For sparkies and apprentices

Free electrical calculators for Australian electricians

Size a cable, assess maximum demand or check voltage drop. Smythy shows the inputs, method and working so you can review the result and keep a PDF for the job.

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No account needed, including for PDF exports.

Switchboard230 V single phase

Shed supply20 A example load

Follow a shed supply through the calculations.Illustrative circuit, not to scale.

Follow a shed supply

01Maximum demand

Work out maximum demand.

Start with what the shed will supply: lighting, socket-outlets and equipment.

Choose a demand method, enter the loads or floor-area details it needs, and review the result per phase.

Calculate maximum demand

LightingSocket-outletsEquipment

Illustrative shed loads: lighting, socket-outlets and equipment.The voltage-drop example that follows uses a fixed 20 A load.

02Voltage drop

Check voltage drop along the run.

Follow the cable from the switchboard to the shed. Change the route length or copper conductor size below to compare voltage drop. The example keeps the load at 20 A.

One-way route 30 m

Copper conductor

Voltage drop2.93%223.27 V at the shed

Voltage along the cable230 V supply · single phase · 20 A example

4 mm² selected

Simplified voltage drop using AS/NZS 3000 Table C8.Fixed 20 A example load.

Open voltage drop with these inputs Play the shed supply in 3D

Voltage drop only. Current capacity, protection and the complete installation need separate checks.

03Cable sizing

Size the cable for its conditions.

A voltage-drop result is one part of cable selection. Current capacity depends on how and where the cable is installed.

Choose the installation method, ambient temperature and grouping in the cable-sizing calculator. Smythy shows the correction factors alongside the cable result.

Check cable sizing

Flat TPS

Flat TPS cable, shown as an illustrative cutaway.Copper V-75 multicore cable screening. Confirm the selection against the standard, manufacturer data and installation.

Installation method

Choose the wiring system and the way the cable is installed.

Ambient temperature

Enter the temperature around the cable.

Grouping

Account for other loaded circuits grouped with it.

04Earth fault-loop impedance

Check the fault return path.

Use the earth fault-loop calculator to check the cable route against the protective device you plan to use.

Choose the device rating, type and disconnection time to find maximum loop impedance. Add conductor sizes and route length for the cable-route check.

Check earth fault loop

Active to equipment

Earth path to supply

Schematic fault return path.Confirm the tool’s assumptions against the actual circuit and protective device data.

05Calculation records

Keep the inputs and the working.

The record follows the voltage-drop example: it updates with the route length and conductor size you choose.

Open the same inputs in the full calculator, review the working and download a PDF for your job notes.

Open this voltage-drop calculation

PDF exports need an internet connection and include a Smythy watermark. A calculation record does not certify the installation.

Voltage drop · Single phase

Shed supply

Voltage drop2.93%

6.73 V along the run

Supply: 230 V
Load: 20 A
One-way route: 30 m
Copper conductor: 4 mm²

Table C8 simplified method

20 × 30 ÷ 205 = 2.93%

Voltage drop only. Current capacity, protection and the complete installation need separate checks.

Change this example

Use your own figures

Choose your next calculation.

Choose what you need to work out, or open the full library for installation, protection and DC calculators. Each tool explains its method and limits.

All electrical calculators

33 calculators. No account needed.

Supply a shed or outbuildingLoad, cable size and the route to the load

Work from the load to the cable run. If you already know the current the circuit must carry (the design current), start at cable sizing.

Work out maximum demand
Assess the connected loads using the applicable demand method.
Have ready: load types, ratings and phases
Check cable sizing
Review current capacity and voltage drop for copper V-75 cable.
Have ready: design current, route length and installation conditions
Check voltage drop along the run
Compare a selected conductor size using the simplified Table C8 method.
Have ready: current, one-way length and cable size
Check protective earth size
Look up the protective conductor size for the selected active conductor.
Have ready: active conductor material and size
Check the fault return path
Review maximum loop impedance or the copper cable route against the protective device.
Have ready: device type, rating, disconnection time and conductor details

Earthing arrangements, fault protection and site conditions need their own assessment. This path does not establish a complete shed supply design.

Read the cable sizing method and limits

Check voltage lost along a cableCheck a conductor size or work towards a voltage-drop limit

Calculate voltage drop for a selected conductor size, or find a minimum size for a chosen voltage-drop limit. Use cable sizing when you also need to assess current capacity.

Open the voltage drop calculator
Calculate the drop, or find a minimum size for a chosen voltage-drop limit.
Have ready: current, one-way route length, phase, and conductor size or voltage-drop limit
Check current capacity as well
Allow for installation method, temperature and cable grouping.
Have ready: design current and installation conditions

The voltage drop tool uses the AS/NZS 3000 Table C8 simplified method. A voltage-drop result alone does not establish cable current capacity.

Check voltage rise from solarChoose a single run or a route with several sections

Start with the cable route between the inverter and the supply. Choose the tool that matches how many cable sections you need to model.

Check single-run solar voltage rise
Estimate voltage rise along a single inverter cable run.
Have ready: export current, one-way length, one-conductor resistance in Ω/m, voltage and phases
Check voltage rise across cable sections
Add the voltage rise across separately entered cable segments.
Have ready: each section’s current, length and conductor details

Verify cable impedance and equipment data. These models do not assess every inverter connection or network requirement.

Work out power or motor currentkW, kVA, power factor or a motor load

Convert a load between kW and kVA, or estimate a motor’s full-load current.

Convert kW and kVA
Relate real power, apparent power and power factor.
Have ready: kW or kVA, plus power factor
Estimate motor full-load current
Estimate current from rated motor power and supply using typical motor data.
Have ready: motor power in kW and single- or three-phase supply

Use motor nameplate and manufacturer data for equipment selection. Full-load current does not describe starting current.

Work through a DC circuitVoltage, current, resistance and resistor values

Start with the quantities you know, or identify a resistor before working through the circuit.

Find voltage, current or resistance
Use Ohm’s law and the DC power relationships.
Have ready: any two of voltage, current and resistance
Read a resistor’s colour bands
Decode its resistance and tolerance.
Have ready: band count and colours in order
Combine resistors
Find equivalent resistance for a series or parallel arrangement.
Have ready: resistor values and the circuit arrangement

These tools cover ideal DC circuit relationships. Check component ratings and tolerances separately.