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Calculation No.: Calc-001
Revision: A
Description:
Created By: Engineer Name
Project: Project Name
Project No.: P001
Client: Client Name

Input Parameters

The following load, conductor, and installation parameters were used for this NEC cable sizing calculation:
Parameter Symbol Value Notes
Load Type Type Real Power (kW) Power specification method
Load Power P 100 kW Load power rating
Power Factor pf 0.9 Load power factor (cos φ)
System Voltage VLL 480 V Three-phase line-to-line voltage
Continuous Load Continuous Yes (≥3 hrs) NEC 210.19 - Continuous requires 125% sizing
Conductor Size AWG/kcmil 2/0 Cu conductor
Conductor Temp Rating Tcond 75 °C Insulation temperature rating
Termination Temp Rating Tterm 75 °C Equipment lug temperature (NEC 110.14)
Ambient Temperature Tamb 30 °C Ambient/ground temperature
Current-Carrying Conductors Nccc 3 Conductors in raceway/cable
Cable Length L 100 ft One-way cable run length
Max Voltage Drop Vd,max 3 % Design limit (NEC recommends 3% branch)

Calculation Methodology

All calculations performed in accordance with NEC 2020 (NFPA 70), including NEC Table 310.15(B)(16) ampacities, ambient temperature corrections (Table 310.15(B)(2)(a)), conductor bundling adjustments (Table 310.15(B)(3)(a)), and termination temperature limits (NEC 110.14).

Step 1: Load Current Calculation

Calculate the three-phase load current from real power (kW).

Load Current (Three-Phase)

Calculate current from power and voltage: The calculated load current is 133.65 A.

Formula: $$ \begin{aligned} I_{load} &= \frac{P \times 1000}{\sqrt{3} \times V_{LL} \times pf} \\ &= \frac{100 \times 1000}{\sqrt{3} \times 480 \times 0.9} \\ &= 134 \; \text{A} \end{aligned} $$

Step 2: Continuous Load Demand

This is a continuous load (expected to run ≥3 hours). Per NEC 210.19 and 215.2, conductor ampacity must be sized at 125% of the continuous load current.

Required Ampacity

Apply 125% continuous load factor: The required conductor ampacity is 167.06 A.

Formula: $$ \begin{aligned} I_{req} &= 1.25 \times I_{load} \\ &= 1.25 \times 134 \\ &= 167 \; \text{A} \end{aligned} $$

Step 3: Base Ampacity from NEC Table 310.15(B)(16)

Selected conductor: 2/0 Cu with 75°C insulation rating. Using 75°C column

NEC Table Lookup

From NEC Table 310.15(B)(16), the base ampacity is 175 A (using 75°C column). The effective ampacity is 175 A.

Step 4: Ambient Temperature Correction

Ambient temperature: 30°C (reference: 30°C). Correction factor from NEC Table 310.15(B)(2)(a): 1.000

Step 5: Conductor Bundling Adjustment

Number of current-carrying conductors in raceway/cable: 3. Adjustment factor from NEC Table 310.15(B)(3)(a): 1.000

Step 6: Calculate Adjusted Ampacity

Apply all correction and adjustment factors to the effective ampacity:

Adjusted Ampacity

Multiply effective ampacity by all factors: The adjusted ampacity is 175.00 A. Margin: 7.94 A (adequate).

Formula: $$ \begin{aligned} I_{adj} &= I_{eff} \times f_{amb} \times f_{bundle} \\ &= 175 \times 1 \times 1 \\ &= 175 \; \text{A} \end{aligned} $$

Ampacity Verification

Required ampacity: 167.06 A Adjusted ampacity: 175.00 A ✓ PASS: The adjusted ampacity (175.00 A) meets or exceeds the required ampacity (167.06 A). Margin: 7.94 A.

Step 7: Voltage Drop Calculation

Cable length: 100 ft. Conductor resistance from NEC Chapter 9, Table 8: 0.0967 Ω/1000ft.

Voltage Drop (Three-Phase)

Calculate voltage drop using three-phase formula: The calculated voltage drop is 2.24 V.

Formula: $$ \begin{aligned} V_d &= \sqrt{3} \times I_{load} \times R_{oneway} \\ &= \sqrt{3} \times 134 \times 0.00967 \\ &= 2.24 \; \text{V} \end{aligned} $$

Voltage Drop Percentage

Express voltage drop as percentage of supply: The voltage drop is 0.466% of the supply voltage.

Formula: $$ \begin{aligned} V_{d\%} &= \frac{V_d}{V_{LL}} \times 100 \\ &= \frac{2.24}{480} \times 100 \\ &= 0.466 \; \% \end{aligned} $$

Voltage Drop Verification

Calculated voltage drop: 0.466% Maximum allowable: 3.0% ✓ PASS: The voltage drop (0.466%) is within the acceptable limit (3.0%). NEC recommends 3% for branch circuits and 5% total for feeder + branch.

Summary of Results

Below is a complete summary of all calculated values:
Parameter Symbol Value Notes
Load Current Iload 134 A Full load current
Continuous Factor fcont 1.25 125% if continuous, 100% if not
Required Ampacity Ireq 167 A After continuous adjustment
Base Ampacity Ibase 175 A NEC Table 310.15(B)(16) - 75°C
Effective Ampacity Ieff 175 A Using 75°C column
Ambient Factor famb 1 NEC Table 310.15(B)(2)(a)
Bundling Factor fbundle 1 NEC Table 310.15(B)(3)(a)
Adjusted Ampacity Iadj 175 A After all correction factors
Ampacity Margin ΔI 7.94 A Adequate
Conductor Resistance R1000 96.7 mΩ/1000ft NEC Chapter 9, Table 8
One-way Resistance Roneway 9.67 mΩ Resistance for cable length
Voltage Drop Vd 2.24 V Three-phase voltage drop
Voltage Drop Percentage Vd% 0.466 % Within limit
Overall Status Status PASS NEC compliance status

Overall Compliance

**Overall Status: PASS** ✓ The selected conductor (2/0 Cu) complies with all NEC requirements: • Adjusted ampacity (175.00 A) ≥ Required ampacity (167.06 A) • Voltage drop (0.466%) ≤ Maximum allowable (3.0%) The installation meets NEC 2020 requirements for cable sizing.

Approval

This calculation has been reviewed and approved by:
Created By
Engineer Name
Date: ..........................
Checked
By: .............................
Date: ..........................
Approved
By: .............................
Date: ..........................
Client Approved
By: .............................
Date: ..........................

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