When upgrading your electrics – particularly when adding things like off-peak heating or new consumer units – questions often arise about sub-mains and whether they now need extra protection under current regulations.
This article aims to explain the topic clearly for non-electricians, focusing on what’s changed (and what hasn’t) under the 18th Edition of the Wiring Regulations.
What Is a Sub-Main?
In simple terms, a sub-main is a cable that feeds power from your main fuse box (consumer unit) to another smaller fuse box elsewhere in your property – perhaps for a garage, extension, or in this case, new electric heating.
Do Submains Need RCD Protection?
The short answer is: not always.
The need for RCD (Residual Current Device) protection depends on how the sub-main cable is installed:
- If it’s buried in walls, like Twin & Earth (T&E) cable often is, then yes – an RCD is typically required.
- If it’s surface-mounted, in steel conduit, or armoured cable (SWA), then an RCD is generally not required.
These requirements haven’t changed significantly from the 17th to the 18th Edition. So, if the job was acceptable two years ago, it’s likely still fine now.
Does SWA Need RCD Protection?
Steel Wire Armoured (SWA) cable is very durable and offers mechanical protection. When it’s installed correctly and not buried in a wall, it usually does not require an RCD. This is especially true if it’s clearly visible and not in a place where a drill or nail might hit it by accident.
Changes in the 18th Edition: What’s New?
Some homeowners have been told that the 18th Edition brought in new requirements for sub-mains. This isn’t strictly true. Here’s what has changed:
- All domestic lighting circuits now need RCD protection, even if the cables are installed in steel conduit.
- There’s no new blanket rule saying that all sub-mains must be RCD protected.
So if someone insists that your sub-main needs an RCD “because of the 18th Edition,” it’s fair to ask them exactly which regulation they’re referring to.
Considerations for TN-C-S and TT Earthing Systems
Your earthing arrangement also affects whether an RCD is needed:
- TN-C-S (most common in UK homes): You may not need RCD protection on the sub-main if final circuits are already protected by RCBOs.
- TT systems: RCD protection is almost always required, and in these cases, a time-delayed (Type S) RCD is used to avoid the whole installation tripping due to a single fault.
Risks of Putting an RCD at the Start of a Sub-Main
One concern raised is that putting an RCD at the very start of the sub-main means everything could switch off if there’s a fault on just one circuit. That’s a valid worry.
To reduce this risk:
- Use RCBOs (individual RCDs for each circuit) at the final consumer unit.
- If needed, use a time-delayed RCD at the start of the sub-main. This allows time for a fault to trip a local RCBO first, keeping the rest of the circuits alive.
Summary: What Should a Homeowner Do?
If you’re being advised that new RCD protection is needed:
- Ask why, and which specific regulation this is based on.
- Check the cable type and installation method – SWA may not need it.
- Get a second opinion, especially if the advice seems different from what was quoted in previous years.
Not all electricians interpret the regulations the same way, and some may be overly cautious or influenced by insurer or organisation guidelines.