Transformer disconnect location

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electro7

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Northern CA, US
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Electrician, Solar and Electrical Contractor
I was wondering if someone could confirm it's okay to mount the disconnect switch for the transformer inside the electrical room as long as the transformer is field marked with its location according to 450.14? See drawing attached. Thanks ahead of time!
43dab86f474ee2d70a21b2d7ac1b1c08.jpg


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Maybe I'm missing something but,
Solar, conduit to roof, transformer, disco,...
Maybe this is a solar install, with transformer to match the voltage in the "MSB"
Disco 'inside' the electric room,; perhaps to conform to the 10'of conductors IF this is a line tap.
"480 volt to roof"; so if it is solar, maybe it's 'from' and the transformer is step-up to match utility voltage.

Does anyone else 'see' this??

This post has been bugging me since Thursday AM
 
John A said:
"480 volt to roof"; so if it is solar, maybe it's 'from' and the transformer is step-up to match utility voltage.
Click to expand...
Very likely that is the case. Due to relative costs of inverters and the DC-side of the system for the tradeoff of 480V inverters vs 208V inverters of the same power, it is a value-engineering decision to opt for a transformer to produce a 480V grid. From the point of view of the inverter output current, it is stepped down to 208V. From the point of view of nighttime standby load current, it is stepped up to 480V.

Definitions of primary and secondary used to be ambiguous in this application. Now, as of NEC 2017 (if I recall correctly), this is clarified in Article 705. The larger source of fault current (i.e. the building grid/utility side) is considered the primary, for a transformer with sources on both sides. This puts the solar inverters & other 480V equipment on the secondary side, so 240.21(C) would govern the inverter side. The inverters essentially are a negative load, that supply current following grid voltage, rather than drawing current following voltage as loads ordinarily do.
 
electro7 said:
I was wondering if someone could confirm it's okay to mount the disconnect switch for the transformer inside the electrical room as long as the transformer is field marked with its location according to 450.14? See drawing attached. Thanks ahead of time!

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Click to expand...

Thank you for providing the sketch, it sure helps us understand a lot.

It would also help to see the corresponding single line sketch, that shows electrically how everything in the scope of this question, interacts with each other.

To answer the question about the location constraints on this disconnect, it would also depend on the following:
A. Is it an interconnection on the supply side [705.12(A) in NEC2023, formerly 705.11] of the service disconnect, or the load side [705.12(B) in NEC2023]?
B. Is the disconnect in question on the building side, or inverter side of the transformer?

There are situations which would have length constraints in this application:
1. A disconnect for a supply side interconnection (now considered a service disconnect in NEC 2023). Its line-side conductors may be limited in length, with specifics that depend on your NEC edition. The most restrictive has been 10 ft, and more recent editions have given more flexibility to this in commercial applications.
2. A disconnect on the inverter side of the transformer (i.e. the secondary) is governed by one of the options in 240.21(C). Inside a building, there are length constraints, and context rules to each length constraint.
 
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