ASB Trade › Guides › How to choose a circuit breaker for a flat or a house
How to choose a circuit breaker for a flat or a house
Rating by cable rather than by load, B or C trip curve, number of poles and 4.5 or 6 kA breaking capacity: a short guide to picking a LEGRAND breaker and knowing when you need an RCD or an RCBO.
Rating follows the cable, not the load
A breaker protects the wire, not the appliance: it has to trip before the cable overheats. So the rating is chosen by the copper conductor's cross-section first, and only then checked against the load. A lighting line in 1.5 mm² gets a 10 A breaker, socket lines in 2.5 mm² get 16 A, an oven, water heater or air conditioner on 4 mm² gets 20–25 A, and a hob or the incoming line on 6 mm² gets 32 A.
The power check is simple: at 220 V the current is roughly the power divided by 220. A socket group where a kettle and a microwave run at the same time (about 3 kW) draws roughly 14 A, so a 16 A breaker on 2.5 mm² cable is fine. If the load is more than the cable can carry, change the cable or add a separate line instead of fitting a bigger breaker.
Trip curve B or C
The letter in front of the rating (B16, C16) shows at what short-term overcurrent the breaker trips instantly: B at 3–5 times the rating, C at 5–10 times. For lighting, sockets and household appliances in a flat both types work; B reacts slightly faster to a short circuit on long lines.
C is chosen where there are motors and compressors: pumps, air conditioners, refrigeration, workshops with power tools. A motor's starting current is several times its running current, and a C breaker lets it through without nuisance tripping. The LEGRAND XS3 and XG3 ranges include both curves in the common ratings.
One, two or three poles
A single-pole breaker interrupts only the phase: the standard choice for group lines in a flat with a single-phase supply. A two-pole breaker switches off both phase and neutral; it goes on the incoming line and on circuits that need to be fully isolated for servicing, such as a boiler, outdoor equipment or a garage.
A three-pole breaker is needed with a three-phase supply: in private houses, workshops, for three-phase boilers, pumps and machines. A four-pole one also interrupts the neutral and is fitted on a three-phase incomer. Three-phase equipment gets one three-pole breaker rather than three single-pole ones, because it disconnects all phases at once.
Breaking capacity: 4.5 or 6 kA
Breaking capacity is the highest short-circuit current the breaker can interrupt and remain serviceable. In a flat in an apartment block, far from the substation, the short-circuit current is usually modest and 4.5 kA is enough for group lines.
6 kA is chosen for incoming breakers, houses and premises close to a substation, shops, offices and boards with a large allocated power. If you have no data on the short-circuit current, 6 kA is the more universal choice. LEGRAND XS3 and XG3 come in 4.5 and 6 kA versions.
RCD and RCBO: protection against electric shock
A breaker protects against overload and short circuit, but not against current leaking to an appliance body or through a person. That is the job of an RCD (residual current device): it compares the current in phase and neutral and cuts the line when the difference reaches, typically, 30 mA.
An RCBO (differential breaker) combines a breaker and an RCD in one housing: it saves space in the board, but after a trip it is not immediately clear whether the cause was overload or leakage. A typical flat layout is an incoming breaker, then a 30 mA RCD for the kitchen, bathroom and socket groups, and a separate breaker on every line. For the washing machine, water heater and bathroom sockets leakage protection matters most.
Checklist before buying
Find out the cable cross-section of every line and the total load. Pick the rating: 10 A for lighting, 16 A for sockets, 20–25 A for heavy appliances, 32 A for a hob. Curve B for household lines, C for motors. Poles by the supply: 1P for groups, 2P on the incomer, 3P/4P with three phases. 4.5 kA for a flat, 6 kA for the incomer and commercial premises. Add an RCD or RCBO for wet areas.
LEGRAND XS3 and XG3 breakers, RCBOs, RCDs, consumer units and Unal power cables are all at ASB Trade; if in doubt about the calculation, a manager will help.
Frequently asked questions
Which breaker for sockets wired in 2.5 mm²?
16 A. That covers about 3.5 kW of appliances running at the same time, and 2.5 mm² cable does not overheat at that current.
The breaker keeps tripping — can I fit a higher rating?
No. The breaker is matched to the cable, and a higher rating leaves the wire unprotected. If it trips on overload, split the load across separate lines; if it trips on a short circuit, look for the fault.
Do I need an RCD if breakers are already installed?
Yes. A breaker does not react to current leaking through a person, while a 30 mA RCD cuts the line in exactly that case. At minimum it goes on the bathroom, kitchen and socket groups.