Wire Voltage Drop Calculator and Equations
This voltage drop calculator is based on public domain formula and will provide an approximate value for use in your electrical system design. The National Electrical Code™ Articles 210.19(A)(1) FPN No.4 and 215.2(A)(3) FPN No.2 suggest that a design with no more than 3% voltage drop for feeders and no more than a total of 5% voltage drop in branch circuits to the farthest outlet will "provide reasonable efficiency of operation".
Even though FPN's (fine print notes) in this case are not code requirements but recommendations, it is still good engineering practice to closely follow these guidelines, and indeed exceed them where needed. In fact, Section 647, Sensitive Electronic Equipment, requires (not an FPN) that voltage drop for feeders and branches shall not exceed 1.5% and 2.5%, respectively, and shall not exceed 1% and 2% respectively for cord-connected equipment. Fire pumps have additional maximum voltage drop requirements and these are outlined in Section 695.
If you do increase the size of your conductors to accommodate for voltage drop, remember to check if the new conductor size is compatible with the lugs to which they will be attached. The circuit breaker manufacturers make available the acceptable conductor sizes, and in some cases offer optional larger lugs for this purpose. Also, Article 250 requires that if the conductors are upsized, the grounding conductor must also be upsized proportionately.