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Industrial lighting by the lumen method

Enter the building dimensions, fixture height and activity. The page works out how many fixtures you need, how to lay them out, the power density and how much replacing the current installation saves each year.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Room
Required illuminance
New luminaire
Operation
Current installation (to compare)

Example: 400 W metal halide, which draws about 458 W with its ballast.

Economic analysis
—Luminaires required
—

Plan view of the luminaire layout

Luminaires in rows and columns with even spacing and half spacing next to the walls.

Tap the chart to see the values.

Luminaires required versus mounting height

Same luminaire and reflectances: mounting higher lowers room index K and the utilization factor.

Tap the chart to see the values.

Project payback

Cash flow of the improvement: investment in year 0 and annual savings escalated at the rate entered, discounted at the MARR.

Tap the chart to see the values.

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Room index: K = L × W ÷ [h × (L + W)], with h = mounting height − work plane (RCR = 5 ÷ K)
Number of luminaires: N = E × area ÷ (Φ × UF × MF); achieved illuminance = N_grid × Φ × UF × MF ÷ area
Layout: n_x × n_y ≥ N, chosen so spacings L ÷ n_x and W ÷ n_y are close; check: spacing ÷ h ≤ limit
Density = N × P ÷ area; energy = N × P × hours; savings = (P_current,total − P_new,total) × hours × price

Assumptions and limits

  • Approximate utilization-factor table for a medium-beam industrial LED luminaire (100% efficiency, since the flux entered is the luminaire flux), interpolated linearly between K = 0.6 and 5. It is a pre-sizing reference: use the photometric table of the chosen model.
  • Maintained illuminance values from the ISO/CIE 8995-1 tables; ANSI/IES RP-7 recommendations for U.S. industrial facilities are of the same order and may differ by task.
  • The lumen method assumes evenly distributed luminaires and gives the average only; uniformity, glare and emergency lighting need their own calculation.
  • Savings count energy only; LED long life also cuts maintenance, which is left out.

Standards and technical documents

  • ISO/CIE 8995-1: Lighting of work places — Part 1: Indoor
  • CIE 97: Guide on the maintenance of indoor electric lighting systems
  • CIE 40: Calculations for interior lighting — Basic method
  • EN 12464-1: Light and lighting — Lighting of work places — Part 1: Indoor work places
  • ANSI/IES RP-7: Recommended Practice for Lighting Industrial Facilities
  • IES Lighting Handbook, 10th ed. (zonal cavity method)

U.S. regulations

  • ANSI/ASHRAE/IES Standard 90.1: lighting power density limits (manufacturing and warehouse spaces)
  • OSHA 29 CFR 1926.56: illumination (minimum foot-candles for construction areas)
  • OSHA 29 CFR 1910.37(b): illumination of exit routes
  • NFPA 70 (NEC): branch circuits and luminaire installation

Frequently asked questions

How many fixtures do I need for an 8,500 ft² plant?

For 28 fc (300 lx, machining), 20,000 lm LED high-bays at 26 ft, a 0.72 utilization factor and 0.8 maintenance factor: about 21 fixtures, in a 7 × 3 grid.

How many foot-candles does each activity need?

ISO/CIE 8995-1 calls for 100 lx (9 fc) in circulation and storage, 300 lx (28 fc) for welding and ordinary machining, 500 lx (46 fc) for precision machining and offices and 750 lx (70 fc) for tool making. Check ANSI/IES RP-7 for U.S. practice.

How much does an LED retrofit save?

In the example, 24 metal halide fixtures of 400 W (458 W with ballast) become 21 of 150 W: power drops from 11 kW to 3.15 kW and, at 6,000 h a year and US$ 0.12 per kWh, the bill falls by about US$ 5,600 a year.

What is lighting power density?

Installed power divided by floor area. A well designed LED plant lands near 0.2 to 0.45 W/ft² for 28 fc; ASHRAE/IES 90.1 sets the code maximum by space type.