# 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