SYSTEM GROUNDING 250.20

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bobby ocampo

Senior Member
250.20 (a) Alternating Currents Systems of Less Than 50 volts. Alternating-current of less than 50 volts shall be grounded under any of the following conditions:
(1) Where supplied by transformers, If the transformer supply exceeds 150 volts to ground.

Will the code allow the system grounding to be UNGROUNDED where the supplied by a transformer, if the transformer supply system will NOT exceed 150 volts to ground?

What does the Code means when it says, "If the supply voltage exceeds 150 volts to ground"?

Does it mean that if the supply voltage is 208/120 volts, and the transformer used is delta-delta with secondary voltage is less than 50 volts the secondary is allowed to be ungrounded?
 

Smart $

Esteemed Member
Location
Ohio
150V to ground means just that... but to be more specific it means the nominal voltage between the grounded conductor and any other system conductor. An ungrounded supply has no [nominal] voltage to ground.

A 208/120V 3? 4W system does not exceed 150V to ground. Nominal is 120V to ground. Your secondary can be, but is not required to be grounded.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
An example would be the 24 volt control voltage transformer in a 480 volt MCC. The secondary of that transformer is required to be grounded.
 

bobby ocampo

Senior Member
150V to ground means just that... but to be more specific it means the nominal voltage between the grounded conductor and any other system conductor. An ungrounded supply has no [nominal] voltage to ground.

A 208/120V 3? 4W system does not exceed 150V to ground. Nominal is 120V to ground. Your secondary can be, but is not required to be grounded.

Just to confirm to avoid misinterpretation, "but is not required to be grounded" means the system can be UNGROUNDED? Yes or No?
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
150V to ground means just that... but to be more specific it means the nominal voltage between the grounded conductor and any other system conductor. An ungrounded supply has no [nominal] voltage to ground.

A 208/120V 3? 4W system does not exceed 150V to ground. Nominal is 120V to ground. Your secondary can be, but is not required to be grounded.

So if fed from a 208 delta high leg from a 120/240 Delta I need a ground?
 

Smart $

Esteemed Member
Location
Ohio
So if fed from a 208 delta high leg from a 120/240 Delta I need a ground?
Well it is where the 120/240 neutral is used. If the neutral is not used, it's debatable since only two of the three phase conductors would not exceed 150V. But I think the intent is for it to be grounded... and I would ground it if there's no substantial to not. At the very least it would save on having to install a ground detector.
 
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