Duct Draw Cord: The Pull Cord Left in Every Empty Duct, and Why It Matters

The groundworker lays a 100mm duct under the new floor slab for the moved electricity supply, backfills the trench, and pours the concrete. Months later the electrician reaches in to pull the supply cable through and finds nothing to pull it with. No cord. Now the only ways through a buried, bent duct are draw rods fed blind, a vacuum and a foam bung, or breaking out the slab. A cord laid in earlier would have turned a half-day problem into a five-minute pull. It is the cheapest thing in the trench and the most often forgotten.

What it is and what it's for

A duct draw cord is a length of cord left inside an empty duct or conduit so that a cable can be pulled through later without digging up the route or chasing the wall open again. It does no work itself. It sits there, tied off at both ends, until someone needs to get a cable through. Then they tie the new cable to one end, pull from the other, and the cable follows the cord through the duct.

The principle is the same whether the duct is a 100mm twinwall pipe buried two feet under a garden for a moved electricity supply, a 54mm telecoms duct running to the boundary for broadband, or a short length of conduit chased into a wall during first fix. In every case the rule is identical: never leave an empty duct without a cord in it. The duct exists precisely so you don't have to excavate to run a cable. A duct with no cord defeats its own purpose, because you then have to get a cord into it before you can pull anything, and that is the hard part.

The cord is sometimes called a draw cord, a pull cord, a draw string, a draw rope, or a cable pulling cord. Electricians and groundworkers use the names interchangeably. On a domestic extension it is almost always a thin polypropylene cord or a flat draw tape, not a rope, because the ducts are short and the cables are not heavy enough to need anything thicker.

Why every empty duct gets one

The reason a duct is laid at all is to leave a clear, protected route for a cable that either isn't ready yet or might be needed in future. The electricity board hasn't connected the moved supply. The broadband isn't ordered. The garden power is "for later". In each case the builder lays the duct now, while the trench is open or the floor is down, because the route is far easier to create while everything is accessible than to retrofit once the slab is poured and the patio is laid.

But a duct on its own is just an empty pipe. To pull a cable through it you need something already running the full length of the duct that you can tie the cable to. That something is the draw cord. Lay the cord in as the duct goes down, and pulling a cable later is trivial. Leave the duct empty, and you are back to fishing a cord through a buried, possibly bent pipe, which is exactly the awkward job the duct was meant to avoid.

This is why the cord goes in at the same moment the duct is laid, not later. Once the duct is buried and the trench backfilled, getting a cord into it is no longer a quick job. The window to do it the easy way closes when the trench is filled.

How it gets installed

There are two ways the cord ends up inside the duct, and which one applies depends on whether the duct is being assembled on the spot or already sealed.

The straightforward method is to lay the cord in as the duct sections are jointed together. The groundworker feeds the cord through each length of duct as it's connected, so by the time the run is complete the cord already runs the full length. This is the normal approach for buried twinwall ducts laid in a trench: each section gets the cord threaded as it goes down.

The second method is used when the duct is already in one piece, or already sealed, or too long to thread by hand. The cord is blown or vacuumed through. A light foam bung or a small parachute is tied to the cord at one end, and a vacuum is applied at the other, or compressed air pushes the bung through from the near end. The bung drags the cord behind it down the length of the duct. This is how cable installers get a cord into a long pre-laid duct without taking it apart.

Either way, the cord is then tied off at both ends with plenty of slack left. Slack matters. A cord pulled tight and tied at each opening with nothing spare can disappear into the duct the moment one knot slips or one end is cut for the pull. Leave a loop of slack coiled at each end, secured to something, so there is always cord to grab.

The leave-a-new-cord rule

This is the rule that separates a job done properly from one that stores up trouble, and almost no homeowner guide mentions it.

When you pull a cable through a duct using the cord, pull a fresh cord through at the same time.

Tie the new cable and a new length of draw cord together at the leading end, and pull both through the duct in one go. The cable goes in, and a brand new cord comes through behind it, ready for the next time. The duct is never left without a cord in it.

Skip this and you've used up the one cord the duct had. The next cable run, whether that's a fault repair, an upgrade, or a future circuit, starts from scratch with no cord. You're rodding or vacuuming a cord back in before you can pull anything. A competent electrician does this automatically. If you're watching the supply cable go in, it's a fair question to ask: "Are you leaving a new cord in behind it?"

Types of draw cord and draw aid

The thing used to pull a cable through a duct or conduit isn't always a cord. There are a few options, and they suit different lengths and situations.

Pre-lubricated polypropylene draw tape. A flat woven tape, often printed with breaking-strain markings or length markers, that's slippery enough to feed through long ducts with minimal friction. The flat profile slides past bends better than a round cord. Sold on reels. This is the default for cable installers working long telecoms and power ducts.

Twisted polypropylene cord or rope. A round cord, usually blue, sold on reels or drums in lengths from tens of metres to 500m. Cheap, strong, and the common choice for leaving in buried ducts. A 6mm polypropylene cord has a breaking strain in the region of 550kg, far more than you need for a domestic supply cable, which is exactly why it's the safe default.

Steel or nylon fish tape. A spring-steel or nylon tape on a winder, pushed through a short empty duct or conduit to fish a cable or cord through. Useful for short runs where there's no cord already in place, but it has limited reach and won't turn tight bends in a long buried duct.

Draw rods. Rigid fibreglass or polyester rods that screw together end to end, fed into a duct to push or pull a cable through. They work well on short, fairly straight runs, a wall conduit or a metre or two of duct, but they bind on bends and have a practical reach limit. They're the fallback for a duct with no cord, not a substitute for one.

For leaving a cord in a buried duct, the cheap polypropylene cord in the table below is all you need. The draw tape and rod options are tools for getting a cable or a cord through a duct that has no cord, which is the situation you're trying to avoid in the first place.

Breaking strain: why a strong cord matters

The cord has to be strong enough to pull the cable through without snapping, and that's not as trivial as it sounds. Pulling a supply cable, especially steel wire armoured (SWA) cable, through a buried duct with bends in it takes real force. The cable is heavy, it drags against the duct wall, and every bend multiplies the friction. A light cord that was fine for threading the empty duct can part under the load of an actual cable pull.

This is the classic failure: someone leaves a thin string in the duct, the electrician ties the SWA to it and heaves, and the string snaps halfway. Now the cable is jammed partway through with no cord on it, and the broken cord is somewhere in the middle of the duct. That's a worse position than starting with an empty duct.

A 6mm polypropylene cord with a breaking strain around 550kg gives a wide margin over the force any domestic cable pull will ever apply. It costs almost nothing more than a thin string and removes the risk entirely. Don't leave a duct with a token bit of garden twine in it. Leave a proper draw cord rated to pull a real cable.

Cable lubricant for long pulls

On a long duct, or one with several bends, friction does most of the work of fighting you. Cable pulling lubricant, a thick water-based gel, coats the cable and the inside of the duct so the cable slides instead of dragging. For a short straight duct under a floor you won't need it. For a long run across a garden with two or three bends, it's the difference between a steady pull and a stuck cable.

The lubricant is wiped onto the cable as it feeds into the duct, or poured into the duct mouth ahead of the pull. It's harmless to the cable sheath and to the duct, and it dries to nothing over time. If your electrician is pulling a heavy supply cable through a long duct and isn't reaching for lubricant, that's worth a question, because a cable forced through dry can have its sheath scuffed or torn against a sharp duct joint.

Where it comes up on an extension

On most extensions the draw cord shows up in one specific, important place: the service duct for a moved or upgraded utility supply.

When the electricity supply is relocated, the meter or the incoming cable often has to move, and the distribution network operator's cable runs through a duct laid by the groundworker. That duct gets a draw cord so the supply cable can be pulled through once the connection is scheduled. The duct and cord go in early, during groundwork, because the trench is open then and the lead time on a supply move runs to weeks. Getting the duct and its cord laid while the ground is open is part of doing the utility move properly.

Telecoms and broadband ducts work the same way. A duct laid to the boundary for a future fibre connection has a cord left in it so the cable can be pulled when the service is ordered, without re-digging the front garden.

And then there are the spare ducts. Standard good practice on any extension or landscaping job is to lay a spare duct or two for the future, garden power, an EV charger, an outbuilding supply, lighting down the garden, and leave a draw cord in each. The cord costs pennies and the duct is going in the trench anyway while it's open. A spare duct with a cord in it turns a future "dig up the patio" job into an afternoon's cable pull. Leaving the cord out of a spare duct wastes most of the value of laying it.

The same item appears at first fix, where it's the draw string left inside a length of conduit. An electrician running an empty conduit to a position that might take a future cable, a spare run to a kitchen island, a conduit to a loft, leaves a draw cord in it for exactly the same reason: so a cable can be pulled later without chasing the wall open again.

Who does what

Two trades touch the draw cord, and they hand off between them.

The groundworker lays the duct and the cord. As the trench goes in and the duct sections are jointed, the cord is threaded through and tied off. This is the right moment, while everything is open and accessible. The groundworker isn't pulling any cable, just leaving the route and the means to use it.

The electrician, or the network operator's jointer for an incoming supply, pulls the cable through later using the cord. They tie the cable on, pull from the far end, and, if they're doing the job properly, pull a fresh cord through behind the cable so the duct is left ready for next time.

The handover is where things slip. The groundworker assumes the duct is enough and leaves no cord. The electrician arrives expecting a cord and finds none. Make sure whoever lays the duct knows a cord goes in with it, and confirm it before the trench is backfilled. Once the slab is down, fixing a missing cord is no longer a quick job.

How the install should run

Common mistakes

Laying a duct with no draw cord. The single most common error. The duct goes in, the trench is filled, and there's nothing inside to pull a cable with. Getting a cord into a buried, bent, backfilled duct then means draw rods or a vacuum, the awkward job the duct was supposed to prevent. Always thread the cord as the duct is laid.

Not leaving a new cord when pulling the cable. Using up the cord on the first pull and leaving the duct empty afterwards. The next cable run starts from nothing. Pull a fresh cord through with the cable every time.

A cord too light to pull the cable. Leaving a thin string or garden twine that snaps under the load of a real cable pull, jamming the cable halfway with the broken cord stranded inside. Use a proper draw cord rated well above the pull force, a 6mm polypropylene cord at around 550kg breaking strain is the safe default.

No slack tied off. Tying the cord tight with nothing spare, so the first slipped knot or cut end lets it vanish into the duct. Leave a coil of slack at each end.

Cord left loose in the duct. Not tying the ends off at all, so the cord slides in and is lost. Both ends tied to something solid, every time.

Where you'll need this

• Utility meter relocation - a draw cord is left in the service duct so the moved supply cable can be pulled through later without re-excavating

A draw cord belongs in any empty duct or conduit on any extension, conversion, or landscaping job: service ducts for moved or upgraded supplies, telecoms and broadband ducts, and spare ducts left for future garden power or an EV charger. The cord costs almost nothing and goes in at the moment the duct is laid, whatever the project type.