120/240 3 phase

[Electric-Light]

This is what I mean by 7 wire wye
This is an example of delta-wye7w transformer.

Essentially it would be a 208/120 transformer consisting of 3 0-120-139 tapped inductors rather than 0-120

It would allow serving 120v from all 3 phases while sections of building that run 240v can be run from end taps.

A clear safety advantage over corner ground(which for shock hazard purpose is comparable to 277v lighting due to 240v over ground) is that it only floats 140v and it allows you to use ground referenced 0-120 from every phase while accommodating 240v only equipment by running wires to the transformer as opposed to installing a point of use transformer.

 

[mivey]

This is what I mean by 7 wire wye ...

You could certainly get one wound if you wanted. Not too far from the Scott transformers with the 86% tap (although either 100% or 86% was brought out at one time). But I doubt you will get the utility to provide one. You would have to have the fourth bushing added anyway to use both the 86% and 100% taps at the same time.

Also, we don't do six-stator metering. We could install two summing meters but we would (assuming the load was the right type) just provide two separate services anyway with two separate transformer banks.

So your options will be to use buck-boost transforermers or have a specialty made transformer. Buck-boost is the solution for small loads. Commercial loads of significant size can have two separate services.

 

[templdl]

Wouldn't this be somewhat confusing to the OP's original question? How can this be applied to the OP?

 

[ggunn]

Not sure why they added the transformer for208 as nothing in this complex runs off of 208.

I would assume it would be so they could draw from all three phases to power 120V loads.

 

[Smart $]

I would assume it would be so they could draw from all three phases to power 120V loads.

And that would likely be to help balance loading as I stated back in post #4.

 

[Electric-Light]

Another thing I've read about is an open-delta-open wye transformer for supplying hefty single phase loads

The current on the primary side is spread 1:2:1 with the B phase (V in that diagram) carrying twice that of A or B current. This is as opposed to imposing the single phase load on one phase like a big 1ph 240v welder.

 

[Electric-Light]

templdl: thats the exact setup to the T. As for stinger correct I'm referring to b phase and is not needed. Not sure why they added the transformer for208 as nothing in this complex runs off of 208. Possibly previos tenant equiptment but as of now 3 phase setup which is fairly new is hardly being used other than shop area which is what I want to clean up. the main 3 phase equiptment has its own load center, but they added a transformer converting to 208 after this which I don't need for what needs to be done here.

Around here, 208Y/120 is very very popular in apartment complexes and professional plazas. PoCo supplies building with a 208Y/120 service and tenants get N and two of the phases. In these places, lights run on 120v. A/C on 208.

X-ray machine at quack's practice in such setup usually run on 1ph 208v while the elevator runs on 208/3ph from a house panel. Pretty much everything else in offices are 120v.

 

[kwired]

have a setup which has 2 operating systems 120/240 single phase setup as well as a newer 120/240 volt 3 phase setup with transformer converting 3 phase to 208. my question is this. they have a work shop with 240 volt welders and such and my plan was to take the 240 3phase to a sub pnl back there which would be supplied from the 240v 3 phase equiptment load center before the 208 conversion transformer. any problems with this? I want them to have the actual 240 back there. there is a nuetral available as well so i plan to send that back also to the sub for gen purpose recepts and lights. im aware of the 240 stinger to ground. just curious if i can utilize the 240 load center before it hits transformer and is converted to 208. i dont think neutral balancing is an issue since there is only one 3 phase drop to bldg and there single phase setup is off of same drop and they just left of one phase to metering equiptment anyways

Wouldn't this be somewhat confusing to the OP's original question? How can this be applied to the OP?

Why wouldn't it be something to consider if it were something commonly done? Apparently OP has a need for 120, 208 as well as 240 volts.

 

[kwired]

Biggest problem I can see with the multi-tapped wye configuration would be providing overcurrent protection for the transformer.

Best thing I can come up with that is relatively simple anyway, would be typical fuses or breakers in each ungrounded line primarily for short circuit and ground fault protection and overload relays like commonly seen for motor overload protection installed between the inside ends of each coil and the actual neutral connection point, this way total current on each phase is monitored.

 

[bobbymari]

I would assume it would be so they could draw from all three phases to power 120V loads.

i would buy this if there were anything comming out of this transformer as well as comment of phase balancing if there were anything comming out of this transformer which there isn't. The shop is now being fed out of 240v 3 phase setup via 2 wire jcord using the ground wire as a neutral BEFORE it hits the transformer which is what I will be changing to a conduit run with proper grounding. My true question as in original post was since the transformer is installed is there any issues with comming out of the load center Before it hits the transformer, I cant think of any reasons that what require me to use the transformer but I like to double check myself before spending thousands on materials.

 

[jim dungar]

Biggest problem I can see with the multi-tapped wye configuration would be providing overcurrent protection for the transformer.

Why?

Overcurrent protection for the transformer is based on the 'single voltage input' on the primary of the transformer.
Overcurrent for each set of secondary conductors would be done similar to any other 'multi-circuit' configuration.

While they are not common in premises wiring systems, multi-tap wye connected autotransformers are not unheard of in machine tools.

 

[templdl]

Why wouldn't it be something to consider if it were something commonly done? Apparently OP has a need for 120, 208 as well as 240 volts.

Please review my response post #25 to the OP and his answer post #26.

 

[kwired]

Why?

Overcurrent protection for the transformer is based on the 'single voltage input' on the primary of the transformer.
Overcurrent for each set of secondary conductors would be done similar to any other 'multi-circuit' configuration.

While they are not common in premises wiring systems, multi-tap wye connected autotransformers are not unheard of in machine tools.

If used for general lighting and power, lets say you had a secondary rated @ 100 amps. If you were to put 100 amp overcurrent protection on both taps of each phase then the common part of each phase is able to be loaded to 200 amps without operating overcurrent protection.

I suppose careful selection of overcurrent devices could work in some instances, but this is similar to more conventional setups in that you can overload a transformer by unbalancing it and still not trip overcurrent devices. Careful selection is easier if the loads are fixed, if they are more general purpose in nature, it can be more difficult to select overcurrent protection, and yet allow for maximum possible loading.

 

[jim dungar]

If used for general lighting and power, lets say you had a secondary rated @ 100 amps. If you were to put 100 amp overcurrent protection on both taps of each phase then the common part of each phase is able to be loaded to 200 amps without operating overcurrent protection.

Transformer overcurrent (running current) protection comes from the primary side. It has always been possible to have multiple sets of secondary conductors where the sum of their protective devices exceeds the transformer's output. Having different outputs taps does not impact the transformer much.

But, many in our industry seem to struggle with 240/120 3PH 4W systems, with the wild-leg to ground. I can't imagine how they would cope with both 120V and 136V to ground.

 

[kwired]

Transformer overcurrent (running current) protection comes from the primary side. It has always been possible to have multiple sets of secondary conductors where the sum of their protective devices exceeds the transformer's output. Having different outputs taps does not impact the transformer much.

But, many in our industry seem to struggle with 240/120 3PH 4W systems, with the wild-leg to ground. I can't imagine how they would cope with both 120V and 136V to ground.

Then why must there be a main overcurrent device on the secondary with exception of a two wire secondary or three phase three wire delta secondary?

I agree many people can't understand wild leg systems, corner grounded systems, ungrounded, and high impedance grounded systems and seem to think of any them as a foreign language, go back to theory classes and they are not that hard to understand, the basics do not change.

 

[jim dungar]

Then why must there be a main overcurrent device on the secondary with exception of a two wire secondary or three phase three wire delta secondary?

There is no requirement, in 450.3, for a secondary main.
As long as the transformer primary device is limited to 125% of the primary current, 450.3 is satisfied.
Each set of secondary conductors must be protected by one of the methods in article 240.21(C).

450.3 does contain a provision/exception for increasing the primary if an optional secondary main is provided. This provision is often mistaken as a requirement.

 

[kwired]

There is no requirement, in 450.3, for a secondary main.
As long as the transformer primary device is limited to 125% of the primary current, 450.3 is satisfied.
Each set of secondary conductors must be protected by one of the methods in article 240.21(C).

450.3 does contain a provision/exception for increasing the primary if an optional secondary main is provided. This provision is often mistaken as a requirement.

After further investigation it is actually 240.21(C)(1) that I had mixed up - but that section only applies to overcurrent protection of the conductors and has nothing to do with protection of the transformer.