Site Electrics — AV & Events

Truby's "Simple" Power Explanation

How to split a 3-phase feed into single-phase circuits, and size the load on each leg so nothing trips — written for LED walls, PA and lighting rigs.

400V L‑L / 230V L‑N BS7671 colour code 80% headroom rule

The feed

A UK 3-phase supply is five conductors: three lines, a neutral, and an earth — L1 / L2 / L3 / N / E. Between any two lines you get 400V. Between any one line and neutral you get 230V — the same as a domestic socket.

Every single-phase circuit you take off a distro — every 16A outlet feeding a LED processor, an amp rack, a dimmer — is just one line + neutral + earth. Breaking out 3-phase isn't a different kind of power, it's choosing which line feeds which socket, then keeping the three totals roughly even.

L1 — Brown

Line 1

L2 — Black

Line 2

L3 — Grey

Line 3

N — Blue

Neutral

E — Green/Yellow

Earth

What a breakout looks like

Each socket is an ordinary 230V single-phase circuit — it just draws its live from a different line than its neighbour.

Do the maths

Amps a single-phase device draws

A = W ÷ (230 × PF)

PF = power factor. Use 1.0 for resistive/tungsten loads, 0.9–0.95 for LED drivers and switch-mode PSUs unless the spec sheet gives a better number. When in doubt, use 0.9 — it's the more conservative (higher-amp) assumption.

Worked example

A mid-size corporate rig — 24-panel LED wall, a stereo PA, and 12 moving heads plus a hazer — broken across a 63A 3-phase distro.

Item	Qty	W each	PF	Total A	Phase
LED wall panels	24	65	0.95	7.2	L1
LED wall processor	1	200	0.9	1.0	L1
PA amp rack (L)	1	2200	0.95	10.1	L2
PA amp rack (R)	1	2200	0.95	10.1	L2
Moving heads	12	400	0.95	22.0	L3
Hazer	1	1500	1.0	6.5	L3
Per-phase total				L1 8.2A · L2 20.2A · L3 28.5A

All three legs sit well under 63A × 80% = 50.4A, but L2 and L3 are carrying most of the show while L1 is nearly idle — worth re-spreading before you patch in, which is exactly what the calculator below does automatically.

Your kit — W & PF reference

Typical running power and power factor for gear on the truck, at 230V. Feed these straight into the calculator below — three items are engineering estimates where the maker doesn't publish a figure (marked est.); always check the rating plate on the actual unit before a show.

Item	W	PF	Notes

Figures are manufacturer running/rated power where published (datasheet, spec sheet or product page), converted to 230V where the source gave a different mains voltage. The Alpha Pack is a dimmer, not a load — its rating is what it can switch downstream, not what it draws itself.

Phase load calculator

Enter your own gear list, set the distro rating, and check every leg before you power up. Figures are held only in this browser.

Distro rating (per phase)

Add from kit

Example rig shown below, built from the kit list above — edit the rows or clear them and enter your own.

Item	Qty	W each	PF	Phase	Amps