Conductor for Single Phase Transformer

[timm333]

How do we size the 120/240V secondary conductor for single phase 3 wire 50kVA transformer? Should it be sized for 120V or 240V?

If sized for 120V, the current is 417A (600kcmil conductor required); and if sized for 240V, the current is 208A (4/0 conductor required).

 

[gar]

121126-2355 EST

Think about the question you asked.

Is 1/2 of the secondary (in other words only 120 V) capable of being continuously loaded with 50 kVA?

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[augie47]

Most of the single phase transformers I encounter give you the option, depending on how you connect the secondary leads, of a 120 volt output or a 240/120, so it might depend on how you wire the secondary.
I would seriously doubt it being wired for 120 volt only, so gar's post applies.
IN addition, you need to look at 240.21(C). The secondary conductor will also be dependent on the over-current device. It is permissible to connect to more than one over-current device and size your conductors according to the 230.21(C) rules.
With proper primary over-current protection, using the secondary rules in 240.21(C), your secondary condiuctors could be sized from a #4 thru 600 kcmil + depending on the secondary device.

 

[erickench]

If this is a residence then I would use NEC Table 310.15(B)(7). The correct conductor size would be 3/0 because you might have 240 volt equipment plugged into the system.

 

[erickench]

Okay, in annex D of the NEC there are examples where in the case of residential or light commercial occupations 240 volts is used to determine the service rating. Though it is not an actual NEC rule I would use 240 volts divided into the KVA to determine the size of the secondary conductors.

 

[Smart $]

Gus (augie47) has provided the most comprehensive reply thus far... so I don't need to repeat it

 

[timm333]

It makes sense to size conductors for 240V; but I want some clarification. This transformer is for commercial application and feeds a panel. There will definitely be 240V single phase circuits like water heaters; but there will be some 120V single circuits as well (for example lighting.) If we size conductors for 240V, then what if somebody in the future changes the breakers and uses all the circuits for 120V (instead of 240V). If all the circuits are 120V, then there will be one neutral conductor for two live conductors which means that the current in the neutral will be double the current in each of live conductors. How should live and neutral conductors be sized in this case? Can we still size the conductors for 240V even if all circuits on the panel are 120V? Thanks for help!

 

[augie47]

You would drive yourself crazy designing for future "what-ifs."
If you size and protect the conductors based on a 240/120 operation you should be fine. In that scenario, if all the breakers are used for 120 volt circuits, as long as both phases are used, the neutral will carry only the unbalanced current.
In most all transformer installs one has to trust that future wiring will be done with the parameters in mind.
The "change" you envision (overloaded neutral) would take some unique wiring changes that are highly unlikely.
The 50 kva transformers I see are normally feeding a 225 amp or 250 amp panel with conductors sized accordingly.

 

[gar]

121127-2210 EDT

If you were to put all the 120 V loads on 1/2 of the transformer so there was a 50 kW load at 120 V, then you would overload that half of the secondary (the primary would not be overloaded).

Doubling the current in 1/2 half of the secondary relative to its rating would increase the power dissipation in that half secondary by 4 times its rating and roughly raise the temperature rise in that area by about 4 times. This would probably fairly quickly burn out that secondary half.

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[ActionDave]

.... If we size conductors for 240V, then what if somebody in the future changes the breakers and uses all the circuits for 120V

Not a problem.

If all the circuits are 120V, then there will be one neutral conductor for two live conductors which means that the current in the neutral will be double the current in each of live conductors.

That's not the way it works.
The neutral only carries the imbalance between the phases. Unless you are facing some extraordinary circumstances there is no way you will overload it.

Can we still size the conductors for 240V even if all circuits on the panel are 120V?

Yes you can.

 

[gar]

121128-2025 EST

timm333:

To provide some more background.

In your first post you stated that the transformer secondary was 3 wire 50 kVA. This implied that the transformer could not be reconnected by paralleling the two secondary halfs to become a 120 V only secondary.

Thus, you have two 25 kVA 120 V sources. Using the neutral terminal as the reference the two hot terminals are 180 deg out of phase with each other.

On a continuous basis you can not load either half of the secondary to more than 25 kVA, and at 120 V this is 208 A. That 208 A can be distributed anyway you want between 120 and 240 V loads. Any 240 V load, for example 67 A, places a current of that value on each half of the secondary. If you add a 45 A 120 V load to phase A, then the total current on phase A is 67 + 45 = 112 A. But phase B still only has a load of 67 A.

On any transformer or distribution wire the only major current limitation is how hot the wire gets, and the capability of its insulation to tolerate that temperature. Temperature rise is approximately proportional to I². The continuous full kVA rating of a transformer will be based on the current that does not cause a maximum hot spot temperature to be exceeded.

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