Hi, I looking for a real good formu 1

[fjconair]

Hi, I looking for a real good formulas for high resistance grounded neutral applications, and figuring correct wire sizes for installation.

fjconair

 

[redtrumpet]

Not much of a formula - the resistors are rated in amperes with a time rating. The NGR conductor should be sized to conduct the full ampere rating (this is usually a code requirement). I size the conductor for contnuous current even if it is only a 10 second resistor. This avoids unfavorable interpretations of the code when the inspector comes around (in case you try to chintz on the conductor size because it only has to last 10 seconds).

Or, maybe you're looking how to size the resistor itself. As a minimum it has to be sized for more than the system charging current. Some sources say 5 times the charging current is a good minimum, with 7 to 10 times required to sense ground faults on the 90% of a machine winding farthest from the neutral. Charging current can be estimated or measured, but is on the order of 0.5 to 1.0 ampere/MVA for most industrial installations.

If you're looking for something else, please be more specific.