An improperly sized residual current device (RCD) in a panel usually doesn't fail on the first day. The problem appears later: nuisance tripping, incompatibilities with variable frequency drives, accumulated leakage currents, or a rating that doesn't match the upstream circuit breaker. If you're reviewing how to size an RCD in a panel, what matters isn't just choosing 30 mA and stopping there. You need to cross-reference nominal current, sensitivity, class, number of poles, and type of load.
What it means to properly size an RCD
Sizing a residual current device isn't just about choosing an amperage value. In practice, it means verifying that the device can withstand the service current of the circuit or protected line, that its sensitivity meets the protection objective, and that its technology matches the foreseeable leakage currents of the installation.
In an electrical panel, the RCD works as part of a set. That's why it's not chosen in isolation. You need to consider the main circuit breaker or associated branch circuit breaker, the distribution scheme, whether the network is single-phase or three-phase, the presence of power electronics, and the level of service continuity the client requires. In a basic home, a simple solution may suffice. In a commercial premise or light industry, not always.
How to size an RCD in a panel step-by-step
1. Determine the nominal current
The nominal current of the RCD, expressed in amperes, must be equal to or greater than the current that can permanently flow through the line where it is installed. The common mistake is to think that the RCD protects against overload. It does not. That function belongs to the circuit breaker. But the RCD must be able to withstand that current without being overstressed.
As a practical criterion, if the circuit or branch is protected by a 40 A circuit breaker, the RCD should not be less than 40 A. In many panels, the same rating or a higher one is installed, for example 40 A or 63 A depending on the anticipated load. Moderate oversizing in current does not impair the RCD function and can provide thermal margin, although it should not become a habit without careful consideration.
If the panel supplies several lines under a single RCD, the correct approach is to assess the foreseeable simultaneous current and the upstream protection. In such cases, a 25 A or 40 A RCD might be insufficient if the installation has grown or if future expansions are planned.
2. Choose the correct sensitivity
Sensitivity, typically 30 mA, 100 mA, 300 mA, or higher, defines at what leakage current the device trips. For personal protection in terminal circuits, the most common standard is 30 mA. In most residential, office, and small commercial panels, this is the reference value.
300 mA RCDs are usually used more for fire protection or as a general RCD in certain schemes, always depending on regulations, selectivity, and installation design. They do not simply replace a 30 mA RCD when additional personal protection is required.
There is no single rule here for all panels. If you group too many electronic loads under a single 30 mA RCD, you can cause tripping due to the sum of leakage currents without an actual fault. In such cases, the solution is usually not to increase to 300 mA to stop tripping, but rather to distribute circuits or use RCDs appropriate for the load.
3. Define the RCD class
This is where most mistakes are made. Not all RCDs detect the same type of residual current.
Type AC, valid for sinusoidal alternating currents, has been the basic option for years. But in installations with electronics, which are the majority today, it often falls short. Washing machines, induction hobs, switched-mode power supplies, air conditioning, chargers, LED lighting with drivers, or variable frequency drives introduce components that make a Type A RCD, at a minimum, more advisable.
Type A detects alternating and pulsating residual currents with a DC component. For current homes and light commercial buildings, it is usually the most logical choice in most panels. If there are also devices with single-phase variable frequency drives or particularly sensitive loads, a Type F might be of interest. And if we are talking about applications with three-phase variable frequency drives, specific electric vehicle chargers, elevators, photovoltaics, or environments with more complex continuous residual current, then Type B scenarios come into play.
It's not about always installing the most advanced type. It's about installing the correct type. A higher type increases the cost of the panel without providing any benefit, but a lower type can generate erratic tripping or, worse, not offer the appropriate response to certain leakage currents.
Number of poles and panel configuration
Single-phase: 2 poles
In single-phase panels, the usual solution is a 2-pole RCD. It must cut both phase and neutral, with current and sensitivity appropriate for the protected set. For standard housing, combinations like 25 A or 40 A with 30 mA are common, but always depending on the main incoming breaker, the branch, and the grouped circuits.
Three-phase: 4 poles
In three-phase or mixed panels, the RCD will usually be 4-pole. Here, not only the rating but also the load distribution and the quality of the neutral must be reviewed. In installations with motors, pumps, air conditioning, or light machinery, the RCD class becomes even more important. A 4P Type AC in an environment with variable frequency drives can be a direct source of problems.
The sum of leakage currents matters more than it seems
When an installer asks why an RCD trips without an apparent fault, the answer often lies in accumulated leakage current. Each connected device contributes a small natural leakage current. A power supply, an EMI filter, a variable frequency drive, an LED driver, or an air conditioner don't have to be faulty to add up milliamperes.
If you group too many electronic loads under the same 30 mA RCD, you can approach the tripping threshold under normal operation. And if a maneuver, a transient peak, or ambient humidity occurs, tripping becomes recurrent. Properly sizing the RCD in the panel also means sectioning. Sometimes it's better to install several RCDs than a single higher-rated one trying to cover everything.
When to use super-immunized or SI RCDs
In panels where service continuity is important, immunized or super-immunized RCDs make sense. They are not a luxury. They are designed to better withstand disturbances, harmonics, and transients, reducing nuisance tripping in installations with electronics, IT equipment, air conditioning, or frequent operations.
They don't compensate for poor panel design, but they help a lot when the installation generates electrical noise or when the environment is sensitive to unwanted outages. For a home heavily loaded with electronics, a technical office, a room with computer equipment, or a small business, this option can clearly pay off.
Coordination with circuit breakers and selectivity
An RCD should not be chosen without considering the associated circuit breaker protection. If the circuit breaker is 40 A and the RCD is 25 A, the setup is poorly designed, except in very specific cases with effective prior protection that justifies it. Basic coordination requires that the RCD can withstand the maximum current allowed by the upstream protection or the actual line design.
When there are several RCDs in cascade, selectivity also comes into play. If sensitivity and delay are not coordinated, a leakage current in a terminal circuit can trip half the installation. In slightly more complex panels, using a selective RCD upstream and instantaneous 30 mA RCDs downstream avoids unnecessary general tripping. Here, technical criteria outweigh the unit price of the device.
Common mistakes when sizing an RCD
The first is choosing only by amperage and forgetting the class. The second is installing a single 30 mA RCD for too many circuits with electronic loads. The third is thinking that all nuisance tripping can be solved with an RCD of higher milliamperes. And the fourth is not checking if the installation needs an auto-reclosing device.
Automatic reclosing doesn't solve poor selection, but in unattended locations, second homes, cameras, outdoor lighting, or services that cannot be out of service for hours, it can make a difference. Always, of course, when regulations and safety analysis permit it.
What combination usually works best
There isn't a single answer, but there are clear criteria. In current housing, often the reasonable starting point is a Type A, 30 mA, 2P RCD, with a nominal current equal to or greater than the upstream protection. In three-phase panels with electronics or motors, 4P Type A, F or B depending on the actual load. And in environments with nuisance tripping, consider SI or super-immunized versions instead of continuing to try basic options.
For those who buy for compatibility rather than trial-and-error, it is cost-effective to get it right the first time. In a specialized catalog like Bogas Electronics, this difference is especially noticeable when looking for a specific 2P or 4P reference, class AC, A, F or B, with an immunized or auto-reclosing option and the exact rating for the panel.
Before finalizing the choice, it's worth asking one last question: what is this RCD really protecting and what behavior do I expect from it when the installation operates under normal conditions. If the answer is clear, the sizing usually is too.