AGL Cable Voltage Drop Calculator | Precision AGL Academy

Cable Voltage Drop

Voltage drop and I²R loss along a primary AGL cable run from conductor size, length, temperature and series current.

About this calculator

In a series circuit the same current flows through the whole cable, so the drop is simply current times the total loop resistance. Add each cable segment, and the tool reports the drop, the power lost as heat, the drop as a share of the load voltage, and how the other conductor sizes would compare. In a constant-current series circuit the lamps still receive the set current: the cable drop appears as extra voltage the CCR must supply, not as dimmer lamps.

Cable Segments

Section / description	Conductor	Length (m)	Drop (V)

Rule note

Add one row per cable section. Lengths are the total loop cable in that section (out and back). For a one-way route, enter double the route length.

Circuit Parameters

Circuit current (A)

Full-brightness series current.

Cable temperature (°C)

Rule note

Copper resistance rises about 0.393 % per °C above 20 °C. Use the expected conductor temperature.

Load voltage (V) (optional)

Rule note

Voltage across the load, excluding the cable drop calculated here. If your figure is already the CCR output including the cable, read the drop percentage only and ignore the last result card. Leave 0 to skip the percentage checks. The 850 V shown is only a placeholder; estimate your own as load kVA × 1000 ÷ circuit current (Calculator #13).

Maximum drop (%)

Your project's own design limit, not a standard value.

Report details

Project Name (for report)

Circuit ID (for report)

Build-up

Total loop length

—

Sum of sections

Total resistance

—

At the cable temperature

Voltage drop

—

I × R

Cable power loss

—

I² × R, lost as heat

Drop of load voltage

—

Needs a load voltage

Load voltage + cable drop

—

Arithmetic sum, slightly conservative

Conductor Comparison

The whole loop length re-run in a single conductor size. The last column is the longest loop that stays within your drop limit.

Conductor	Resistance (Ω)	Drop (V)	Drop (%)	Loss (W)	Max length at limit (m)

Calculator #15 | Rev. 1.2 | 26 Oct 2026

Engineering reference tool. Final design shall be verified against the cable manufacturer's data and the applicable project specification.

Method & Reference

How the drop is calculated

Every step is shown so the result can be checked by hand.

1. Section resistance

Each section's resistance comes from the conductor's nominal resistance per km at 20 °C, corrected for temperature and scaled by length.

R = R₂₀ × [1 + 0.00393 × (T − 20)] × L ÷ 1000

2. Drop and loss

The series current is the same in every section, so the section values add directly.

V_drop = I × ΣR P_loss = I² × ΣR

3. Share of load voltage

Drop % = V_drop ÷ V_load × 100

Compared with your design limit. If a load voltage is not entered, only the volts and watts are reported.

4. Maximum length at the limit

L_max = (limit × V_load ÷ I) ÷ (R₂₀ × k_T ÷ 1000)

Here k_T is the temperature factor 1 + 0.00393 × (T − 20). This assumes one conductor size over the whole loop.

Worked example

Item	Value
#8 AWG, 3,000 m loop, 20 °C	R = 6.306 Ω
Circuit current	6.6 A
Voltage drop	41.6 V
Cable loss	275 W
Load voltage	850 V
Drop share	4.90 %
Longest loop at 5 %	3,063 m

What the drop means in a series circuit

The CCR holds the current constant, so a larger cable drop does not dim the lamps. It raises the output voltage the CCR must supply (load voltage plus drop, before any margin) and so the kVA it must deliver, and it adds heat in the cable. Compare the sum with the rated output voltage and kVA of the CCR.

What this tool does not do

It does not include cable reactance, secondary cable tails, connectors or the effect of brightness steps. Resistance values are nominal DC figures, so confirm them against your cable data sheet. AWG sizes use nominal stranded-copper values at 20 °C (ASTM B3/B8 basis, about 2 % above the ideal-copper figures); metric sizes use the IEC 60228 Class 2 maximum.

Cable Voltage Drop Calculator: Frequently Asked Questions

Quick answers on voltage drop and I squared R loss along a primary AGL cable run.

What does the Cable Voltage Drop calculator do?

It calculates voltage drop and I squared R loss along a primary AGL cable run from conductor size, length, temperature and series current. In a series circuit the same current flows through the whole cable, so the drop is the current times the total loop resistance. It also reports the drop as a share of the load voltage and compares the other conductor sizes.

How is the section resistance worked out?

Each section's resistance comes from the conductor's nominal resistance per km at 20 degrees C, corrected for temperature and scaled by length. Copper resistance rises about 0.393 percent per degree C above 20, so the correction is 1 plus 0.00393 times the temperature minus 20. Use the expected conductor temperature.

Should the length be one-way or the whole loop?

Enter the total loop cable in each section, which is out and back. For a one-way route, enter double the route length. Add one row per cable section, and the section values add directly because the series current is the same in every section.

How are the drop and the loss calculated?

Voltage drop is the circuit current times the sum of the section resistances, and cable power loss is the current squared times that resistance. The loss is lost as heat. Enter the full-brightness series current.

What are the load voltage and maximum drop settings?

The load voltage is optional and is the voltage across the load, excluding the cable drop calculated here. The 850 V shown is only a placeholder, so estimate your own as load kVA times 1,000 divided by circuit current. The maximum drop is your project's own design limit, not a standard value. Leave the load voltage at 0 to skip the percentage checks.

What does the conductor comparison show?

It re-runs the whole loop length in a single conductor size and shows the resistance, drop, drop percentage and loss for each. The last column is the longest loop that stays within your drop limit. It assumes one conductor size over the whole loop.

Can you give a worked example?

A #8 AWG loop of 3,000 m at 20 degrees C has a resistance of 6.306 ohms. At 6.6 A the drop is 41.6 V and the cable loss is 275 W. Against a load voltage of 850 V that is a drop of 4.90 percent, and the longest loop at a 5 percent limit is 3,063 m.

What does the tool not do?

It does not include cable reactance, secondary cable tails, connectors or the effect of brightness steps. Resistance values are nominal DC figures, so confirm them against your cable data sheet. Final design shall be verified against the cable manufacturer's data and the applicable project specification.

Important Notice, Disclaimer & Limitation of Liability

Please read before relying on any content on this page or on any result or record produced by it.

Specific to this page: Applies to the Cable Voltage Drop (calculator), including its PDF reports.

Calculated results. Results, tables, charts, status indicators (such as “OK”, “within limit” or “pass”/“fail”), recommended or selected values and the example presets on this page are first-pass planning and training estimates calculated in your browser from the values you enter, using simplified formulas, generic tables and typical values. They do not take account of every factor of a real installation, may contain errors or rounding differences, and are not a design, specification, selection, sizing, procurement or acceptance decision. A competent person must check them against the approved design, the manufacturer’s data and the applicable standard before any use. “Recommended” means an indicative estimate, not an endorsement of any rating, product or supplier.

Electrical values. Voltage, current, resistance, impedance, power, power factor, voltage drop, loss, load, capacity and efficiency figures depend on cable data, temperature, connector and transformer losses, CCR type, tolerances and other site conditions that a simplified calculation cannot fully capture. Default and example values are typical only. Do not rely on a result to load, energise, re-rate, tap or modify any circuit, CCR, cable or transformer.

Reports, downloads and printouts. PDF reports, CSV files and printed pages produced by this tool are uncontrolled working records generated in your browser. They are not approved designs, drawings, controlled documents, certificates, permits or regulatory records. Calculator numbers, revision labels, dates, project names, signature or approval fields on them are layout only and carry no authority unless made under your own approved document-control system. You must not present an output as an approved, verified or certified document. If you enter personal or confidential information, you are responsible for handling any exported file lawfully, including under the Digital Personal Data Protection Act, 2023.

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Notice v2.9 · 27 October 2026 · Full terms: Legal & Disclaimer

Cable Voltage Drop

Cable Segments

Circuit Parameters

Report details

Build-up

Conductor Comparison

How the drop is calculated

1. Section resistance

2. Drop and loss

3. Share of load voltage

4. Maximum length at the limit

Worked example

What the drop means in a series circuit

What this tool does not do

Cable Voltage Drop Calculator: Frequently Asked Questions

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Important Notice, Disclaimer & Limitation of Liability