RCD Size Calculator
Determine the suitable residual current device (RCD) rating using circuit load current, supply arrangement, and protection requirements in accordance with BS 7671 UK electrical installation practices.
RCD Size Calculator
How to Use the RCD Size Calculator
Correctly sizing a Residual Current Device (RCD) is critical to ensuring safety and operational reliability in compliance with the IET Wiring Regulations. Follow this practical, step-by-step workflow to use the sizing tool effectively:
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1Enter design current. Input the calculated design current (Ib) of the circuit in Amperes. This represents the maximum current expected during normal service.
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2Enter protective device rating. Input the nominal rating of the upstream overcurrent protective device (In) in Amperes, such as the MCB or fuse.
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3Select circuit type. Select the specific circuit application. This helps determine the default sensitivity requirements (e.g., 30mA for additional shock protection).
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4Select supply system. Choose the earthing system configuration (TN-C-S, TN-S, or TT) of the installation. This affects compliance recommendations.
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5Calculate results. Click "Calculate RCD Size" to view the recommended continuous current rating (In), trip sensitivity (IΔn), and specific BS 7671 references.
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6Review recommended RCD rating and sensitivity. Inspect the output details to verify if your device meets standard current carrying capacities and safety regulations.
How to Calculate RCD Size
Sizing an RCD requires evaluating two independent criteria: the continuous current carrying capacity (load rating) and the residual operating current (sensitivity) of the device. These must be determined based on overcurrent coordination and protection needs.
Formula 1 — Recommended RCD Current Rating
To protect the internal contacts of the RCD from thermal damage due to overloads, the rated current of the RCD (IRCD) must be equal to or greater than the nominal rating of the upstream overcurrent protective device (In). This is expressed by the relationship:
Where:
- IRCD = Rated continuous current of the RCD (standard UK ratings are 25A, 40A, 63A, 80A, 100A, 125A).
- In = Rated current of the upstream overcurrent protective device (MCB/fuse).
- Ib = Design current of the circuit.
Formula 2 — Residual Current Sensitivity
RCD sensitivity, or rated residual operating current (IΔn), is chosen based on whether the device is providing additional protection, fault protection, or fire protection:
- Additional Protection (Personal Safety): Standard sensitivity is 30 mA. BS 7671 requires 30mA RCDs for socket outlets, lighting in domestic dwellings, EV chargers, and locations containing showers or baths.
- Fault Protection (Automatic Disconnection): Typically 100 mA or 300 mA. Used in TT installations at the origin, or on distribution circuits where the earth fault loop impedance (Zs) is too high to trip overcurrent devices but personal shock protection is handled downstream.
- Fire Protection: Maximum of 300 mA under Regulation 422.3.9 for areas with high fire risk.
Real UK domestic worked example
Scenario: Sizing an RCD protecting a 32A Ring Final Circuit supplying socket outlets in a domestic property with a TN-C-S earthing arrangement. The design current is calculated at 26A.
Step-by-Step Calculation:
- Step 1: Check overcurrent coordination. The upstream MCB rating is 32A (In = 32A). According to the sizing formula:
IRCD ≥ In → IRCD ≥ 32 AThe next standard RCD current rating above 32A is 40 A.
- Step 2: Determine sensitivity. Under BS 7671 Regulation 411.3.3, all socket-outlets up to 32A in general installations must have additional protection via an RCD with a rated residual operating current not exceeding 30mA:
IΔn = 30 mA
- Step 3: Verification against earthing system. In a TN-C-S system, fault protection is primarily achieved by MCBs. The 30mA RCD provides additional shock protection. Therefore, a standard 40A 30mA RCD (or RCBO) is the correct selection.
Final Selection Output
- Recommended RCD Rating: 40 A
- Recommended Sensitivity: 30 mA
- Application: Ring Final Socket Circuit
- Compliance Status: BS 7671 Compliant
RCD Size Selection Chart
This reference table details the standard RCD current ratings and sensitivity limits recommended for typical UK circuit protective devices under normal load configurations. Values comply with coordination guidelines to prevent overload damage.
| Protective Device Rating (In) | Recommended RCD Rating (IRCD) | Typical Application | Mandated Sensitivity (IΔn) |
|---|---|---|---|
| 6 A | 25 A | Domestic Lighting Circuit | 30 mA |
| 16 A | 25 A | Radial Power Circuit | 30 mA |
| 20 A | 40 A | Socket Outlet Radial Circuit | 30 mA |
| 32 A | 40 A | Ring Final Socket Circuit | 30 mA |
| 40 A | 63 A | Cooker Radial Circuit | 30 mA |
| 50 A | 63 A | Small Distribution Board Submain | 100 mA / 300 mA (S-Type) |
| 63 A | 80 A | Submain Feeder Cable | 100 mA / 300 mA (S-Type) |
| 80 A | 100 A | Commercial Distribution Circuit | 100 mA / 300 mA (S-Type) |
| 100 A | 100 A | Main Switch Intake Protection | 100 mA / 300 mA (S-Type) |
Note: Actual selection depends on manufacturer ratings, diversity, fault protection requirements and BS 7671 compliance. Upstream devices on submain feeders should incorporate time-delayed (S-type) features to maintain discrimination.
RCD Size Calculator Frequently Asked Questions
For a 32A circuit (such as a ring final circuit or cooker radial), you typically need a 40A RCD. The RCD's continuous current rating (load rating) must equal or exceed the rating of the upstream protective device (MCB) to prevent thermal overload of the RCD's internal contacts under maximum load conditions.
Yes, the continuous current rating of an RCD (e.g., 40A, 63A, 80A) should always be equal to or greater than the rating of the upstream circuit breaker (MCB) protecting it. Sizing the RCD higher prevents overload currents from overheating and damaging the RCD switch mechanism during normal operation or prolonged minor overloads.
Yes, under BS 7671 Regulation 411.3.3, a 30mA residual current device is mandatory for additional protection on all socket-outlets with a rated current not exceeding 32A in general installations. This sensitivity rating is crucial as it trips fast enough to prevent lethal electric shocks to humans.
EV charger installations require a 30mA RCD. Under BS 7671 Section 722, it must be a Type A device at a minimum, combined with equipment that detects DC residual leakage currents exceeding 6mA. Alternatively, a Type B RCD can be used to handle both smooth DC and high-frequency AC earth leakage.
Yes, a 63A RCD can protect a circuit with a 40A MCB. Since 63A is the maximum continuous current the RCD can safely carry, protecting it with a lower-rated 40A overcurrent device is completely safe. The RCD will protect against earth leakage faults, while the MCB protects against overloads.
A 30mA RCD has high sensitivity and is used for additional personal protection (tripping within 40ms at 5x fault current to prevent electrocution). A 100mA RCD has lower sensitivity and is primarily used for fault protection (automatic disconnection) or fire protection, often in TT earthing systems or submains.
