Wire Voltage Drop Calculator and Equations

Instrumentation & Electronics Design Data | Voltage Drop Calculations Review

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.

Voltage Drop


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
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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