Hi all,

If I have to add a new sub-panel on the 10th floor of an existing building, and let's say the new feeder to this sub-panel (originating from a distribution board ON the 10th floor) is 100 feet long, how does one know what volt drop percentage to use when calculating the wire size for this new feeder? The voltage drop from the cellar service up to the 10th floor distribution board may be at 2%, or 2.5%, or even already at the max of 3%. How would i know what percentage to use for the new feeder serving the sub-panel, and make sure I don't exceed 3% from the cellar service up the new sub-panel? It's an existing building, and let's say existing drawings and calculations don't exist (as sometimes is the case). Does the NEC cover this?

Thanks.

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