MikeHolt_Section05_Voltage Drop Calculations

Voltage Drop Circuit Calculations

  • Equipment is designed to operate within a voltage range, typically no less than 10% and no more than 5% from voltage rating.
  • When circuit conductors are installed, conductor voltage drop is determined by 1 of 2 methods:
  1. Ohm’s law

    • Multiplying the circuit current by the circuit conductors total resistance
    • 1Ø only, not for 3Ø
    • VD = I x R
      • “I” is equal to the load in amperes and
      • ”R” is equal to the resistance of the conductor as listed in:
        • Chapter 9, Table 8 for direct current circuit, or
        • Chapter 9, Table 9 for alternating current circuits
    • Math Examples:
      • 120V
        • 120V / 16A / 200′ (100′ from PS) / #12AWG
        • VD=I*R
        • i=16
        • R=2Ω per 1000′ / 5 = 0.4Ω
          • VD = 16*0.4 = 6.4
          • 6.4/120V = 5.3% VD
          • 120-6.4 = 113.6 Operating Voltage
      • 240V
        • 240V / 44A / 320′ (160’=1-way) / #6 AWG
        • VD = I*R
          • I=44
          • R=0.49Ω/1000′ (NEC-Ch9-Tbl9) = 0.157
            • VD = 44*0.157 = 6.91 VD
            • 6.91/240V = 2.8% VD
            • 240-6.91 = 233 Operating Voltage
  2. Voltage Drop formula

      • VD = (2*K*Q*I*D)/Cmils
        • Note: #2 is for 2 wires)
      • VD = (1.732*K*Q*I*D)/Cmils
    • Note: For VD, K, Q, I, D, & Cmils explanation, see Formula Tables
    • Math Examples
        • #6AWG / 44A / 240V / 1Ø / 160′-1Way
        • VD=2KQID/CM
          • K=12.9 (copper constant)
          • Q=N/A
          • I=44
          • D=160
          • CM=26240 (NEC-Ch9-Tbl8)
          • VD=2*12.9*44*160/26240 = 6.92 VD
          • 6.92/240 = 2.9% VD
          • 240-6.92 = 233 Operating Voltage
        • #6AWG / 44A / 240V / 1Ø / 160′-1Way
        • VD=2KQID/CM
          • K=21.2 (aluminum constant)
          • Q=N/A
          • I=100 ? how to transfer fractions numerators and denominators [100 kVA = √3 × (50 × I) to 100 kVA ÷ ( √3 × 50) = I]
          • D=80
          • CM=83690 (NEC-Ch9-Tbl8)
          • VD=1.732*21.2*100*80/83690 = 3.5 VD
          • 3.5/208 = 1.7% VD
          • 208-3.5 = 204.5 Operating Voltage

Unit 08: Voltage Drop Calculations

NEC Article Chapter 9-Table 8-DC & Cmils, Table 9-AC

  • NEC Ch. 9, Table 8
    • DC circuit conductor resistances
    • conductor circular-mils (cmils)
  • NEC Ch. 9, Table 9
    • AC circuit conductor resistances
    • reactance
  • NEC Voltage Drop ‘recommendations’
    • 210.19(A)(Note 4)
    • 215.2(A)(Note 2)
    • 310.15(A)(1)(Note 1)
      • NEC Voltage Drop Recommendations:
        • 5% VD for Feeder and Branch Circuits
        • 3% VD for Feeder or Branch-Circuits
  • DC Conductor Ω Formula
    • DC Conductor Ω = (Conductor Ω / 1000′) * Conductor Length
  • AC Conductor Ω Formula
    • AC Conductor Ω = (Conductor ohms-to-neutral Ω / 1000′) * Conductor Length
  • Conductor opposition to current flow
    • conductor material
    • conductor cross-sectional area
      • Conductor Ω varies inversely with its size
        • ↑ cross-sectional area, ↓ conductor Ω
        • ↓ cross-sectional area, ↑ conductor Ω
    • conductor length
      • conductor Ω is directly proportional to its length
    • operating temperature
  • AC & DC Resistance total is based on the total length so be sure to use source and return conductors
  • AC & DC Resistance Differences to ignore
    • ≤ less than 1/0 AWG conductors
    • stranded conductors are the same as solid conductors
    • uncoated & coated conductors
      • This means Ch.9-Table 8 &/or  9 can be used for all these conductors
  • EVD = I * R
    • EVD = conductor voltage drop expressed in volts
    • I = amps at 100% (not 125%)
    • R = Conductor Ω from NEC-table 8 or 9
  • Sizing Conductors, accounting for VD, 1 Ø & 3Ø circuits
    • 1 Ø
      • Cmils = (2*K*I*D) / VD
    • 3 Ø
      • Cmils = (1.732*K*I*D) / VD
  • Determining Conductors VD
    • 1 Ø
      • VD = (2*K*I*D) / Cmils
    • 3 Ø
      • VD = (1.732*K*I*D) / Cmils
  • Determining Maximum Conductor Limiting Voltage Drop
    • 1 Ø
      • D = (Cmils * VD) / (2 * K * I)
    • 3 Ø
      • D = (Cmils * VD) / (1.732 * K * I)
  • Limit Current to Limit Voltage Drop / Determining the Maximum Load
    • 1 Ø
      • I = (Cmils * VD) / (2 * K * D)
    • 3 Ø
      • I = (Cmils * VD) / (1.732 * K * D)

 

  • True K = conductor Cmil area * conductor Ω  per foot

Mike Holt_Voltage Drop Calculations_P1

Mike Holt_Voltage Drop Calculations_P1