Electrical Load Schedule Calculator — Circuit-by-Circuit Demand Analysis

An electrical load schedule is a tabulated list of every circuit in an electrical installation, showing the connected load, protective device rating, cable size, and design current for each circuit. It is the master document that captures the entire electrical demand of a building, from individual lighting circuits to heavy power feeds.

The load schedule serves multiple purposes. For the designer, it is the basis for calculating the total maximum demand and verifying that the incoming supply is adequate. For the installer, it is the specification that defines what to install. For the inspector, it is the reference document checked during EICR inspections and initial verification. For the building owner, it is a permanent record of the electrical installation.

A properly prepared load schedule includes the circuit reference number, circuit description (what the circuit supplies), the connected load in watts, the design current in amps, the protective device type and rating, the cable type and size, the wiring method, and the phase allocation for three-phase installations. It is one of the most important design documents an electrician produces, yet it is also one of the most time-consuming to prepare manually.

Load schedules are not optional documentation — they are a fundamental part of electrical design and certification. The following scenarios all depend on an accurate load schedule:

• Supply assessment — the DNO requires a maximum demand figure when you apply for a new supply or an upgrade. This figure comes directly from the load schedule with diversity applied.
• EIC and EICR certificates — Section 6 of the EIC and Section F of the EICR require the assessed maximum demand. You cannot complete these certificates accurately without a load schedule.
• Main cable and switchgear sizing — the load schedule determines the size of the main tails, the rating of the main switch, and the busbar rating of the distribution board.
• Generator and UPS sizing — if the installation has standby generation or battery backup, the load schedule identifies which circuits are essential and their combined demand.
• Three-phase balancing — for three-phase installations, the load schedule ensures loads are distributed evenly across phases, preventing neutral overloading and voltage imbalance.

Each row in the load schedule represents one circuit. The following columns are standard for UK electrical design:

For domestic installations, the load schedule is typically simple — 6 to 12 circuits covering lighting, socket outlets, cooker, shower, immersion heater, and EV charger. For commercial installations, the schedule may run to 50 or more circuits across multiple distribution boards. Elec-Mate supports both, with the ability to create sub-schedules for each distribution board and roll up to a main schedule. The diversity factor calculator applies the correct percentages for each load type.

Once every circuit is listed with its connected load, diversity is applied to calculate the assessed maximum demand. Without diversity, the total connected load of a typical domestic installation might exceed 30 kW. With diversity applied per IET On-Site Guide Table 1A, the assessed maximum demand typically falls to 12-18 kW.

Diversity is applied by load type, not by circuit. The main categories and their diversity allowances are:

• Lighting: 66% of total connected load
• Socket outlets: first 10 A at 100%, then 50% of remainder
• Cooking appliances: 10 A + 30% of remainder + 5 A if socket outlet on cooker unit
• Showers, immersion heaters, storage heating: 100% (no diversity)
• EV charge points: 100% for single domestic unit

Elec-Mate applies these rules automatically as you build the schedule. Each circuit is categorised by load type, and the diversity percentages are applied in the summary section. The result is the assessed maximum demand — the figure you enter on the EIC certificate and compare against the supply rating.

For three-phase installations, the load schedule must show the phase allocation for each circuit. The aim is to balance the total load as evenly as possible across the three phases (L1, L2, L3) to minimise neutral current and prevent voltage imbalance.

Perfect balance is rarely achievable in practice because individual circuit loads vary, but the designer should aim for the highest loaded phase to be within 10-15% of the lowest loaded phase. Significant imbalance causes excessive neutral current (which can overheat the neutral conductor in a system without an oversized neutral), voltage differences between phases (which can damage sensitive equipment), and reduced capacity from the supply transformer.

Elec-Mate's load schedule calculator includes a phase balance display showing the total load on each phase and the percentage imbalance. As you allocate circuits to phases, the display updates in real time, helping you achieve good balance before installation begins. This is particularly important when combined with the three-phase power calculator for commercial and industrial projects.