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[Solved] Determining Wire Cross-Section Between Switchgear Elements: Disconnector, RCD, Circuit Breaker

piotrkartuz 23316 18
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How should the bridge wire cross-section be chosen between switchgear elements, and can it be the same size or larger than the incoming 4 mm² conductors?

The bridge should not be reduced below the incoming conductor size; keep the same cross-section up to the outgoing point, and then size each circuit according to its own load and protection. A 4 mm² feed can therefore be bridged with 4 mm² or larger, and oversizing is not a problem [#16881746][#16881699][#16881841][#16881805] In the thread, it is also noted that ready-made combs are commonly used for such connections, often around 10–12 mm², so using a larger bridge than the feed is acceptable [#16881821][#16881833] Between the RCD and the miniature circuit breaker, the same rule applies: do not reduce the bridge just because the downstream circuit is smaller; the reduction belongs in the protected outgoing circuit, not in the internal switchgear link [#16881699][#16881841]
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  • #1 16881631
    piotrkartuz
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    Hello
    What cross-section should the bridge have between the elements of the switchgear?

    E.g. 5x4 mm2 is connected to the disconnector
    1) the bridge between the disconnector and the RCD should be 4 mm2 (1 phase)
    2) not less than 4 mm2
    3) larger than 4 mm2

    Should the same cross-section be continued between the RCD and the miniature circuit breaker or should the cross-section of the conductor of the circuit protected by the miniature circuit breaker be present?
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  • #2 16881690
    kokapetyl
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    piotrkartuz wrote:
    5x4mm2 is connected to the disconnector

    This disconnector is probably not five-pole?
    From the disconnector, the circuits diverge in several directions, there you must use cross-sections in accordance with the load on the given line.
  • #3 16881699
    Anonymous
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  • #4 16881741
    piotrkartuz
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    kokapetyl wrote:
    piotrkartuz wrote:
    5x4mm2 is connected to the disconnector

    This disconnector, I don't think is five poles?
    From the disconnector, the circuits diverge in several directions, there you need to apply cross-sections in accordance with the load on the given line.


    The disconnector is 3-pole (PE and N are not disconnected), my question is about phase conductors.
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    #5 16881746
    elpapiotr
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    The bridges should have a cross section of 4 mm?
  • #6 16881749
    kokapetyl
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    Koderr wrote:
    The fact that a bulb is connected there, for example, with a 1.5-wire cable does not mean that in a while there will be no change to e.g. an induction furnace - the cross-section is much larger

    Did you encounter such a situation that an induction stove is connected to the lighting?
    The system disconnects from the disconnector over several circuits and here you need to maintain the appropriate cross-section of the cable depending on the load. And so, for lighting 1.5 mm?, for 2.5mm? sockets and for special receivers, a separate power cord also minimum 2.5mm?.
  • #7 16881773
    Anonymous
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  • #8 16881788
    piotrkartuz
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    The reason for my question is the use of bridges with a larger cross-section than 4mm2 (i.e. with a cross-section larger than the cross-section of the cable before RCD).
    I was wondering if there could be any problems with "over-dimensioning".
  • #9 16881797
    elpapiotr
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    piotrkartuz wrote:
    I was wondering if there could be any problems with "over-dimensioning".
    And what problems can these be?
    Too heavy distribution or what?
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  • #10 16881805
    Anonymous
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  • #11 16881811
    kokapetyl
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    Koderr wrote:
    I meant justifying not reducing the cross-section of the wires.

    Does it mean how WLZ is 10mm?. is it supposed to be this way?
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  • #12 16881821
    xury
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    And no one thought that these bridges are dedicated "combs" usually 10-12mm2?
  • #13 16881833
    retrofood
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    Koderr wrote:

    But why bridge the cross section? Well, unless you have an excess of e.g. 25 square ;-)
    For what? After that the bridge can be bought ready. The standard comb is also a bridge.
  • #14 16881835
    Anonymous
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  • #15 16881841
    retrofood
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    kokapetyl wrote:
    Koderr wrote:
    I meant justifying not reducing the cross-section of the wires.

    Does it mean how WLZ is 10mm?. is it supposed to be this way?

    The principle is to install the protection in a place where the cable cross-section is reduced.
  • #16 16881872
    kokapetyl
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    Koderr wrote:
    There is no question of WLZ.

    And what comes to the "disconnector" (I mean the cross-section of the wire), because earlier there is an energy meter.
    We switch from the disconnector into (some circuits) and there does not have to be the same cross-section as at the entrance.
    ! 0 mm?, for 5x X for a flat is enough
  • #17 16886740
    piotrkartuz
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    Ok, thanks to everyone for the answer.
    I was wondering if RCD bridging with 4 "S" using different cross-sections is dictated by something? The other two RCDs are connected by a 12mm2 comb.
  • #18 16886753
    elpapiotr
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    piotrkartuz wrote:
    Ok, thanks to everyone for the answer.
    I was wondering if RCD bridging with 4 "S" using different cross-sections is dictated by something? The other two RCDs are connected by a 12mm2 comb.

    The question is how it's done.
    Through the distribution block, double wire sleeves or else?
  • #19 16886880
    piotrkartuz
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    lines in double sleeves, judging by the thickness of the lines, the cross-section is different
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Topic summary

✨ The discussion revolves around determining the appropriate wire cross-section for bridges connecting switchgear elements, specifically between a disconnector, RCD, and circuit breaker. Participants emphasize that the cross-section should not be reduced at terminals where circuits diverge, and it should match the load requirements of the connected devices. The author clarifies that the disconnector is 3-pole and inquires about maintaining a consistent cross-section, particularly when using larger cross-sections than 4 mm². Responses indicate that while oversizing the cross-section may not cause issues, it is essential to ensure compatibility with the load and to avoid unnecessary weight. The conversation also touches on the use of standard combs for bridging connections and the importance of maintaining appropriate cross-sections throughout the circuit.
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FAQ

TL;DR: Typical switchgear bridges use 10–12 mm² comb busbars; “these bridges are dedicated ‘combs’ usually 10–12 mm².” [Elektroda, xury, post #16881821]

Why it matters: Correct bridge cross‑section prevents overheating, preserves protective selectivity, and simplifies future load changes for DIYers and pros.

Quick Facts

What cross‑section should I use between the disconnector and the RCD?

Keep the same cross‑section as the supply up to the protected circuit’s “exit,” or apply a rated comb busbar. This avoids premature downsizing and supports future load changes without rewiring. If you must reduce size, place protective devices at the reduction point. [Elektroda, Anonymous, post #16881773]

Is it okay to oversize the bridge (e.g., larger than the feeder)?

Yes. Oversizing a short bridge is fine and does not harm operation. It can run cooler under load and offers headroom for upgrades. “With such oversize it will work and nothing will happen.” [Elektroda, Anonymous, post #16881805]

Should I continue the same cross‑section from the RCD to the MCBs?

From the disconnector or RCD, branch circuits may use cross‑sections appropriate to their loads. Size each outgoing line for its expected current and protective device. Do not reduce conductor size upstream of protection. [Elektroda, kokapetyl, post #16881690]

Are ready‑made comb busbars acceptable as bridges?

Yes. A standard comb busbar is a bridge designed for linking devices on a DIN rail. It simplifies wiring, ensures uniform cross‑section, and fits manufacturer terminals. [Elektroda, retrofood, post #16881833]

What cross‑section do comb busbars typically have?

Forum practitioners note that dedicated comb busbars are usually 10–12 mm². This size supports typical residential rows of MCBs under an upstream RCD. [Elektroda, xury, post #16881821]

Where must I place protection if I reduce the conductor cross‑section?

Install the protective device exactly where the conductor cross‑section is reduced. This prevents the smaller downstream conductor from overheating during faults. “The principle is to install the protection in a place where the cable cross‑section is reduced.” [Elektroda, retrofood, post #16881841]

Is a 3‑pole disconnector acceptable when N and PE stay connected?

Yes. The thread example uses a 3‑pole disconnector with N and PE unswitched. Phase conductors are the focus for bridging and protection in that setup. [Elektroda, piotrkartuz, post #16881741]

How should I size branches to sockets and lighting from the RCD/MCB row?

Use practical guides cited: 1.5 mm² for lighting circuits, 2.5 mm² for socket circuits, and larger dedicated lines for special appliances. Match MCB ratings accordingly. [Elektroda, kokapetyl, post #16881749]

Can I downsize a bridge because the load is small today (e.g., just a bulb)?

Avoid downsizing on that basis. Loads can change to high‑demand appliances later. Keeping the larger cross‑section preserves safety margins and flexibility. [Elektroda, Anonymous, post #16881699]

Can an induction hob run on a lighting‑grade line?

No. Heavy appliances require their own appropriately sized circuits. Lighting‑grade conductors risk overheating and nuisance trips with such loads. Use a dedicated, larger cable. [Elektroda, kokapetyl, post #16881749]

What methods can bridge one RCD to several MCBs cleanly?

Use a distribution block, double wire ferrules, or a comb busbar. Choose one method and keep terminations consistent with device specifications for reliability. [Elektroda, elpapiotr, post #16886753]

What does “comb busbar” mean in this context?

It’s a ready‑made bridging bar that links adjacent devices, replacing hand‑made jumpers. It standardizes cross‑section and reduces wiring faults on DIN assemblies. [Elektroda, retrofood, post #16881833]

Example: I have a 5×4 mm² feeder to the disconnector; what next?

Maintain 4 mm² phase bridges to the RCD or use a rated comb. Then size each outgoing circuit per its load and protective device. [Elektroda, piotrkartuz, post #16881631]

How do I size bridges between disconnector, RCD, and MCBs?

  1. Keep incoming cross‑section to the protection point.
  2. If reducing size, place the protective device at that junction.
  3. For multiple MCBs, use a rated comb busbar to match device spacing. [Elektroda, retrofood, post #16881841]

Why do some RCDs feed four MCBs using mixed cross‑sections?

Installers may parallel outputs with double ferrules or connect via blocks, resulting in mixed wire sizes. Consistency and proper terminations remain crucial. [Elektroda, piotrkartuz, post #16886880]
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