Underground Cable Pit Sizing: Cable Count & Access Guide

A contractor running a new underground supply to a rear switchboard hits a direction change halfway through the trench, and there's no clean way to pull the cable through without an access point right there. Order a pit that's too small for the number of cables passing through, and the job stalls while a bigger one gets delivered. Getting the size right the first time comes down to two things: how many cables are actually going through it, and how often someone's going to need to open it back up.

Size and depth are decided together on most jobs, but they're not the same question. If you haven't locked in confirming your burial depth yet, that's worth sorting first — this guide picks up from there, on what happens once the pit goes in and which size actually holds up to being opened again and again.

Cable Count Decides the Pit, Not the Trench Width

It's a common mistake to size a pit off the trench it's sitting in, rather than what's actually going to be coiled and joined inside it. A single service cable passing straight through needs very little room. The same pit trying to hold a joint, a spare loop for future testing, and a second cable entering from a different angle needs a lot more — even though the trench either side looks identical.

Before ordering, count what's actually going in: how many cables pass through, whether any of them terminate or joint at this point rather than just running through, and whether there's a spare loop being left for future work. A pit sized for a straight pass-through will feel undersized the moment a joint or a second cable gets added later, and re-digging to swap it out costs more than ordering one size up would have.

Matching Pit Size to the Job

Schnap's underground cable pit range runs from compact single-service pits through to large distribution pits built for multiple heavy runs. As a general starting point, here's how the range breaks down by what each size is typically suited to:

These pits sit within Schnap's broader range of underground enclosures, and the sizing logic above is a starting point — a Medium pit with one joint and no spare loop can behave more like a Small one in practice, while a Small pit pressed into service for a junction it wasn't sized for gets cramped fast.

Access Frequency Changes the Calculation

A pit that only ever gets opened for an emergency fault doesn't need to be worked in comfortably. A pit that's going to have a technician kneeling over it every time cabling gets tested or extended does — cramped access means more time on site and a higher chance of damaging a joint while working around it. If maintenance or testing access is going to be routine rather than rare, it's generally worth sizing up from what the cable count alone would suggest, purely to leave room to actually work.

Load rating follows a similar logic to access frequency, but it's about what sits on top of the pit rather than what happens inside it. A pit set into a garden bed or verge that never sees vehicle traffic has different requirements to one installed in a driveway or car park. As a general rule, pits expected to carry any vehicle loading need a higher load class than pedestrian-only locations — check the specific load rating listed against each product before ordering if the pit is going anywhere near driveway or trafficable ground.

Common Mistakes

Sizing off the trench instead of the cable count. A wide trench doesn't mean a wide pit is needed, and a narrow one doesn't rule out a joint being added later. Count what's going in, not what's around it.

Not leaving room for a spare loop. A pit sized exactly for the cables going through it today leaves no slack for re-termination or future testing — a common regret once the pit's already backfilled around.

Ignoring how often the pit will actually be opened. A pit that's technically big enough but painful to kneel in front of turns a quick job into a slow one every single time it's accessed.

Assuming pedestrian load rating covers a driveway install. If there's any chance a vehicle will drive over the pit — including during construction, before landscaping is finished — the load rating needs to match that, not the finished use.

Frequently Asked Questions

Pit Sizing & Selection

how many cables can go through a small underground pit?

It depends on the cable diameter and whether the pit is a straight pass-through or a joint point. As a general rule, a small pit around 320-420mm is comfortable for a single service cable running straight through, but gets tight fast once a joint, spare loop, or second cable is added.

is a small pit big enough for a single-phase service run?

For a straight single-phase service cable with no joint at that point, a compact pit such as the 320 x 530mm pit is generally sufficient. If a joint or a spare loop is planned for that access point, size up to a medium pit instead.

Installation & Compliance

do I need a licensed electrician to install an underground cable pit?

Installing the pit itself as a civil element generally doesn't require an electrical licence, but any cabling work connected to it — joints, terminations, or connecting into a switchboard — does need to be carried out or supervised by a licensed electrician under Australian regulations. Check your state's specific licensing requirements before starting.

does the pit depth need to match my conduit depth?

Yes — the pit needs to sit at the same depth as the cable or conduit run it's connecting to, rather than being set at a convenient depth on its own. If you haven't confirmed matching pit and conduit depth for your run yet, that's worth locking in before finalising which pit to order.

Shop Underground Cable Pit at Schnap

Trade pricing and same-day dispatch from Kingsgrove NSW across the full underground cable pit range, from single-service sizes through to large distribution pits.

• Small
• 419 x 155 x 350mm underground cable pit — compact single-cable pass-through
• 320 x 530mm underground cable pit — small single-service joint pit

• Medium
• 553 x 180 x 514mm underground cable pit — narrow multi-cable run
• 582 x 320 x 525mm underground cable pit — medium duty junction
• 582 x 393 x 580mm underground cable pit — medium duty with room for a joint
• 663 x 243 x 570mm underground cable pit — longer narrow run
• Underground cable pit (Pit 6) — mid-capacity junction point

• Large
• 584 x 281 x 760mm underground cable pit — deeper install, regular access
• 1240 x 420 x 834mm underground cable pit — major junction point

• Extra-Large
• 910 x 440 x 1075mm underground cable pit — heavy-duty distribution
• 2040 x 585 x 900mm underground cable pit — largest, major distribution runs

Find the full underground cable pit range at Schnap — trade pricing and same-day dispatch from Kingsgrove NSW.