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Allowable Overvoltage Limit

Allowable Overvoltage Limit

Allowable Overvoltage Limit

(OP)
ANSI C84.1-1989 voltage tolerance limits specifies a maximum overvoltage of 218Y/126V for a 208Y/120V nominal base. Our system includes a 600V-208Y/120V transformer feeding a panelboard. Due to maximum demand loading at a worse case scenario, a primary tap of -2.5% is required to maintain the voltage drop at the 208V panelboard to within 3~4%. However, if I simulate a black start, on the 208V panelboard with minimum loading on the upstream system, the overvoltage on the panelboard bus is 105.8% which exceeds the 105% maximum imposed by ANSI C84.1-1989.

The 208Y/120V panelboard is feeding office loads and I have concerns during weekends when the loading is almost neglgible (all loads are shut-off).

Is there a way I can strike a balance or compromise?

RE: Allowable Overvoltage Limit

(OP)
Also with reference to CAN-3-C235-83

RE: Allowable Overvoltage Limit

How about an autotransformer or an isolation transformer?  Introducing another place to further step down the voltage could fix the problem.  

You could also replace your current transformer with one that has a lower tap.

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If it is broken, fix it.  If it isn't broken, I'll soon fix that.

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