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
- Use dedicated comb busbars for bridging RCD to MCBs; typical cross‑section is 10–12 mm². [Elektroda, xury, post #16881821]
- Do not downsize conductors before the protection point; place protection where cross‑section reduces. [Elektroda, retrofood, post #16881841]
- Oversizing a short bridge is acceptable; it “will work and nothing will happen.” [Elektroda, Anonymous, post #16881805]
- Example feeder: 5×4 mm² entering a disconnector; size downstream bridges accordingly. [Elektroda, piotrkartuz, post #16881631]
- Typical final‑circuit guides used: 1.5 mm² lighting, 2.5 mm² sockets, larger for dedicated loads. [Elektroda, kokapetyl, post #16881749]
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?
- Keep incoming cross‑section to the protection point.
- If reducing size, place the protective device at that junction.
- 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]
