in parallel at a disconnect that is fused at 400 Amps.

Question: if at the load side of source 3 is ampered at 320 Amp will source 1& 2 tripp. Please explain either way.

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[augie47]

No sure if I follow, but it seems you have 2--200 amp over-current devices in parallel feeding a 400 amp swicth. If so, that in itself is a violation of 240.8

 

[arrail.thomas]

No sure if I follow, but it seems you have 2--200 amp over-current devices in parallel feeding a 400 amp swicth. If so, that in itself is a violation of 240.8

This will not be inspected.Its a engineering theory but its being used as a temporary source. What I looking for is not Code but a visual of how this would work or not . im looking for the science of it all.

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[kwired]

If you parallel two source coils you will want them to be same characteristics to give them as close as possible same impedance so current will divide evenly. You also want same out of any conductors that link them together as well as for the primary coils involved.

Next thing you want is for them to be same "polarity" and "in phase" with one another or you will get objectionable current flow even with no load connected.

I have mostly only encountered this with small control transformers when trying to "twin" HVAC units, but things I just mentioned would apply to any size transformer.

 

[publicgood]

See NEC 450 for requirements. You can parallel xfrms, yes.conditions apply.

 

[kwired]

See NEC 450 for requirements. You can parallel xfrms, yes.conditions apply.

Does anyone ever do that outside of certain control transformer applications and maybe to attain a larger capacity then what is typically readily available for under 600 volt applications?

 

[publicgood]

Does anyone ever do that outside of certain control transformer applications and maybe to attain a larger capacity then what is typically readily available for under 600 volt applications?

Typically not under 600V. Primarily because of increased short circuit capacity, causing higher equipment ratings and potentially affecting arc flash.

I have seen this done under 600V for cutovers to avoid shutdowns. Same precautions need to be made for transformer characteristics.