Add up the wattage of your spots and stay under roughly 80 percent of what the driver can handle. On our 40 watt driver that works out at ten 3 watt spots, on the 20 watt version at six. The limit is not the number of spots but the total power.
First this: do you even need a driver?
Only low voltage spots need one. If your spot runs on 230 volts, as everything with a GU10 socket does, it goes straight onto the outlet and no driver is involved. If it runs on 12 volts, mains voltage has to come down first and you do need one.
That distinction is set out more fully in Which LED driver do you need? Put bluntly: if there is a block in the box, you have 12 volts.
The sum
Add up, then leave margin. Six 3 watt spots is 18 watts. On paper that fits a 20 watt driver, but you are then at 90 percent of the maximum.
The rule of thumb in the trade is 70 to 80 percent of the rated power. A driver running flat out continuously gets hotter, and heat is the main reason electronics give up early. On top of that, an LED driver briefly draws far more current at switch-on than while running. That peak has to fit too.
In practice:
- 20 watt driver: six 3 watt spots at most, and that is tight. Four or five is more comfortable.
- 40 watt driver: ten 3 watt spots at most. That is 30 watts, so 75 percent, which sits exactly right.
The Dovo set of six spots comes to 18 watts in total. Six 3 watt spots on a 20 watt driver works, but if you might add a spot later, start with the bigger driver.
There is a lower limit too
Less well known, and the cause of a lot of flickering: many dimmable drivers need a minimum load. Hang two 3 watt spots on a 40 watt driver and you are at 15 percent, and it may not regulate cleanly or may start pulsing when dimmed low.
So you do not pick a driver as large as possible to be safe. You pick it to fit: generous enough for the load plus margin, small enough that you do not fall below its floor.
Cable length counts
This gets forgotten at 12 volts. Power is voltage times current, so a low voltage means a high current. Ten 3 watt spots is 30 watts, and at 12 volts that is over 2.5 amps. The same 30 watts at 230 volts is less than 0.15 amps.
That current has to travel through your wire, and every metre of wire has resistance. Over ten metres of 0.75 mm² installation wire you lose about 1.2 volts at 2.5 amps. That is ten percent of your 12 volts, and you see it as spots at the end of the run burning visibly weaker.
Two fixes: thicker wire, or put the driver closer to the spots and run the 230 volts as far as possible. The second is almost always the better one. With a long run of spots, two smaller drivers in two places beat one big one at the start.
Several drivers on one switch
That is fine. On the 230 volt side a driver is just a load, so you hang as many on one circuit as the circuit can take. Do watch out if there is a dimmer in the wall: every driver then has to be dimmable and the combined power has to fall within that dimmer's range.
Our two drivers are dimmed with the remote supplied rather than a wall dimmer, so that does not come up. The accessories are grouped together.
In short
Add up the watts, keep 20 to 30 percent spare, and stay above the driver's minimum. At 12 volts, check the cable length as well, because that is the mistake that only shows up once everything is already in the ceiling. The spots themselves are at recessed spotlights.
Can I use a bigger driver than I need?
A little more headroom is good, but not without limit. Many dimmable drivers need a minimum load. Well below it, the light will flicker when dimmed low.
Can I link two drivers together?
Not on the 12 volt side, that is not how they are meant to work. On the 230 volt side you can: connect each one separately to mains and split the spots between them.
Why do my last spots burn weaker?
Almost always voltage drop over 12 volt wiring that is too thin or too long. Thicker wire, or moving the driver closer to the spots, solves it.