By Kent Traders · Updated 21 October 2026 · 8 min read · England (Part P)

Most guides answer “how many downlights per circuit” with a single number, usually 10 or 12, and stop there. That figure is a rule of thumb, not a calculation. On an LED circuit the breaker amps almost never decide it. The dimmer, the switch-on surge and the way you split the house do.

How many downlights per circuit does the maths allow?

Electrically, far more than anyone should install. The design rule in BS 7671 is simple: the load current (Ib) must not exceed the breaker rating (In), and that rating must not exceed what the cable can carry (Iz). Lighting circuits are usually protected at 6A on 1.5mm² cable, so the arithmetic is short.

A 6A breaker at 230V covers 1,380W. A fire-rated downlight such as the Luceco EFTF5W40 is rated at 5W with a power factor above 0.9, so each one draws roughly 6VA. Divide 1,380 by 6 and the amps ceiling is above 200 fittings. Nobody should wire 200 downlights on one way, so amps are not the constraint.

Browse the full fire-rated downlights range to see the ratings you are working with.

From watts to amps: the sum

Run this sum for every fitting you plan to buy.

1. Find the driver rating in watts. Use the fitting’s own rating, not the “equivalent to 50W” claim on the box.
2. Divide by the power factor. Where the datasheet says above 0.9, use 0.9. That converts watts to volt-amps.
3. Divide by 230. The answer is the current each fitting draws, in amps.
4. Multiply by the fitting count. That is the circuit’s load current (Ib).
5. Compare with the breaker. Keep Ib comfortably below the 6A rating, then check the dimmer and inrush limits below.

The halogen row is the warning. A GU10 fitting marked “50W max” can hold a lamp ten times hungrier than the LED you fitted. Twenty halogens still fit inside 6A, but with little margin.

What really limits you: inrush current

Inrush is the short current spike when an LED driver switches on. All drivers contain capacitors, and capacitors take a large gulp of current for a fraction of a millisecond. The breaker sees that spike, not the steady 0.02A.

ABB published a test with twelve 150W LED drivers on one C16 breaker. It tripped at switch-on. ABB concluded eleven drivers could share that breaker, though its own 80% safety margin points nearer ten. That is one set of drivers, not a rule to copy, but it shows why watts alone mislead. Read the ABB note on protecting LED lighting circuits.

Dimmers set the practical ceiling

Most dimmers hit their limit long before the breaker does. A dimmer carries its own LED rating, and it is lower than the number on the front. The BG 881 trailing edge dimmer is listed as 200W rated, 100W for LED. Twenty 5W downlights is exactly 100W, which leaves no margin.

We would stay near 80% of the LED rating. On that dimmer, that is sixteen 5W fittings. Trailing edge types are the usual choice for LEDs, and our guide to LED dimmer flicker explains the difference in the related reading below. All 66 products in the dimmer switches collection are in stock at the time of writing.

Worked example: 24 downlights in a house

Take a ground floor with 24 fire-rated 5W downlights: 12 in the kitchen-diner, 8 in the living room and 4 in the hall.

One circuit could carry all 24, but we would not wire it that way. Circuit 1 takes the kitchen-diner and hall: 16 fittings, 80W, 0.39A. Circuit 2 takes the living room: 8 fittings, 40W, 0.19A. A fault in the kitchen then leaves the living room lit.

Buying 24 of the Luceco EFTF5W40 at £7.97 each costs £191.28, checked on 20 October 2026. Keep one colour temperature per room, since mixed whites look like a mistake.

How to split downlights across circuits

Split by zone first, then by dimmer group. The aim is that one fault never leaves the whole house dark.

• Put each main room on a switch group of its own, sized to the dimmer’s LED rating.
• Keep hall and landing lights on a different way from the rooms they serve, so you can always reach the consumer unit.
• Leave spare capacity on each way for the extra fittings you will want in two years.
• Label every circuit at the consumer unit with the rooms it feeds.

Where downlight circuits go wrong

Three mistakes cause most of the call-backs.

First, sizing on the fitting’s “max” label. A GU10 holder rated 50W max carries 5W when you fit an LED lamp. If someone later swaps in halogens, the load jumps tenfold. Choose dimmable GU10 LED lamps and tell the customer what to buy next time.

Second, putting non-dimmable fittings on a dimmer. They flicker, buzz or fail early. Third, ignoring the dimmer’s minimum load.

What to buy: Luceco fire-rated downlights

Four Luceco downlights are in stock in the TEO down lights collection. Prices were checked on 20 October 2026 and may change.

For the wider choice, see the Luceco range. Check before you order:

• Fire rating in minutes for your joist type.
• Cut-out size against your ceiling. The EFTF5W40 needs 76mm.
• IP rating for kitchens and bathrooms. The Essence range is IP65.
• Dimmable driver, if the room will be dimmed.
• One colour temperature per room.

FAQs

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