How many machines can be installed on 1 RCD: PUE standards, calculations and errors

Connecting several circuit breakers to one RCD (residual current device) is a common practice when assembling electrical panels, but it requires strict adherence to the rules. If you exceed the permissible number of circuit breakers per RCD, the system will falsely trigger at the slightest current leakage, and in the worst case, it will not work in a real accident. Main criterion: the total leakage current of all connected lines should not exceed 30% of the nominal value of the RCD (for household devices usually 10–30 mA). For example, for a 30 mA RCD, the maximum leakage current of all machines should be ≤9 mA.

The problem is that each circuit breaker protects a line with a certain load, and each load (especially with switching power supplies) creates a background leakage current. If you connect 10 16A circuit breakers to a 25A RCD, the total leakage may exceed the threshold even without an accident. To avoid this, you need to take into account not only the number of machines, but also the type of load (lighting, sockets, powerful appliances), as well as characteristics of the RCD itself (electromechanical or electronic). Next, we will analyze the rules of the PUE, examples of calculations and typical errors.

1. PUE standards: what the law says about the number of machines on RCDs

B Electrical Installation Rules (PUE) There is no direct limit on the number of machines connected to one RCD. However, there are key requirements that indirectly regulate this issue:

• 📜 PUE 7.1.71 — The RCD should operate at a leakage current of no more than 30% of the rated one (to protect against electric shock). For example, for a 30 mA RCD, the response threshold is 10 mA.
• ⚡ PUE 7.1.73 — the total network leakage current in normal mode should not exceed 1/3 of the RCD setting. For household networks, this is critical, since modern devices (computers, washing machines) create a background leakage of 0.3–1 mA per device.
• 🔧 PUE 7.1.83 — it is recommended to install separate RCDs for groups of sockets in bathrooms, kitchens and children’s rooms (due to increased danger).

In practice, this means that one 30 mA RCD can be connected to no more than 3–5 machines (depending on load). For example:

• 💡 For lighting line (small leakage) - up to 5 machines.
• 🔌 For outlet groups with household appliances - no more than 3 machines.
• ⚠️ For powerful appliances (boiler, washing machine) - a separate RCD for each line.

Important: electronic RCDs (unlike electromechanical) require power to operate. If the zero is broken, they lose functionality, so they are not recommended for use on critical lines (for example, a refrigerator or alarm system).

2. How to calculate the maximum number of machines on an RCD

The calculation formula is based on the total leakage current of all connected lines. Algorithm:

1. Define rated leakage current of RCD (I_Δn). For consumer devices this is usually 10, 30 or 100 mA.
2. Find out background leakage current for each line (I_background). For modern devices:
   • 💻 Computer, TV - 0.5–1 mA.
   • 🧺 Washing machine - 1-3 mA.
   • 🔥 Electric stove - 2–5 mA.
   • 💡 Lighting (LED) - 0.1–0.3 mA per lamp.

Calculate the total background current of all lines (ΣI_background).

Make sure ΣI_background ≤ 0,3 × I_Δn. For example, for a 30 mA RCD: 0.3 × 30 = 9 mA.

Calculation example: The following are connected to the 30 mA RCD:

• 3 outlet groups (2 mA leakage each) = 6 mA.
• 1 lighting line (0.5 mA) = 0.5 mA.
• Total: 6.5 mA ≤ 9 mA (normal).

In this case, you can add 1-2 more machines with low load.

Load type	Leakage current per line, mA	Max. number of lines on RCD 30 mA
Lighting (LED, fluorescent)	0,1–0,5	up to 10
Sockets (without powerful devices)	0,5–1	up to 5
Washing machine, dishwasher	1–3	1–2
Electric stove, boiler	2–5	1 (separate RCD)

⚠️ Attention: If the total leakage exceeds 30% of the RCD rating, the device will trip for no apparent reason. For example, when connecting 4 washing machines to one 30 mA RCD (4 × 3 mA = 12 mA > 9 mA), a false trip will occur.

1. Check the RCD rating (10/30/100 mA).

2. Calculate the total leakage of all lines.

3. Make sure the leakage is ≤ 30% of the RCD rating.

4. Divide powerful devices into separate RCDs.

5. Use an electromechanical RCD for critical lines.

-->

3. Typical mistakes when connecting several machines to an RCD

Even experienced electricians make mistakes that lead to false alarms or, conversely, to a lack of protection. Common mistakes:

• 🔌 Connecting zero to the common bus after the RCD — leads to imbalance of currents and false alarms. The zero of each line must pass through an RCD.
• ⚡ Exceeding the total current of machines. For example, 3 25A circuit breakers are connected to a 40A RCD (75A > 40A). In this case, the RCD will burn out when overloaded.
• 🛠️ Using one RCD for all lines in the house — increases the risk of false alarms and complicates troubleshooting.
• 🔄 Incorrect phase and zero alternation - if you mix up the connection at the RCD terminals, it will not work.

Consequences of errors:

• 🔥 Overheating and fire when the RCD fails due to overload.
• 💥 False positives when turning on household appliances.
• ⚡ Electric shock due to incorrect operation of the protection.

How to avoid problems:

1. Always check the connection diagram with a multimeter (test mode).
2. Use an RCD with a rating one step higher than the total current of the machines (for example, for 3 machines of 16A each - a 40A RCD).
3. For powerful devices (boiler, air conditioner), install separate RCDs with a setting of 10–30 mA.

4. Examples of connection diagrams: how many machines per RCD

Let's consider real schemes for different types of objects:

Scheme 1: Apartment (2 rooms, kitchen)

Typical wiring:

• 🔌 Sockets: 3 groups (living room, bedroom, kitchen) - 3 circuit breakers 16A each → 1 RCD 40A/30 mA.
• 💡 Lighting: 2 groups (rooms + kitchen/bathroom) - 2 circuit breakers 10A each → 1 RCD 25A/30 mA.
• 🚿 Washing machine: separate machine 16A + RCD 16A/10 mA (due to a wet room).

Scheme 2: Private house (3 floors)

More RCDs are required here due to the increased load:

• 🏠 1st floor: 4 socket groups + lighting → 2 RCDs (30 mA each).
• 🔥 Kitchen: separate RCDs for stove (40A/30 mA), dishwasher (16A/10 mA), refrigerator (16A/30 mA).
• 🛁 Bathroom: sockets + boiler → RCD 25A/10 mA (high humidity).

Key principle: the more floors and devices, the more RCDs are required. For houses >100 m² it is recommended modular system with separate RCDs for each floor or zone.

Why does a washing machine need a 10 mA RCD?

Washing machines operate in damp environments where the risk of electric shock is higher. An RCD with a setting of 10 mA triggers faster (in 0.04–0.3 s), which is critical for human protection. A 30 mA RCD may not have time to turn off when it touches a bare live wire.

5. How to choose an RCD for the number of machines: 3 key parameters

When purchasing an RCD, consider:

1. Rated current (In) - must be equal to or higher total current of connected machines. For example:
   • For 3 machines of 16A (48A) → RCD for 63A.
   • For 2 machines of 25A (50A) → RCD for 63A.
2. Leakage current (IΔn):
   • 10 mA - for wet rooms (bathroom, sauna).
   • 30 mA - for socket groups in living rooms.
   • 100–300 mA - for fire protection (does not protect against electric shock!).
3. RCD type:
   • 🔄 Electromechanical — works without power, more reliable.
   • 📱 Electronic - cheaper, but requires voltage to operate.

Critical error: use an RCD with a nominal value lessthan the total current of the machines. For example, connect 3 25A (75A) circuit breakers to a 40A RCD. In this case, the RCD will overheat and fail at a load >40A.

Number of machines	Total current of machines, A	Recommended RCD rating, A
1–2	16–32	25–40
3–4	48–64	63
5+	80+	80–100 (or divided into groups)

6. Frequently asked questions about connecting machines to RCDs

Is it possible to connect 10 machines to one 30 mA RCD?

Theoretically, it is possible if the total leakage of all lines is ≤9 mA. However, in practice this is unlikely: even 10 lighting lines with 0.3 mA leakage each will produce 3 mA, but adding sockets or appliances will exceed the limit. Optimally - no more than 5 machines per 30 mA RCD.

What happens if you connect circuit breakers with a total current greater than its nominal value to the RCD?

The RCD will not turn off the circuit when overloaded (this is the task of automatic machines), but it itself can overheat and fail. For example, if you connect 63A circuit breakers to a 40A RCD, at a current of 50A the RCD will burn out, but will not turn off.

Do I need an RCD for a line with one machine?

Yes, if the line powers sockets or appliances in wet areas (bathroom, kitchen). An RCD is not necessary for lighting in dry rooms, but is recommended for general safety.

Is it possible to connect machines of different ratings (16A and 25A) to the RCD?

Yes, the main thing is that the total current of the machines does not exceed the rating of the RCD. For example, to a 63A RCD you can connect 2 16A and 1 25A circuit breakers (total current 57A).

Why does the RCD trip when the washing machine is turned on?

Most likely, the total leakage exceeds 30% of the RCD rating. Solutions:

1. Connect the washing machine to a separate 10 mA RCD.
2. Check the machine for insulation breakdown (the heating element may be faulty).
3. Replace the RCD with 30 mA (if it is currently 10 mA).

7. Conclusion: the optimal number of machines per RCD

So, there is no universal answer to the question “how many machines can be installed on 1 RCD” - it all depends on load type, RCD rating and total leakage. General recommendations:

• 🏠 For an apartment: 3–5 machines for one RCD 30 mA (sockets + lighting).
• 🏡 For a private home: 2–3 machines on RCDs, plus separate RCDs for powerful devices.
• ⚡ For wet rooms: 1 machine = 1 RCD 10 mA.
• 🔥 For fire protection: 100–300 mA RCD (does not protect against electric shock!).

Remember: RCD is not a panacea. It must be paired with circuit breakers and a proper grounding circuit. If you are in doubt about the calculations, use selective RCDs (with a delay) or divide the networks into several protection devices.

⚠️ Attention: After installation, be sure to check the RCD by pressing the "Test" button. If it does not turn off, the device is faulty or not connected correctly.