DC Cable Sizing Calculator

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System voltage *

DC system nominal voltage in volts

Continuous current *

Maximum continuous operating current in amperes

Cable length *

One-way cable length in meters

Application type

Type of DC application

Max voltage drop

Maximum acceptable voltage drop percentage

Safety factor

Current safety multiplier for design current

Installation method

Number of grouped circuits

Number of circuits bundled together

Insulation type

Ambient temperature

°C

Ambient temperature for installation

Conductor temperature rating (°C)

Conductor operating temperature

°C

Assumed conductor operating temperature

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Calc number

Tag number

Title

Description

Revision

Company name

Project number

Prepared by

Project name

Client company

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Your Company

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

Project: Project Name
Project No.: P001
Client: Client Name

Input Parameters

The following design parameters were considered for this calculation:

Parameter	Symbol	Value	Notes
System Voltage	V_sys	48 V	DC nominal voltage
Continuous Current	I_c	100 A	Maximum continuous current
Cable Length (One-way)	L	3 m	One-way distance
Maximum Voltage Drop	V_d,max	3 %	Percentage limit
Ambient Temperature	T_amb	30 °C	Installation environment
Installation Method	—	Open Air (Clipped Direct)	Cable routing method

Calculations

All calculations were performed in accordance with IEC standards and good engineering practice.

Step 1: Design Current Calculation

The design current includes a safety factor for continuous loads.

Formula: $$ I_d = I_c \times \text{SF} = 100.0 \times 1.25 = 125.0 \; \text{A} $$

Step 2: Maximum Allowable Voltage Drop

Convert percentage limit to volts.

Formula: $$ V_{d,max} = \frac{V_{sys} \times V_{d,\%}}{100} = \frac{48 \times 3}{100} = 1.44 \; \text{V} $$

Step 3: Copper Resistivity at Operating Temperature

Resistivity varies with temperature. At 70 degrees C, the copper resistivity is adjusted from the 20 degrees C reference value.

Formula: $$ \begin{aligned} \rho_{T} &= \rho_{20} \times [1 + \alpha(T - 20)] $$ \\ &= 0.017241 \times [1 + 0.00393 \times (70 - 20)] = 0.02063 \; \Omega \cdot \text{mm}^2/\text{m} \end{aligned} $$

Step 4: Minimum Conductor Size (Voltage Drop Basis)

For DC two-conductor circuit, voltage drop is V_d = 2 * rho * I * L / A. Solving for minimum cross-sectional area gives the formula above. Next standard size: 35 mm²

Formula: $$ \begin{aligned} V_d &= \frac{2 \times \rho \times I_d \times L}{A} $$ \\ &= \frac{2 \times \rho \times I_d \times L}{V_{d,max}} $$ \\ &= \frac{2 \times 0.02063 \times 125.0 \times 3}{1.44} = 10.74 \; \text{mm}^2 \end{aligned} $$

Step 5: Ampacity Derating Factors

Conductor ampacity must be adjusted for installation conditions. Temperature Correction Factor (Ct): 1.000 Grouping Factor (Cg): 1.00 Installation Method Factor (Cc): 1.00

Formula: $$ \begin{aligned} C_t &= \sqrt{\frac{T_{max} - T_{amb}}{T_{max} - T_{ref}}} = \sqrt{\frac{75 - 30}{75 - 30}} = 1.000 $$ \\ &= C_t \times C_g \times C_c = 1.000 \times 1.00 \times 1.00 = 1.000 \end{aligned} $$

Step 6: Ampacity Verification

Tabulated ampacity for 35 mm² conductor: 148 A Adjusted ampacity with derating factors applied. Verification: Iz (148.0 A) ≥ Id (125.0 A) — PASS

Formula: $$ I_z = I_t \times C_{total} = 148 \times 1.000 = 148.0 \; \text{A} $$

Step 7: Actual Voltage Drop

Conductor resistance per kilometer, voltage drop for two-conductor DC circuit, and voltage drop percentage. Verification: Vd,% (0.92%) ≤ Vd,max% (3%) — PASS

Formula: $$ \begin{aligned} R &= \frac{\rho \times 1000}{A} = \frac{0.02063 \times 1000}{35} = 0.589 \; \Omega/\text{km} $$ \\ &= \frac{2 \times I_d \times R \times L}{1000} = \frac{2 \times 125.0 \times 0.589 \times 3}{1000} = 0.442 \; \text{V} $$ \\ &= \frac{V_d}{V_{sys}} \times 100 = \frac{0.442}{48} \times 100 = 0.92\% \end{aligned} $$

Step 8: Power Loss in Conductors

Power dissipated as heat in the cable, and as percentage of transmitted power.

Formula: $$ \begin{aligned} P_{loss} &= V_d \times I_d = 0.442 \times 125.0 = 55.3 \; \text{W} $$ \\ &= \frac{P_{loss}}{V_{sys} \times I_d} \times 100 = 0.92\% \end{aligned} $$

Final Selection Summary

Selected Conductor Size: 35 mm² Copper Insulation: XLPE (75°C rating) Installation: Open Air (Clipped Direct) Voltage Drop: 0.92% (Max: 3%) — Margin: 2.08% Ampacity: 148.0 A (Design: 125.0 A) — Margin: 15.5% Compliance: COMPLIANT

DC Cable Sizing Overview

The cable sizing diagram illustrates the DC circuit layout showing conductor size, voltage drop, and current capacity for the selected cable.

Figure 1: DC cable sizing diagram

Summary of Results

Below is the summary of calculated values for ease of reference:

Parameter	Symbol	Value	Notes
Design Current	I_d	125 A	With 1.25× safety factor
Selected Conductor Size	A	35 mm²	Copper conductor
Actual Voltage Drop	V_d	0.921 %	0.442 V
Adjusted Ampacity	I_z	148 A	With all derating factors
Power Loss	P_loss	55.3 W	0.92% of transmitted power
Conductor Resistance	R	589 mΩ/km	At 70°C

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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