Guide

Cable Routes and Lengths: How Do You Determine a Cable Length on an Electrical Plan?

The straight line is systematically too short. Installation zones under 5.2.9.10, installation method and route, splitting at junction boxes, allowances as trade practice, and when the AREI/RGIE requires the length on the diagram.

Published on 31 August 2026 12 min

How do you determine a cable length on an electrical plan?

Not as a straight line, but as the path the cable actually takes: horizontally along the walls, vertically at both ends up or down to the device, with the detours around doors and windows and with the slack left at the board and in the box. The straight line is not a rough estimate of that path — it is a lower bound, and indoors it routinely sits 30 to 60 % below the real value.

This is not cosmetic. The length is the input to the voltage-drop calculation and therefore a criterion in choosing the conductor cross-section (sub-section 5.2.1.2 point b), and it is the unit of measure of your material costing. A guessed length makes both calculations worthless without anyone noticing.

Part of the route	What determines it	Where the value comes from
Horizontal run along the walls	Floor plan and the permitted installation zone	measurable from the plan
Vertical run between the zone band and the device	Mounting height of the device relative to the horizontal band	mounting height minus band height
Vertical run to the next storey	Storey height and position of the riser	architectural plan, section
Detour around doors, windows and openings	sub-section 5.2.9.10 point b	measurable from the plan
Buried run to an outbuilding	Trench route plus one descent and one ascent to burial depth	minimum 0.60 m per sub-section 5.2.9.2 point a
Slack at the termination point in the board	Terminal layout, wiring, later modifiability	company convention — not a standard
Slack in the box, switch or appliance	Connection method and space inside the enclosure	company convention — not a standard

The last two rows are marked that way deliberately: the AREI/RGIE names no value at all for allowances or slack lengths. Why an obligation nevertheless follows from it is explained further down.

Why is the straight line systematically too short?

Because a cable embedded in a wall is not allowed to run diagonally. Sub-section 5.2.9.10 point b ties electrical lines embedded without conduit in the walls of rooms to a fixed pattern:

The route consists exclusively of horizontal and vertical runs; horizontal runs in a ceiling are perpendicular to the vertical walls.
Horizontal runs lie 25 to 35 cm from the floor or from the ceiling, and likewise 25 to 35 cm above the underside of window lintels — provided the lines then stay at least 25 cm below the ceiling.
Vertical runs lie as close as possible to a corner of the room, or 10 to 20 cm from door frames and jambs.
Installation outside this zone pattern is carried out vertically above a visible electrical machine or appliance.
The covering plaster layer is not thinner than 0.4 cm.

The pattern serves a safety purpose — whoever drills later should know where nothing runs — and has a side effect on length: every diagonal becomes a staircase of one horizontal and one vertical leg, and every doorway forces a detour above or below it.

A worked example. A switch sits 6.0 m from the board in a straight line, diagonally across the room. Along the walls it is 4.2 m plus 3.6 m, so 7.8 m. Add the climb from the termination point in the board up to the horizontal band, say 0.9 m, and the drop from the band down to the switch height of 110 cm, roughly 0.8 m. Total: 9.5 m — 58 % more than the straight line, and that is before any slack.

For a socket outlet the last step almost disappears: if the horizontal band sits 25 to 35 cm above the floor and the socket at 30 cm, the vertical drop is close to zero. That is exactly why it pays to treat switches and sockets separately instead of applying one flat percentage to everything.

What does the installation method mean for the length?

It decides whether the zone pattern applies at all, and which geometric constraints are added.

Installation method	Effect on the route	Reference
Embedded without conduit, in walls	Bound to the zone pattern; plaster cover ≥ 0.4 cm	5.2.9.10 point b
Embedded in concrete or cement	Cover ≥ 3 cm; the route follows formwork and reinforcement, not the shortest line	5.2.9.10 point a
Embedded in conduit	The zone pattern of 5.2.9.10 b does not apply by the letter of the text; instead minimum bending radii and the duty to be able to pull in and remove conductors at any time	5.2.9.3 point e
Surface, cable tray, cable ducting	Freer routing, but tensile forces from self-weight must not act on the terminals in vertical sections	5.2.9.5, 5.2.9.6
Under mouldings and skirting boards	The route follows the skirting, so in practice the room perimeter	5.2.9.4
Buried	At least 0.60 m deep unless technically impossible ⇒ two vertical legs on top of the trench length	5.2.9.2 point a
Extra-low voltage (ELV)	Point b of 5.2.9.10 is expressly excluded — bell and SELV wiring is not bound to the zone pattern	5.2.9.15 point b

The bending radii for conduits bent on site are explicit: not less than ten times the outer diameter for metal conduits, eight times for flexible thermoplastic and five times for rigid thermoplastic conduits (sub-section 5.2.9.3 point e). For a 25 mm conduit that is 12.5 to 25 cm of radius per corner. In a dwelling with a dozen changes of direction per circuit, that adds up.

Which method suits which case is covered in Preflex versus loose wiring; the cable types themselves in XVB versus VOB.

Do you count the one-way distance or the round trip?

The one-way distance, from the board to the load. The return conductor is in the formula, not in the input: single-phase carries a factor of 2, three-phase a factor of √3. Entering twice the length counts the return conductor twice and oversizes the cross-section for no reason.

The criterion itself is not a number in the AREI/RGIE. Section 5.2.5 — "Chute de tension" in French, "Spanningsval" in Dutch, "Spannungsänderung" in German — only requires voltage drop to be limited to the values described in the rules of the trade. The widely used 3 % for lighting and 5 % for other final circuits are an interpretation following IEC/NBN HD 60364-5-52. Calculation method, network arrangements and examples are in Calculating voltage drop; the cross-section to protection mapping in Cable cross-section per breaker.

How do you count a cable split at a junction box?

Physically it is several cables; electrically it is one path. For voltage drop, what counts is the sum over the entire run from the upstream protective device to the load, not the longest individual segment. Three sections of 12 m each make 36 m, and calculating only the last leg understates the result by two thirds.

Why the split still has to exist in the model:

Each section can have its own cross-section and its own installation method. Both are mandatory single-line diagram content for domestic installations (sub-section 3.1.2.2 point a), and both weigh differently in the calculation.
Connection and junction boxes are themselves mandatory diagram content and must additionally appear with their position on the position plan (sub-section 3.1.2.3 point a).
Connections may only be made at defined places: in distribution and control assemblies, in connection or junction boxes, at the terminals of switches and socket outlets, or in sufficiently large ceiling boxes (sub-section 5.2.6.1). A joint buried in the middle of a wall is not a routing option, it is a breach.
For embedded conduit installations there is more: in connection, junction and draw boxes the conductors and cables remain accessible, and tees and elbows are forbidden (sub-section 5.2.9.3 point i.2). The box is therefore also a geometric waypoint, not merely a terminating point.

What allowances and slack are customary — and what does the AREI/RGIE say about them?

On the size of an allowance the AREI/RGIE says nothing. Neither section 5.2.5, nor section 5.2.9, nor the documentation requirements in section 3.1.2 mention a percentage, a slack length or a waste factor. Any figure you use for it is a convention of your own firm — and belongs in your calculation sheet as exactly that.

What the regulation does fix are the conditions from which slack follows in the first place:

Connections to appliances must not be subjected to tensile and torsional forces (sub-section 5.2.6.2 point b).
In vertical sections, tensile forces from the self-weight of the lines must not act on the connection terminals (sub-section 5.2.9.5).
In conduits it must be possible at any time to pull in or remove conductors or cables (sub-section 5.2.9.3 point e). A conduit route calculated to the millimetre makes precisely that impossible.
In boxes the conductors remain accessible (sub-section 5.2.9.3 point i.2) — which does not work without a loop inside the enclosure.

In practice this means: the allowance is not a safety cushion on an uncertain calculation, it is a material item with a purpose. If you apply it as a flat percentage of the total length, document the percentage, where it comes from and which design stage it belongs to. A value without provenance is no longer traceable at the next stage and quietly finds its way into quotations.

Does the cable length have to appear on the single-line diagram?

Here the AREI/RGIE distinguishes two cases, and the distinction is often overlooked.

Document and installation type	Required data about the line	Length required?	Reference
Single-line diagram, domestic installation	Type, cross-section, number of conductors, installation method	no	3.1.2.2 point a
Circuit diagram, non-domestic installation	Installation method, nature, number and cross-section of conductors and the length of the lines	yes	3.1.2.2 point b
Common parts of a residential building	May be limited to the requirements for domestic installations	no	3.1.2.1 point e
Position plan, domestic installation	The position of boards, connection and junction boxes, socket outlets, light points, switches, fixed machines and appliances and sources	no — position, not length	3.1.2.3 point a

In housing, the length is therefore a design quantity, not a drawing obligation. That is precisely what makes it dangerous: because it need not appear anywhere on the diagram, nobody checks it — while the cross-section that follows from it does appear and is checked. Without a recorded length, you cannot justify afterwards why 2.5 mm² was chosen in one place and 4 mm² in another.

For a non-domestic installation the position is unambiguous: without a length figure, the circuit diagram is incomplete.

From length to material list and price

Through two separate outputs that use the same resolved length.

The cable list gives one row per cable: type, number of cores, cross-section, length, From and To, the installation methods encountered along the route, and the CPR fire class; plus current, voltage drop in volts and percent, end voltage and the limit applied. If a length is missing, the row stays without a figure rather than showing an estimate.

The material list groups the same cables by specification and reports them in metres; where a price per metre exists, the line item follows from it. Two points belong explicitly with that:

The lengths shown are net lengths from the model; waste and installation allowances are not included. The allowance from the previous section is therefore not in there.
A circuit without a cable route does not disappear: it appears with 0 m and the note "Routing missing". A material list that looks suspiciously lean for a whole house usually contains exactly those rows.

The project overview draws the same distinction: it shows either a complete total length, or a subtotal marked "≥" together with the number of missing lengths, or no figure at all. An actual total of 0 m and an unknown length are two different states.

How does PlanElec route cables automatically, and where are waypoints still needed?

In the situation plan, under Linking & cabling → Cable route. If cables exist in the topology, you route the whole active storey at once or one selected line individually.

The algorithm looks for the shortest path along the drawn walls, not across the room: start and end points are projected onto the nearest wall edge, then a shortest-path search runs over the wall nodes. If there is no wall path — because the floor plan is interrupted there, for instance — the route falls back to the direct line. The reported length follows the visible polyline exactly; deliberately slanted wall segments are not artificially stretched to an orthogonal measure.

To adjust, select a cable line: a click on a section inserts a waypoint and starts dragging, a double-click removes it. A dragged waypoint is stored only on release and can be undone in a single step. When a circuit is rebuilt, the cable specification, manual length and saved route are preserved.

Four limits worth knowing:

The automatic route is two-dimensional. It lies in the plane of the storey; height differences within one storey — the climb into the zone band, the drop to the switch — do not enter the length. That is what waypoints, or the manual length, are for.
Between storeys you explicitly pick an existing staircase, shaft or riser. PlanElec invents no vertical connection. If one is missing, or reliable heights are missing, the route stays incomplete instead of reporting an apparent length. Where the connection exists, the height difference is counted.
The manual length takes precedence over the calculated route. That is the deliberate escape hatch for measured distances — and exactly the place where an old value can quietly survive if the route changes later.
Routing along the walls is an assumption about the execution, not a finding. Whether the cable runs in the zone, in conduit or on the surface is decided by the installation method you set per section — not by the router.

Step-by-step operation is in the guide Switch chains, circuits and cable routes. Which switch type sits at the end of the chain is covered in Two-way, intermediate or double switch.

What the AREI self-check does not do about length

The self-check is advisory and evaluates only supported rules against the data entered. Specifically on length:

It calculates voltage drop only when the path is complete. If the length of even one section between the protective device and the load is missing, or the path cannot be traced back without a break, it reports a data gap instead of a result. That is intentional: a value computed from partial data would be systematically too favourable.
It works from assumptions, not measurements: 80 % of the rated current of the upstream protective device as the load case, cos φ 0.95, copper, a purely resistive model with no reactance and no temperature correction.
The 3 % and 5 % limits are an IEC/NBN interpretation, not the wording of the AREI/RGIE. Section 5.2.5 refers to the rules of the trade.
It does not check whether the drawn route matches the actual installation, whether the zone pattern of 5.2.9.10 point b was respected, or whether slack was provided. No drawing reveals that.
It does not check current-carrying capacity under the real installation method, grouping and ambient temperature. Sub-section 4.4.1.4 names those influencing factors explicitly; the Table 4.11 found there only caps the protective device per conductor cross-section and is not a complete verification.
It replaces neither measurement nor inspection. The official assessment is made exclusively by an approved body.