Three questions come up on every prep: how much does it draw, will the line take it, and do we need a generator. All three answer to simple maths, provided you do it before ordering rather than at the moment of plugging in.
This article gives the method and the orders of magnitude. It does not replace a qualified electrician: connecting to an installation, choosing the protection and meeting regulations are their responsibility.
1. Turning watts into amps
A breaker does not count watts, it counts amps. On single phase 230 V the conversion is direct:
current (A) = power (W) ÷ 230
A 2.5 kW head therefore draws about 11 A, a 4 kW about 17 A, a 575 W about 2.5 A. On three phase 400 V the formula becomes power ÷ (400 × 1.73), which spreads the load across three phases.
Two corrections matter in practice. Discharge sources (HMI) pull a starting current well above their running draw, and their ballast is not a hundred per cent efficient: count generously. Modern LEDs are far better behaved, but their switching supplies cope badly at the end of a long run.
2. Add up, then add margin
The total is worked out location by location, never across the whole package. What counts is what will be switched on at the same time in the same place.
And the critical moment is not the take: it is the look, when everything is on to judge a balance, before the unused fixtures get killed. Your figure has to cover that peak, not the average across the day.
The rule of thumb fits in a sentence: add up the wattage, add thirty per cent, and compare with what the location can genuinely supply. If the two numbers are close, the generator question has already answered itself.
| Supply | Usable power, single phase | What that means |
|---|---|---|
| 16 A | about 3.5 kW | One medium fixture and its fill |
| 32 A | about 7 kW | A small interior |
| 63 A | about 14 kW | A comfortable location |
| Three phase 32 A | about 22 kW | A stage, if the load is balanced |
These are orders of magnitude, not regulatory design figures. They are there to decide what to order, not to sign off an installation.
3. Balancing the phases
On three phase, the available power only counts if it is spread. Three phases of 32 A do not give you 22 kW on any one of them: they give you three lots of about 7 kW.
In practice you split the big fixtures across different phases, and you write down what is on which. It is the kind of thing you think you will remember and forget the moment one fixture moves.
A bad imbalance trips one phase while the total looks comfortably within limits, and it is one of the most frustrating faults on set precisely because the overall figure was right.
4. Choosing the cable size
Two constraints stack up: heating, which depends on current, and voltage drop, which depends on current and length.
Over short distances heating governs. Over long ones voltage drop decides, and that is the one people underestimate: a run that never gets warm can still deliver a voltage that is too low at the far end.
| Cross section | Typical use | Length to watch |
|---|---|---|
| 1.5 mm² | Small fixtures, short extensions | Beyond roughly 25 m |
| 2.5 mm² | Standard 16 A extension | Beyond roughly 40 m |
| 6 mm² | 32 A run | Beyond roughly 50 m |
| 16 mm² and up | Feeding a distro box or a generator run | By calculation |
Low voltage does not show up as a clean failure but as misleading symptoms: an LED that refuses to start although it worked elsewhere, an HMI that will not strike, a dimmer behaving oddly. You go looking for the fault in the fixture when it is in the fifty metres of cable upstream.
5. Mains or generator
Mains is free, silent and already there. A generator costs money, makes noise, needs fuel and somebody to watch it. You only take one when there is no alternative, but you take it properly when there is not.
The situations that force it are well known: the location cannot supply enough, the installation is too old to connect to with any confidence, you are on an exterior with no supply, or production will not accept any risk of a cut on an expensive setup.
The point people overlook is noise. A generator has to be moved away, and moving it away means cable length, which means cross section, which means budget and rigging time. That distance gets decided on the recce, not on the morning of the shoot.
A generator is sized like a supply: peak load plus margin, phases balanced, and headroom for discharge sources striking.
6. What to bring to check
A multimeter lets you check the voltage at the supply and at the far end of the run, which is the single most useful measurement of the day. A socket tester confirms polarity and the presence of an earth in two seconds. A clamp meter shows what is actually flowing, and often surprises you.
Checking the earth is not optional: on an old installation or an improvised setup, it is the first thing to verify before plugging anything in.
This maths takes ten minutes during prep and saves the question asked out loud on set: "why did that trip?". You do it once, write it into the project, and read it back when you place the order.
In Cinecyclop, the Power tool works on the current list: it adds up what is actually in the order, location by location. The lighting prep checklist covers these points at the moment they get decided.
Frequently asked questions
How do I convert watts to amps on a shoot?
On single phase 230 V, divide the wattage by 230. A 2.5 kW head therefore draws about 11 A. In practice, divide by 200 instead: you get a slightly inflated figure that leaves you the margin you need. On three phase 400 V the formula becomes power ÷ (400 × 1.73).
How much margin should I allow on a shoot's power?
Around thirty per cent above the total of the fixtures switched on at the same time in the same location. The peak draw does not happen during the take but during the look, when everything is on at once to judge a balance.
When do I need a generator?
When the available supply does not cover the peak plus margin, when the installation is too old to connect to with confidence, on exteriors with no supply, or when production will not accept any risk of a cut. Noise means it has to sit further away, which turns into cable length you need to plan from the recce.
Why will an LED not start at the end of a long run?
Usually voltage drop. Over a long distance with an undersized cable, the voltage at the far end falls below the supply's starting threshold. The fixture works perfectly well elsewhere. The habit is to go up one cross section beyond thirty to forty metres.