Gas and water bonding is the thick green and yellow cable clamped to the metal pipes where your gas and water come into the house. Most people only hear about it when an electrician, a gas engineer or an EV charger installer says it is missing or too thin. This guide answers the questions that come up most: what bonding does, what size cable it needs, where the clamps have to go, and why it matters for your EICR.
What Is Earth Bonding?
Earth bonding, properly called main protective bonding, is a cable that connects metal pipes entering your home to the main earthing terminal next to your consumer unit. It keeps those pipes at the same voltage as the earthed parts of your electrical installation. If a fault makes something live, the voltage between a tap and a metal appliance stays low enough to be safe while the fuse or circuit breaker disconnects the fault.
Bonding is not the same as earthing. Earthing gives fault current a path back to the supply so the fuse or circuit breaker trips. Bonding stops a dangerous voltage building up between metal parts while that happens. A safe installation needs both.
The pipes that normally need bonding are the incoming gas pipe, the incoming water pipe and, where you have one, an oil supply pipe. Exposed structural steelwork and some central heating pipework can also need it.
What Size Earth Cable for Bonding Gas and Water Pipes?
For almost every UK home the answer is 10mm² copper. The exact rule depends on how your supply is earthed:
| Your supply | How the size is worked out | Typical home |
|---|---|---|
| PME (TN-C-S), the most common in the UK | Set by the size of the supply neutral (BS 7671 Table 54.8). A neutral up to 35mm² needs 10mm² bonding. | 10mm² |
| TN-S, an earth taken from the metal sheath of the supply cable | Half the size of the earthing conductor, never less than 6mm². | 10mm² (half of a 16mm² earthing conductor, rounded up to the next cable size) |
| TT, your own earth rod | Half the size of the earthing conductor, never less than 6mm². | 6mm² or 10mm², depending on the earthing conductor |
Bonding is sized from the supply and the earthing arrangement, not from how many appliances you have. Some cable sellers' guides suggest bigger bonding for bigger houses or heavier loads, which is not how the Wiring Regulations work. If you are not sure which supply you have, the label on your consumer unit or your last EICR will say, or an electrician can tell you in a minute by looking at the cut-out.
Where Gas and Water Bonding Clamps Must Go
The Wiring Regulations (Regulation 544.1.2) set out where the connection to each pipe is made:
- Within 600mm of the meter outlet, or of the point where the pipe enters the building if the meter is outside.
- On the house side of the meter or point of entry, on hard metal pipework, not on a flexible connector or a plastic section.
- Before the pipe branches off, so every branch in the house is covered by one connection.
- With a proper BS 951 earth clamp and a permanent label reading "Safety Electrical Connection - Do Not Remove".
Can You Bond Gas and Water Pipes With One Cable?
Yes. IET guidance allows one conductor to bond both the gas and the water, as long as the cable is continuous. It should loop through the first clamp without being cut, so that taking one clamp off can never disconnect the other pipe. Two separate cables, each run back to the main earthing terminal, are also fine.
What is not acceptable is joining the gas and water pipes to each other and only running one short cable back, or cutting the cable at the first clamp and starting a new length. Both are common findings on older installations.
Do Plastic Water Pipes Need Bonding?
Not automatically. Many homes now have a blue plastic water main coming in from the street, and a plastic pipe cannot bring an earth potential into the house.
The catch is the metal pipework inside the house. If that copper pipe touches the ground somewhere else, for example an outside tap or a pipe running through a solid floor, it can still be an extraneous-conductive-part and need bonding. An electrician confirms this with a resistance test between the pipe and the main earthing terminal, rather than assuming either way.
Do You Still Need Bonding in the Bathroom?
Often not. The cables you sometimes see clamped to pipes under a bath or basin are supplementary bonding, which is separate from the main bonding at the meters. Under Regulation 701.415.2, supplementary bonding in a bathroom can be left out when every circuit in the room is protected by a 30mA RCD, main bonding is in place, and the circuits meet their disconnection times. In a home with a modern consumer unit, that is usually the case. Do not remove existing bathroom bonding yourself, though. Leave that call to an electrician who has tested the installation.
How to Check Your Gas and Water Bonding in Two Minutes
You can do a quick visual check without touching anything electrical:
- Find your gas meter and look at the pipe leaving it. Within a hand span or two, you should see a metal clamp with a green and yellow cable attached.
- Find your water stopcock, often under the kitchen sink. Look for the same clamp and cable on the pipe just after it.
- Check each clamp has a label reading "Safety Electrical Connection - Do Not Remove".
- Look at the cable thickness. 10mm² cable is roughly as thick as a pencil, while 6mm² is noticeably thinner.
- Note anything loose, painted over, corroded or clamped onto a plastic section of pipe.
If a clamp is missing or loose, do not try to refit it while the pipe is in use. Book an electrician to check it.
Why Gas and Water Bonding Comes Up on an EICR
Earthing and bonding are part of every electrical installation condition report, and they are among the most common findings in older homes. As a general guide:
- No main bonding at all is normally coded C2, potentially dangerous, which makes the report unsatisfactory.
- A clamp that has come loose, or one fitted after a plastic section of pipe, is usually treated the same way, because the bond is not doing its job.
- Bonding that is present but thinner than today's rules require, such as 6mm² on a PME supply, doesn't get a code, as long as it's at least 6mm² and shows no sign of heat damage. Thinner than 6mm² is a C2.
The inspector decides each case on what they find, so treat these as typical outcomes rather than fixed rules. Our guide to EICR codes C1, C2 and C3 explains what each code means, and landlords can see the deadlines that apply in our guide to EICR requirements for landlords.
Gas engineers also notice bonding. It is often flagged when a gas meter is swapped or a new boiler is fitted, even though the fix itself is electrical work.
Why EV Charger Installers Check Your Bonding First
Before fitting a home charger, installers check the main bonding, because the Wiring Regulations require the existing earthing and bonding to be adequate before anything is added to an installation (Regulation 132.16), and most homes are on PME supplies, which have extra earthing rules for EV charging. If the bonding is missing, under 10mm² or more than 600mm from where the pipes enter, the installer will usually ask for it to be put right first. Cord EV, a national charger installer, says it cannot proceed until the bonding is 10mm² and within 600mm of the pipe entry, and lists the upgrade at £125 per pipe on its bonding guide.
If you are planning a charger, ask for the bonding to be checked at the survey so any upgrade is done before installation day. Find out more about our EV charger installation service, or estimate the running cost with our EV charging cost calculator.
What Gas and Water Bonding Costs
Bonding upgrades are one of the smaller jobs an electrician does. The price depends mainly on how far the cable has to run from the meters to the consumer unit, how easily it can be routed out of sight, and whether one or both pipes need new clamps. The figure above from Cord EV gives an idea of the going rate when it is quoted alongside a charger installation.
We carry out bonding checks and upgrades across Tunbridge Wells and Kent, either on their own or as remedial work after an EICR, and every job is tested and certified. As a NAPIT registered electrician, we can confirm your supply type, check the existing bonding and give you a clear price. Get in touch for a free quote.