Automatic machine for 3 phases: how to choose, connect and avoid 7 critical mistakes

If when you turn on a welding machine, compressor or machine in a garage with a three-phase network the input machine is triggered or the voltage noticeably sags, the problem in 90% of cases lies in an incorrectly selected circuit breaker. Three-phase machines (for example, ABB S203 C40 or IEK BA47-63 3P 25A) require taking into account not only the total load power, but also simultaneity factor, type of release (B, C, D) and even the length of the cable from the switchboard to the consumer. An error in selection leads to false alarms, overheating of the contacts, or, worse, a fire due to the inability to turn off the short circuit.

This article contains specific calculations for selecting a machine for 3 phases, analysis of connection diagrams (star/delta), as well as 5 prohibited mistakes, which even experienced electricians allow during installation. We will separately dwell on the nuances for garages, workshops and car services, where the load is often pulsed (for example, starting currents of compressors up to 500% of the nominal value).

1. How to calculate the rating of a machine for 3 phases: formulas and examples

The rated current of a three-phase machine is determined by the formula:

Inom = P / (√3 × U × cosφ)

where:

• 🔹 P — total power of all consumers (in watts). For a garage with a compressor (3 kW), a welder (5 kW) and lighting (0.5 kW) this is 3000 + 5000 + 500 = 8500 W.
• 🔹 U — linear voltage (380 V for 3 phases).
• 🔹 cosφ — power factor (0.8 for electric motors, 1 for heaters).
• 🔹 √3 ≈ 1.73 — constant for three-phase networks.

For the example above:

Inom = 8500 / (1.73 × 380 × 0.85) ≈ 15.2 A

But this not the final value of the machine. Need to consider:

• 🔹 Simultaneity factor (0.7–0.9 for garages). Multiply 15.2 A × 0.8 = 12.16 A.
• 🔹 Starting currents. For compressor with cosφ = 0.8 and starting current 5× rated, we take the machine with a margin: 12.16 A × 1.5 ≈ 18 A. The closest standard denomination is 20 A.

Important: if the load is asymmetrical (for example, one phase is loaded by 80%, and the other two by 30%), the current in the neutral wire may exceed the phase one. In such cases, you need an automatic machine with high breaking capacity (for example, Schneider Electric Multi9 C60N 3P 25A with breaking capacity 6 kA).

2. Types of releases: which one to choose for a garage, workshop or home

Three-phase machines are divided into response characteristics (type of release):

Type	Response range	For what loads	Application example
B	3–5×In	Lighting, heaters, socket groups	Garage lighting, heater
C	5–10×In	Moderate starting currents (motors up to 5 kW)	Compressor, circular saw
D	10–20×In	High starting currents (welders, powerful machines)	Welding machine, lathe
K	8–12×In	Inductive loads with sudden surges	Transformers, chokes

A mistake many people make is installing a machine like B to the welding machine. When starting, the current can reach 15×IN, and the machine will fire falsely. The optimal type for a garage C (if the equipment power is up to 10 kW) or D (over 10 kW).

For car services with several posts and asynchronous motors (for example, lifts), it is better to use machines with electronic releases (for example, ABB Tmax T4S160 PR222DS-LS). They allow you to more accurately adjust the time-current characteristic and avoid false shutdowns during short-term overloads.

3. Connection diagrams: star vs delta

Three-phase machines are connected according to two main schemes:

• 🔺 Star (Y): the phases are connected to the load through a common point (neutral). Used for 220V loads (eg lighting, sockets). Automatic in this case must be 4-pole (3 phases + neutral).
• 🔺 Triangle (Δ): phases are closed in a ring, there is no neutral. Suitable for 380 V motors. There is enough 3-pole circuit breaker.

A critical error is connecting a 3-pole circuit breaker in a star circuit without neutral protection. If zero is lost on a load with asymmetrical consumption (for example, one phase is loaded at 100%, the other two are loaded at 20% each), the voltage on the phases can reach 380 V instead of 220 V, which will damage the equipment.

What happens if you mix up a star and a triangle?

When connecting a motor designed for 220 V (star) to a 380 V delta circuit, the windings will burn out in a few seconds. And vice versa: a 380 V (delta) motor connected to a 220 V star does not develop full power (torque drops up to 30%).

A typical combination scheme for garages is:

• 🔹 Lighting and sockets - star (220 V), protected by a 4-pole machine.
• 🔹 Machines and compressors - triangle (380 V), protected by a separate 3-pole machine.

4. 5 mistakes when installing a three-phase machine

Even experienced electricians make these mistakes, which lead to fire or equipment failure:

1. 🔴 Using single-pole circuit breakers per phase. In a three-phase network all phases must be switched off simultaneously. Otherwise, if one phase breaks, the motor switches to “two-phase power supply” mode and burns out.
2. 🔴 Wrong choice of cable cross-section. For a 25 A machine, you need a cable of at least 4 mm² (copper) or 6 mm² (aluminium). Using a 2.5 mm² cable leads to overheating and melting of the insulation.
3. 🔴 Lack of RCD or differential circuit breaker. In garages with metal floors and moisture, current leakage to the frame is a common cause of damage. Standard for RCD - 30 mA.
4. 🔴 Connecting zero to the ground terminal. This leads to tripping of the RCD and the risk of electric shock when touching the equipment body.
5. 🔴 Ignoring temperature conditions. The machines in the panel must be located with a gap of at least 5 mm from each other. If they are tightly packed, they overheat and the rated current is reduced by 10–15%.

Special attention - phase sequence. For motors, the direction of rotation is important. If after connection the motor spins in the opposite direction, it is enough to swap any two phases on the terminal block of the machine.

5. TOP 5 machines for 3 phases: comparison by price and characteristics

When choosing a machine for a garage or workshop, focus on:

• 🔹 Breaking capacity: for household networks 4.5 kA is enough, for industrial networks - from 6 kA.
• 🔹 Current limiting class: 3rd class reduces the arc time during short circuit to 3 ms (for example, Legrand DX³ 3P 40A).
• 🔹 Availability of an operation indicator: simplifies diagnosis (available in Hager NH000C40).

Model	Denomination (A)	Release type	Off capacity (kA)	Price (RUB)	Features
ABB S203 C40	40	C	6	2 800	Double busbar clamps, current limiting class 3
IEK BA47-63 3P 25A	25	C	4.5	850	Budget option, suitable for lighting
Schneider Multi9 C60N 3P 32A	32	C	6	2 200	Comb terminals, certified for Russia
Legrand DX³ 3P 50A	50	D	10	4 500	For powerful machines, contact position indicator
Hager NH000C40	40	C	6	3 100	Modular design, compatible with Hager accessories

For garages with unstable voltage (for example, in cooperatives), automatic machines with wide operating temperature range (from –25°C to +50°C), such as KEAZ OptiDin VM63 3P. They are less sensitive to changes and vibrations.

6. How to connect a three-phase machine: step-by-step instructions

For installation you will need:

• 🔧 Automatic (for example, ABB S201 C25 for lighting or Legrand DX³ D32 for machines).
• 🔧 Comb bus (for connecting several machines) or PV-3 mounting wires with a cross-section of 6 mm².
• 🔧 DIN rail and limiters.
• 🔧 Insulated screwdriver and tester (eg. Fluke T5-1000).

Procedure:

1. Turn off the input switch and check the absence of voltage with a tester on all phases.
2. Install the machine on the DIN rail and secure it with stops.
3. Connect the phases (L1, L2, L3) to the upper terminals of the machine. For a star circuit, connect the neutral (N) to the fourth terminal (if the circuit breaker is 4-pole).
4. Connect the bottom terminals to the load. For delta motors, use jumpers between the terminals L1→L2, L2→L3, L3→L1.
5. Check the reliability of tightening the terminals with a torque of 2.5 Nm (for machines up to 63 A).
6. Turn on the input switch and check the voltage at the output of the machine.

If the machine operates immediately after switching on:

• 🔹 Check that there is no short circuit in the load.
• 🔹 Make sure that the machine’s rating is not underestimated (see section 1).
• 🔹 For engines, check the health of the starting capacitor (if there is one).

7. Frequent faults and diagnostic methods

If a three-phase machine behaves unstable, the reasons may be as follows:

Symptom	Probable Cause	How to check	Solution
The machine heats up, but does not turn off	Poor terminal contact or overload	Measure the current with clamps, check the tightness of the terminals	Tighten the terminals, reduce the load or replace the circuit breaker
Triggers when the engine is turned on	Underrated rating or type of release	Check the motor starting current (usually 5–7×In)	Replace the machine with type D or increase the denomination
One phase disappears after heating	Contact oxidation or cable damage	Check the phases with a tester, inspect the terminals	Clean contacts or replace cable
The machine does not turn on after triggering	Mechanical wear or jamming of the release	Try manually returning the lever to the "On" position.	Replace the machine

Particularly dangerous neutral break in a star design. In this case, in phases with less load, the voltage can rise to 380 V, which will damage lighting and electronics. To avoid this, use phase control relay (for example, RKF-3M), which will turn off the power if unbalanced.

8. FAQ: answers to frequently asked questions

Is it possible to replace a three-phase circuit breaker with three single-pole ones?

No. Three single-pole circuit breakers do not guarantee simultaneous disconnection of all phases. If one phase breaks, the motor will switch to “two-phase power” mode and burn out. In addition, such a scheme violates the PUE (clause 3.1.18).

Which machine should I put on a 380 V, 7 kW welding machine?

Calculate the current: I = 7000 / (1.73 × 380 × 0.85) ≈ 12.5 A. Taking into account starting currents (up to 200% for welders), take an automatic type D on 25 A (for example, IEK BA47-63 D25). Cable cross-section is at least 4 mm².

What happens if you install a machine with an inflated denomination?

The machine will not protect the cable from overheating. For example, if to the machine 50 A connect a 2.5 mm² cable (maximum 27 A), the insulation will melt long before the protection is triggered. Always select a machine according to to the weakest link - cable cross-section.

Is it necessary to install an RCD on a three-phase socket group in the garage?

Yes, especially if the floor is metal or concrete. Choose three-phase RCD with leakage current 30 mA (for example, ABB FH202 AC 40A/0.03A). For engines it is better to use differential machine (for example, Schneider Electric C60D 3P 25A/30mA).

How to check a three-phase machine without load?

Use a machine tester (for example, Sonel MPT-3) or assemble a circuit with three 100 W lamps connected in a star configuration. If, when voltage is applied, all lamps light up evenly, the machine is working. To check operation, use the "Test" button (if available) or apply a current exceeding the rated current.

⚠️ Attention: Never use three-phase circuit breakers as the main protection for single-phase networks (for example, connecting a 220 V load to one phase). This disrupts selectivity and can lead to electric shock if the neutral is broken.