Worked example: a 600 m² office floor with continuous light lines

Starting points for the example

The example is fictional, the figures are realistic. An open-plan office of 30 by 20 metres, so 600 m², with a white suspended ceiling at 3.0 m, light-coloured walls and a dark floor (reflectances 0.7 / 0.5 / 0.2). One long side is a glazed facade. There are 48 workstations in rows parallel to the long side.

The requirements follow from EN 12464-1 for writing, reading and screen work: a maintained 500 lux on the working plane at 0.75 m, UGR 19, uniformity Uo 0.60 and Ra 80. As the luminaire we choose the linear luminaire from LINEA: 24, 32 or 40 W per module via dip switch, 120 lm/W, 4000 K, Ra > 80, in white. We suspend the lines 0.4 m below the ceiling, without the uplight module. A module is 1,208 mm long; for the layout we work with 1.2 m per module. Check the dimension and the connectors on the product page before you finalise the parts list.

Step 1: from lux to lumens

The lumen method starts at the working plane: 600 m² × 500 lux = 300,000 lumens that have to arrive on the desks. Not all of the luminous flux ends up there and the installation ages. So you divide by two factors: the utilisation factor and the maintenance factor.

The utilisation factor depends on the room index and the reflectances. This room is large and low (1.85 m between luminaire and working plane) with a light ceiling; for a direct-emitting light line, 0.65 is then a realistic value. We set the maintenance factor at 0.8: a clean office with a two-year cleaning interval.

Installed luminous flux required: 300,000 / (0.65 × 0.8) = 300,000 / 0.52 ≈ 577,000 lumens. At 120 lm/W a LINEA module delivers around 2,900 lumens at 24 W, 3,800 lumens at 32 W and 4,800 lumens at 40 W. That gives a lower limit of 121 modules at 40 W or 152 modules at 32 W.

Step 2: quantities and layout

The lines run lengthways, parallel to the facade and to the viewing direction of the staff. A line of 24 modules is 29.0 m long and leaves 0.5 m free at both end walls. That gives two variants:

• Variant A, 6 lines of 24 modules at 40 W: 144 modules, 691,200 lumens installed, around 600 lux on the working plane. Line spacing 3.33 m, first line 1.67 m from the facade.
• Variant B, 7 lines of 24 modules at 32 W: 168 modules, 638,400 lumens, around 553 lux. Line spacing 2.86 m, first line 1.43 m from the facade.

Variant A needs 24 fewer modules and has almost 20 per cent margin, but its 3.33 m line spacing is 1.8 times the height above the working plane. For a direct-emitting light line, around 1.5 times that height is a common rule of thumb; beyond it the illuminance between the lines falls too far for Uo 0.60. Variant B stays within that rule of thumb at 2.86 m, keeps a margin of over 10 per cent and has a setting in reserve: at 40 W the same installation gives around 700 lux, should the client later want more light on the working plane. In this example we choose variant B.

Step 3: dip-switch setting and energy

You set the wattage before mounting, with the supply isolated, and record the setting per room in the handover file. For the open-plan office that is the 32 W setting. The same module goes to the adjacent corridor and kitchenette at 24 W, where 100 to 200 lux is enough, so you order a single item for the whole floor.

The connected load of variant B is 168 × 32 W = 5,376 W, or 9.0 W/m². Variant A comes to 144 × 40 W = 5,760 W and 9.6 W/m². At 2,500 operating hours a year that is 13,440 against 14,400 kWh: despite the extra modules, variant B uses almost 1,000 kWh a year less, because it sits closer to 500 lux rather than well above it.

Glare, walls and the check in the lighting calculation

The UGR<21 of LINEA is a table value for a reference room. In this office three things work in your favour: the lines run parallel to the viewing direction, the ceiling is white and the lines hang beside the desks, not directly above the screens. Whether the calculated UGR per workstation comes out at 19 or lower has to be shown by the lighting calculation; the same applies to the 75 lux on the walls and 50 lux on the ceiling that the standard asks for in offices.

If the ceiling comes out too dark in the calculation, the indirect uplight module from LINEA is the answer. Then recalculate: part of the flux goes upwards and the utilisation factor drops, so you add modules or go one setting higher. For 350 euros excl. VAT you receive lux, UGR and uniformity per room plus a parts list.

Parts list and final circuits

The order for variant B consists of:

• 168 linear luminaire modules LINEA 24-40 W, white, dip switch at 32 W
• 7 supply points, one per line, with suspension kits and connectors as listed on the product page
• optional uplight modules, only if the calculation calls for them
• additional modules at 24 W for the corridor and kitchenette, counted separately

With 168 units of a single item you are above the 100-unit threshold at which Lichtplein applies project pricing; request it via the quote request. Luminaires are delivered from European stock within 5 working days, with a 5-year warranty.

Size the final circuits for the highest setting, even if you hand over at 32 W: 24 × 40 W = 960 W per line, 6,720 W for seven lines, roughly 29 A at 230 V. That is at least two 16 A circuits; take four, so that the facade lines can be switched off separately in daylight. Allow for the inrush current of the drivers when choosing the circuit breakers.

Frequently asked questions