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EV Charging Infrastructure

EV charger sizing, demand factors, and load management

IEC 61851NEC 625BS 7671AS/NZS 3000

Standard

Supply Parameters

Charger Groups

Load Management

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EV charging load is the electrical demand imposed by an electric vehicle supply equipment unit on the installation. IEC 61851-1 defines charging modes from Mode 1 through Mode 4, with power levels ranging from 3.7 kilowatts single-phase to over 350 kilowatts DC fast charging. Circuit design must account for continuous duty and diversity among multiple chargers.

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How to Size an EV Charger Circuit

  1. 1
    Determine the charging mode — Identify the IEC 61851-1 charging mode: Mode 2 uses a portable EVSE with domestic socket, Mode 3 uses a dedicated wallbox with Type 1 or Type 2 connector, and Mode 4 is DC fast charging.[IEC 61851-1]
  2. 2
    Establish the continuous load current — EV charging is classified as a continuous load because it can operate for more than three hours. Apply a 1.25 multiplier to the charger rated current when sizing the cable and protective device.[BS 7671 Regulation 433.1]
  3. 3
    Select cable size with derating — Size the cable for the continuous load current including all applicable derating factors. A typical 7.4 kW single-phase charger draws 32A, requiring a minimum 6mm2 cable for most installation methods.[IEC 60364-5-52 Clause 523]
  4. 4
    Select the protective device — Install an RCBO or MCB plus RCD combination with Type A or Type B RCD sensitivity of 30 mA for personal protection. Type B RCD is recommended for Mode 3 chargers with DC fault current capability.[IEC 62955]
  5. 5
    Apply diversity for multiple chargers — For installations with multiple EV chargers, apply diversity factors to reduce the total supply demand. Typical factors range from 0.8 for two chargers to 0.4 for ten or more, based on usage patterns.[BS 7671 Appendix 15]
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How EV Charging Works

The EV charging calculator sizes the electrical infrastructure for electric vehicle charging installations, including charger selection, cable sizing, demand management, and supply capacity assessment.

The calculator supports all IEC 61851-1 charging modes: Mode 1 (domestic socket), Mode 2 (portable EVSE), Mode 3 (dedicated AC), and Mode 4 (DC fast charging). Demand factors are applied per the selected standard — BS 7671 Section 722 provides specific diversity factors for multiple EV chargers, NEC Article 625 establishes feeder demand factors per Table 625.42, and AS/NZS 3000 applies general maximum demand rules.

Static and dynamic load management options reduce the required supply capacity by distributing available power across chargers. Results include recommended charger types, per-charger and total demand, cable sizes per circuit, supply capacity assessment, load management schedule, and cost estimation.

EV Charging Modes and Power Levels

ModeConnectorPower RangeCurrentStandard
Mode 2Domestic socket2.3–3.7 kW10–16AIEC 61851-1
Mode 3 (single-phase)Type 23.7–7.4 kW16–32AIEC 61851-1
Mode 3 (three-phase)Type 211–22 kW16–32AIEC 61851-1
Mode 4 (DC)CCS/CHAdeMO50–350 kW125–500AIEC 61851-23

Source: IEC 61851-1 Clause 6, BS 7671 Section 722

Frequently Asked Questions

How do I size a circuit for an EV charger per NEC Article 625?
NEC Article 625.40 requires that EV charging equipment be treated as a continuous load, meaning the branch circuit must be rated at 125% of the maximum load. For a 40A Level 2 EVSE, the circuit must be rated for at least 50A (40A x 1.25), requiring a 50A breaker, 6 AWG copper conductors (per Table 310.16 at 75 degrees C), and a NEMA 14-50 or hardwired connection. NEC 625.42 specifies that each EVSE must have its own dedicated circuit unless the energy management system provisions of 625.42(A) are used.
What is the continuous load rating requirement for EV charging?
EV charging is classified as a continuous load (operating for 3 hours or more) by IEC 61851-1, NEC 625.40, and BS 7671 Section 722.531.1. This means the circuit conductors and protective devices must be rated at 100% of the EVSE rated current using the appropriate installation method, or where the protective device is not rated for continuous duty, at 125% of the load. For example, a 32A Type 2 charger under BS 7671 requires a 40A protective device on a circuit rated for at least 32A continuous using reference method C or better.
What is EV load management and when is it required?
EV load management (also called smart charging or demand management) distributes available supply capacity among multiple EVSEs to avoid exceeding the building supply limit. BS 7671 Section 722.311.1 explicitly addresses this, requiring that the maximum demand of the installation including EVSEs be assessed. NEC 625.42(A) permits a power management system to limit the total EVSE load. IEC 61851-1 defines Mode 3 with communication (via pilot signal) enabling dynamic load adjustment. For a 10-charger car park with 100A supply, static management might limit each to 10A, while dynamic management allocates current based on actual usage.
What RCD/GFCI protection is required for EV chargers?
IEC 61851-1 requires Type B or Type A + Type B RCD protection for EV charging to detect both AC and DC residual currents (EVSEs with onboard rectifiers can produce smooth DC fault currents). BS 7671 Regulation 722.531.3 mandates a 30mA Type A RCD minimum, with Type B for circuits feeding EVSEs without DC leakage protection. NEC 625.54 requires GFCI protection for all EV charging outlets. AS/NZS 3000 requires RCD protection per Clause 2.6.3. Many modern EVSEs include built-in DC 6mA detection, allowing use of a Type A RCD instead of the more expensive Type B.
How many EV chargers can I install on my existing supply?
The number of chargers depends on your available supply capacity after existing load diversity. First determine your maximum demand (per AS/NZS 3000, BS 7671 Appendix A, or NEC Article 220) and subtract from your supply capacity. Each charger at full load draws its rated current continuously (e.g., 7.4kW single-phase = 32A, 22kW three-phase = 32A per phase). With dynamic load management per IEC 61851-1, you can install more chargers than the raw capacity allows, as the system throttles charging rates based on real-time demand. ECalPro's calculator models both static and dynamic scenarios.
What are the IEC 61851 charging modes?
IEC 61851-1 defines four charging modes: Mode 1 is slow charging from a standard socket outlet without dedicated protection (prohibited in many countries including the US and UK). Mode 2 is charging from a standard socket with an in-cable control and protection device (ICCPD), limited to 8-13A. Mode 3 is dedicated EVSE with control pilot communication (the standard for permanent installations, 16-63A). Mode 4 is DC fast charging (CHAdeMO, CCS) with the charger converting AC to DC externally. BS 7671 Section 722 and NEC Article 625 primarily address Mode 3 fixed installations.

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Key Terms

ev charging loadcable sizingrcd operating timetype b rcd

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Standards Reference

  • IEC 61851-1 — EV conductive charging
  • NEC Article 625 — EV charging
  • BS 7671:2018+A2 — Section 722