### 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.

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 Only the yellow cells need be modified. Enter the system voltage L-L (3ph) or L-N (1-ph) Wire AWG Ohms/kft Enter the maximum rated current for the system # 12 2.0500 -- If parallel runs, divide max amps by the # of runs # 10 1.2900 Enter the 1-way length of the conduit run # 8 0.8090 Select the wire size exactly as it is in the table # 6 0.5100 Read the voltage drop below # 4 0.3210 # 3 0.2540 INPUT INPUT DATA # 2 0.2010 SYSTEM VOLTAGE Volts # 1 0.1600 MAX CURRENT Amps 1/0 0.1270 ONE-WAY LENGTH Lin. Ft. 2/0 0.1010 WIRE SIZE # 12 # 10 # 8 # 6 # 4 # 3 # 2 # 1 1/0 2/0 3/0 4/0 250kCM 300kCM 350kCM 400kCM 500kCM AWG 3/0 0.0797 4/0 0.0626 READ RESULTS 250kCM 0.0535 Resistance per Ft Ohms/kFt 300kCM 0.0446 VOLTAGE DROP Volts 350kCM 0.0382 PERCENTAGE DROP % 400kCM 0.0331 500kCM 0.0265 This calculation assumes an unbalanced load, with phase and neutral conductors of the same size. VOLTAGE DROP CALCULATOR FORMULA VD = ( 2 · A · L · R ) / 1000 Where: VD = Voltage Drop (Volts) per unit circuit length A = Full Load Current (Amps) L = One-Way Circuit Length (ft) R = Resistance (Ohms/Kft)

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