Multiple Breakers on One RCD

When designing an electrical system for a home or small business, you may consider connecting multiple circuit breakers to a single RCD (Residual-Current Device) for protection. However, careful planning is needed to ensure safety and compliance with electrical codes such as the NEC.

What Is an RCD and How Does It Work?

An RCD (or GFCI, Ground-Fault Circuit Interrupter, in the U.S.) monitors the balance between the current flowing in the live and neutral conductors. If an imbalance occurs—indicating a leakage current—it trips and disconnects the circuit, protecting against electric shock. In this configuration, each branch circuit is typically protected by its own circuit breaker, while the RCD oversees multiple circuits.

Connecting Multiple Breakers to One RCD

Key Factors:

1. Maximum Load Current:
   The RCD must handle the sum of the currents from all connected circuits. For residential installations, this is typically around 25 amperes (A).

2. Differential (Leakage) Trip Current:
   The RCD trips when the leakage current exceeds its sensitivity threshold, commonly 30 milliamps (mA) for standard residential protection. However, to ensure proper protection, the total natural leakage (I_leak) from all circuits should not exceed one-third of the trip current—about 10 mA.

3. Leakage Calculation:
   Leakage current can arise naturally due to the resistance in the wiring. The approximate formula for estimating leakage is:

   $$I_{\text{leak}} = 0.4 \times I_{\text{load}} + 0.0031 \times L_{\text{ft}} \quad \text{(in mA)}$$
   • $I_{\text{load}}$ is the load current in amperes.
   • $L_{\text{ft}}$ is the length of the phase conductors in feet.

   For short cable runs, the second term (0.0031 × L) is often negligible, simplifying the calculation to:

   $$I_{\text{leak}} \approx 0.4 \times I_{\text{load}}$$

   To keep $I_{\text{leak}}$ below 10 mA, the load current should be limited to around 25 A (since 0.4 × 25 = 10 mA).

Practical Examples

Depending on your design, you might consider one of the following configurations:

• Four Breakers at 6 A Each:
  Combined load equals 24 A—within safe limits.
• Two Breakers at 10 A Each:
  Combined load equals 20 A.
• A Single Breaker at 25 A:
  Ideal if the circuit’s total load does not exceed 25 A.

The total rating of the RCD should be at least equal to the sum of the currents from all the connected circuits—in our example, a minimum of 25 A.

Pros and Cons

• Pros:
  • Efficient Wiring: A single RCD for multiple circuits reduces the number of devices and simplifies the installation.
  • Cost Savings: Fewer RCDs mean lower material and labor costs.
• Cons:
  • Nuisance Tripping: If the cumulative natural leakage exceeds the threshold, the RCD may trip unnecessarily, cutting power to all connected circuits.
  • Troubleshooting Complexity: When a trip occurs, pinpointing the faulty circuit becomes more challenging.
  • Safety Risks: Overloading the RCD’s leakage capacity may compromise protection, potentially allowing hazardous conditions to persist.

Final Thoughts

While it is technically possible to connect several circuit breakers to one RCD, doing so requires careful calculation and consideration. Ensure that the sum of the natural leakage currents remains well below the RCD’s trip threshold (ideally under 10 mA) to prevent nuisance tripping and maintain safety. Always adhere to NEC guidelines and perform regular testing to verify proper operation.

For expert advice on electrical system design, troubleshooting, and product recommendations, visit safsale.com. Our team is here to help you build a safe, compliant, and efficient electrical system for your home or business.