Hi,
I have a 3 phase supply for a greenhouse and currently have 120 lamps of 400 watts each with a 50 ampere supply (of that perhaps 10 amperes are needed for ancillary equipment). If I increase the number of lamps to 300 how many more amperes do I need?

We need to know the voltage of the system and light fixtures to do any calculations.

Hi, so far as I know its 220V and 50Hz. At the moment I have 40 lamps on each phase.
As well as for a total of 300 lamps I would also like to know how many extra amperes I would need if I increase to 600 lamps. And if any of you out there are experts on greenhouses - as a general rule for growing herbs, how many lamps should I have per m2 (or per 10ft2)?

So, I understand your numbers correctly, 40 lamps at 220 volts each phase, that is 42 amps per phase, and you have a 50 amp supply, that supply is at capacity.

If each fixture draws 400 watts at 220 volts, each fixture draws 1.82 amps.

Is that enough to answer your question?

Regarding lighting a greenhouse, you should post a question perhaps at the Gardening and Plants forum, however, before anyone could answer, they will need to know more about the lamp other than the watts.

Lumen output, or in metric, Lux output, and color of light output in Kelvin.

Hi tkrussell. Thanks for your posts. What I don't understand is that if each lamp draws 1.82 amps then for 120 I would need 218.4 amps. Even if you divide it in 3 (for each phase) it would still need 72.8 amps. Isn't it something to do with the square root of 3 (=1.73)?
I'll take your advice re the Gardening & Plants forum.

I would like to know if your 220 volt circuits use a single pole or a 2 pole breaker.

Single would mean you have one leg of 220 volts grounded, 2 pole would be two live legs.

To arrive at the load in amps per phase, total the single phase load:

1.82 amps x 120 units = 218.4, then convert to 3 Phase KVA

218.4 amps x 220 volts x 1.73 = 83,123 VA divide by 3 = 27,707 VA per phase divide by 220 volts = 126 amps per phase.

Clear as mud?

Hello and thanks again. As you can tell I don't know much about electric so I'm not sure about the single or 2 pole breaker. What I can tell is that the 5 strand wire was split up into 3 lines with one phase on each line and also earth and neutral to each line. Therefore each lamp has 3 wires, 1 live, 1 earth and 1 neutral. It's actually split into a total of 4 groups of 3 lines at the junction box but that I have no idea about - there's a 25A breaker on each line (10 lamps) so far as I know.
If I understand your answer correctly, if I had 120 lamps on 1 phase I'd need 126 amps. Therefore does it follow that if I had 200 lamps on each phase I'd need to have 210amps /phase or a total of 630amps?