How do I calculate the maximum demand for a domestic property?

For a typical domestic installation, the IET On-Site Guide (OSG) provides a simplified approach using diversity factors from BS 7671 Appendix 1. The method adds up the connected loads in each category (lighting, socket outlets, cooking appliances, water heating, space heating), applies the appropriate diversity factor to each category, and sums the results to give the assessed maximum demand. For example: lighting (100% of largest + 50% of remaining circuits), socket outlets (100% of largest + 40% of remaining), immersion heater (100%), cooker (10A + 30% of remainder + diversity for second appliance if present). The result is used to select the consumer unit main switch, incoming supply fuse, and supply cable. A typical 3-bed semi with all-electric services typically calculates at 40 to 60A maximum demand.

What are diversity factors and why are they used?

Diversity factors reflect the fact that not all electrical loads in an installation operate simultaneously at full power. If every circuit in a house ran at full load at the same moment, the total demand would far exceed the supply capacity — but in practice, loads are switched on and off at different times and rarely all reach full power simultaneously. BS 7671 Appendix 1 provides diversity factors that reduce the design current below the sum of all connected loads. Applying diversity factors allows the designer to select an appropriately sized main protective device and supply cable without the cost and impracticality of sizing for the theoretical worst case. However, diversity factors must not be applied where it is foreseeable that loads will actually run simultaneously — for example, EV chargers with fixed timed charging schedules.

How does cable current-carrying capacity relate to load calculations?

Once the maximum demand (design current, Ib) for a circuit has been established, the cable must be selected with a current-carrying capacity (CCC or Iz) at least equal to Ib. However, the tabulated CCC values in BS 7671 Appendix 4 assume installation at 30°C ambient temperature in free air or a single circuit in a standard enclosure. In practice, cables are often grouped with others (reducing CCC due to mutual heating), installed in warmer locations such as roof spaces (requiring derating for ambient temperature above 30°C), or clipped to a thermal insulation surface (requiring derating for partial or full enclosure in insulation). The correction factors Ca (ambient temperature), Cg (grouping), Ci (insulation), and Cc (buried cables) must all be applied. The corrected CCC = tabulated CCC × Ca × Cg × Ci must be not less than Ib.

When do I need to do a full load calculation rather than use the IET OSG simplified method?

The IET On-Site Guide simplified diversity method is intended for standard domestic installations. You should perform a full load calculation (using first principles from BS 7671 Appendix 1 and the IET Wiring Regulations) in the following circumstances: commercial or industrial premises of any size; domestic premises with unusually high connected loads (multiple EV chargers, heat pumps, electric showers, large cooking ranges); any premises where the supply authority's cutout fuse is 100A or larger; three-phase domestic installations; or any premises where the designer cannot confirm that the OSG diversity assumptions are realistic for the actual usage pattern. Full calculations should be documented and retained with the design information for the installation.

How do I size a supply for a new EV charger without overloading the existing installation?

EV charger load calculation requires knowing the charger rated output (typically 7kW for a single-phase 32A EVSE, or 22kW for three-phase 32A per phase), the existing installation maximum demand, and the available supply capacity. Unlike socket outlet circuits, EV charger circuits should be treated as full demand (no diversity) unless a load management system is installed that actively limits charger output when other loads are high. For a 7kW (32A) single-phase charger: the existing 60A supply may be adequate if the existing maximum demand is calculated at 28A or less. If inadequate, a supply upgrade to 80A or 100A should be requested from the supply company, or a dynamic load management (DLM) system should be installed to share available capacity between the charger and other loads. This must be documented in the installation design and reflected on the EIC.

What is the IET On-Site Guide and where does it cover load calculations?

The IET On-Site Guide (OSG) is a practical companion to BS 7671, produced by the Institution of Engineering and Technology. It provides simplified tables and methods for standard domestic and small commercial installations. Load calculation and diversity factor guidance is found in Section 1.2 (maximum demand) and Appendix B (diversity factors). The OSG is updated to reflect each amendment to BS 7671 — ensure you are using the edition corresponding to BS 7671:2018+A4:2026 (the 2024 edition of the OSG). The OSG is available from the IET Shop and is a standard reference for the AM2, City and Guilds 2382, and NVQ Level 3 exams.

How do I balance a three-phase supply across phases for a commercial installation?

Three-phase phase balancing is achieved by distributing single-phase loads as evenly as possible across the three phases (L1, L2, L3) during the circuit design stage. The goal is to minimise the neutral current, which is the vector sum of the three phase currents. For perfectly balanced loading, neutral current is zero. For significantly unbalanced loading, the neutral current can approach the phase current, requiring the neutral conductor to be sized accordingly under BS 7671 Regulation 523.6. In practice, exact balance is rarely achievable — aim for phase currents within 10% of each other. Heavily unbalanced single-phase loads (such as all EV chargers on one phase) create significant neutral current and increased losses. Document the phase allocation in the distribution board schedule and on the EIC.

TECHNICAL GUIDE

Electrical Load Calculation — UK Electrician Guide

A complete guide to calculating electrical load for domestic and commercial premises: diversity factors from BS 7671 Appendix 1, maximum demand, cable sizing correction factors, three-phase calculations, and IET On-Site Guide methods with worked examples.

Start 7-day free trial

Free for 7 days · No charge until day 8 · Cancel anytime · Used by 1,000+ UK electricians

10 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

ShareX in W

1,000+

UK electricians

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer — DP Electrical

Key Takeaways

1Electrical load calculation determines the maximum demand on a circuit, distribution board, or supply in order to select correctly rated protective devices, cables, and supply capacity. Under-sizing any of these elements creates a fire and shock risk.
2BS 7671:2018+A4:2026 Appendix 1 provides diversity factors for domestic and commercial installations, allowing the calculated maximum demand to be reduced below the simple sum of all connected loads to reflect the realistic simultaneous usage pattern.
3The IET On-Site Guide (OSG) provides tabulated diversity factors and simplified methods for domestic load calculations. For commercial and industrial premises, full calculations using first principles are required.
4Cable current-carrying capacity (CCC) must be derated for grouping, ambient temperature, and installation method using the correction factors in BS 7671 Appendix 4. The corrected CCC must not be less than the design current of the circuit.
5For three-phase supplies, phase balance should be considered during load allocation. Unbalanced loading increases neutral current, which must be accounted for in neutral conductor sizing under BS 7671 Regulation 523.6.
6Maximum demand calculations must be reviewed when significant new loads are added to an existing installation — for example, EV chargers, heat pumps, or electric showers. The supply capacity and protective device ratings must be adequate for the revised maximum demand.

01 · Technical Guide

What is Electrical Load?

Electrical load is the total power demand placed on a circuit, distribution board, or supply by the connected electrical equipment. It is expressed in amperes (A) for current, watts (W) or kilowatts (kW) for power, or volt-amperes (VA) for apparent power. The design current (Ib) is the current that a circuit is expected to carry under normal operating conditions.

Load calculations are required when designing a new installation, when modifying an existing installation to add significant new loads, and when assessing whether an existing installation can safely accommodate additional loads. Incorrectly sized circuits, distribution boards, or supply connections are a significant cause of electrical fires and equipment damage.

Design current (Ib): The current the circuit is designed to carry. Must not exceed the current-carrying capacity (Iz) of the cable after applying correction factors.
Maximum demand: The highest assessed demand that the installation will draw simultaneously. Used to size the main switch, supply cable, and supply fuse.
Connected load: The sum of the rated power of all connected equipment. The maximum demand is always less than the connected load for most installations because not all loads run simultaneously.

Free download

Get the BS 7671 A4:2026 Cheat Sheet — free

Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.

Every regulation change summarised
New model forms (EIC + MEIWC)
Free PDF — no subscription

02 · Technical Guide

Diversity Factors — BS 7671 Appendix 1

BS 7671:2018+A4:2026 Appendix 1 provides guidance on diversity factors for assessing the maximum demand of domestic and commercial installations. Diversity factors reduce the calculated maximum demand to reflect realistic simultaneous usage, rather than the theoretical worst-case where all loads run simultaneously.

Domestic Diversity Factors (from BS 7671 Appendix 1)

Lighting: 66% of total connected lighting load (or 100% of the largest circuit + 50% of remaining, per IET OSG method)
Heating and water heating (thermostatically controlled): 100% of the largest + 50% of remaining
Cooking appliances: 10A + 30% of connected load above 10A + 5A if second outlet on cooker control unit
Socket outlet circuits: 100% of largest + 40% of remaining (residential); 100% of total in commercial

EV chargers and heat pumps: These loads should generally be assessed at 100% (no diversity) unless a load management system is installed that actively limits output. Always discuss with the client and document the assessment basis in the design.

03 · Technical Guide

Maximum Demand Calculation

Maximum demand calculation brings together connected loads and diversity factors to establish the design basis for a supply, distribution board, or circuit. The result is used to select the main protective device rating and the supply cable CCC.

Domestic Example — 3-Bed Semi

Lighting (8 × 100W equivalent): 66% of 800W = 528W / 230V = 2.3A
Ring final circuits (2 × 32A rings): 32A + (32A × 40%) = 44.8A (but limited by supply)
Cooker (7kW): 10A + (30% × 20.4A) = 16.1A
Immersion heater (3kW): 100% = 13A
Assessed maximum demand: approximately 52A — 60A supply adequate

The actual calculation must be performed using the connected loads specific to the property. Document the calculation and retain it with the EIC for the installation. Use

Try it free for 7 days

to record design details on the EIC.

04 · Technical Guide

Cable Sizing from Load Calculations

Once the design current (Ib) for a circuit is established, the cable must be selected with sufficient current-carrying capacity (Iz) after applying all relevant correction factors. The fundamental relationship is:

Iz (corrected) = tabulated Iz × Ca × Cg × Ci ≥ Ib

Ca — Ambient temperature correction: Applied when the ambient temperature at the cable location exceeds 30°C. Cables in roof spaces in summer can reach 60°C+ ambient — Ca can reduce CCC by 40% or more.
Cg — Grouping correction: Applied when cables are grouped together or run in a common enclosure. For 4 touching cables, Cg = 0.65 (35% reduction in CCC).
Ci — Insulation enclosure correction: Applied to cables fully enclosed in thermal insulation. For full enclosure, Ci = 0.5 for some installation methods — a 50% reduction in CCC.

BS 7671 Appendix 4 contains the full tables of CCC and correction factors. The IET On-Site Guide provides simplified tables for common installation methods. Always check voltage drop (Regulation 525) as well as CCC — the voltage drop from supply to load must not exceed 3% for lighting circuits and 5% for other circuits under normal operating conditions.

Try Elec-Mate free for 7 days

16 certificate types, 70+ calculators, RAMS, quoting, invoicing, AI agents, and 46+ training courses — from £6.99/mo.

Start free trial

05 · Technical Guide

Three-Phase vs Single-Phase Load Calculations

Three-phase supplies are standard for commercial and industrial premises, and are increasingly used in domestic premises with large electrical loads (EV charging, large heat pumps). The load calculation approach differs from single-phase.

Three-phase balanced load: For balanced three-phase loads (three-phase motors, three-phase heaters), P = √3 × V_L × I × pf, where V_L is the line voltage (400V in the UK). A 22kW three-phase EV charger draws 22,000 / (√3 × 400 × 1.0) = 31.8A per phase.
Single-phase loads on a three-phase supply: Each single-phase load draws current on one phase only. Phase balance must be considered during circuit design — allocate single-phase circuits to phases to minimise the difference in loading between L1, L2, and L3.
Neutral conductor sizing: For balanced three-phase loading with no significant harmonic distortion, the neutral current is zero or negligible. Where significant single-phase loads cause phase imbalance, or where non-linear loads (LEDs, VFDs, switched-mode power supplies) create third-harmonic currents, the neutral current may exceed the phase current — requiring the neutral to be the same cross-section as the phase conductors (BS 7671 Regulation 523.6.3).

06 · Technical Guide

Commercial and Industrial Load Calculations

Commercial and industrial load calculations follow the same principles as domestic calculations, but the scale and complexity is greater and the consequences of errors more significant. Full first-principles calculations are required — not simplified OSG methods.

Distribution board schedules: Each distribution board requires a schedule documenting circuit designation, protective device rating, cable size, design current, and diversity. This schedule forms part of the EIC documentation.
Demand diversity in commercial premises: BS 7671 Appendix 1 provides diversity factors for offices and commercial premises. For socket outlet circuits, 100% of total connected load is often assumed in commercial locations — unlike the 40% diversity applied in domestic.
Power factor correction: Industrial premises with large motor loads have a lagging power factor — the apparent power (kVA) exceeds the true power (kW). Power factor correction (PFC) capacitors reduce the reactive demand and can avoid penalty charges from the supply authority. Cable sizing uses apparent current (kVA / V), not true power current.

07 · Technical Guide

Worked Examples

The following worked examples illustrate the application of diversity factors and load calculation methods for common UK electrical installation scenarios.

Example 1 — EV Charger Added to Existing Installation

Existing assessed maximum demand: 52A (from previous calculation)
New 7kW EV charger (32A): added at 100% demand (no diversity) = 32A
Revised maximum demand: 52A + 32A = 84A — exceeds 60A/80A supply
Action: Apply for 100A supply upgrade from DNO, or install DLM system limiting charger to 28A or less

Example 2 — Small Office Distribution Board

Lighting (6 × 2A circuits): 100% = 12A
Socket outlet circuits (4 × 32A): 100% of total = 128A (commercial — no diversity)
Air conditioning (3-phase, 12kW): 12,000 / (√3 × 400) = 17.3A per phase
Assessed maximum demand: 157A — select 160A MCCB, 95mm² supply cable

See for how undersized cables and protective devices are typically recorded as C2 observations on electrical inspection reports.

Electrical Load Calculation — Frequently Asked Questions

What electricians say

Verified reviews from the UK App Store.

One App for Everything!

Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.

Fantastic app for electricians

I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.

Absolutely amazing

I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.

Trusted by electricians across the UK

Real feedback from real sparks

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer

Sole Trader · DP Electrical

“I've won two contracts this month because I could turn quotes around same-day with the AI cost engineer.”

Nathan Perry

Electrician · NP Electrical Services

“The study centre got me through my AM2. Mock exams and flashcards are brilliant.”

Jake Pizey

3rd Year Apprentice · Apprentice

7-Day Free Trial — Cancel Anytime, No Hassle

Issue Electrical Installation Certificates with load calculation records

Generate compliant EIC documents on your phone with Elec-Mate. Start your free 7-day trial.

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer, DP Electrical

From £6.99/mo after trial — less than a coffee a week

or download the app

7 days free, then from £6.99/moCancel in one tap — no calls, no hassleiOS, Android & WebBS 7671 compliant

Electrical Load Calculation — UK Electrician Guide

Key Takeaways

What is Electrical Load?

Get the BS 7671 A4:2026 Cheat Sheet — free

Diversity Factors — BS 7671 Appendix 1

Domestic Diversity Factors (from BS 7671 Appendix 1)

Maximum Demand Calculation

Domestic Example — 3-Bed Semi

Cable Sizing from Load Calculations

Try Elec-Mate free for 7 days

Three-Phase vs Single-Phase Load Calculations

Commercial and Industrial Load Calculations

Worked Examples

Example 1 — EV Charger Added to Existing Installation

Example 2 — Small Office Distribution Board

Electrical Load Calculation — Frequently Asked Questions

How do I calculate the maximum demand for a domestic property?

What are diversity factors and why are they used?

How does cable current-carrying capacity relate to load calculations?

When do I need to do a full load calculation rather than use the IET OSG simplified method?

How do I size a supply for a new EV charger without overloading the existing installation?

What is the IET On-Site Guide and where does it cover load calculations?

How do I balance a three-phase supply across phases for a commercial installation?

Continue reading

Solar PV System Sizing

Cable Management Systems

EICR Observation Codes Explained

EICR for Landlords

Kitchen Electrical Requirements

What electricians say

One App for Everything!

Fantastic app for electricians

Absolutely amazing

Trusted by electricians across the UK

Issue Electrical Installation Certificates with load calculation records