# Single-phase vs three-phase generator

I am in India. We have 3 phase power coming in from the grid. They don't do single phase for anything over 3 kVA. 4 wires from the pole to a fuse box. 415V, 3 wires each going to a separate fuse and neutral. All appliances are single phase. They are distributed across all 3 phases. All sockets have a ground, neutral and live. Is this a 3 phase wye? That would mean 240V at each socket which sounds correct.

Now if you were to get a generator would you do a single phase or 3 phase generator? 3 phase is slightly cheaper. How would the single phase generator be connected in this scenario?

• In what scenario do you refer to? Jul 29, 2020 at 12:39
• If I got a 3phase generator I'd have to balance appliances across the 3 phases. How would single phase generator output be wired to supply to the 3 phases as set up? Would that be preferable?
– VM7
Jul 29, 2020 at 12:43
• @VM7 If you connect a 1 phase generator to 3 phase network none of the three phase loads would function. Jul 29, 2020 at 18:46

How would the single phase generator be connected in this scenario?

If you have a few critical loads that you want to power with a generator during an outage, a single-phase generator would be ok if those loads are all on one phase or if they can conveniently re-connected to one phase.

However you connect the generator, you must have either contactors or a switch that can safely transfer the loads to the generator and assure that the generator does not connect to the incoming feeder. If you want to connect more than one phase to a single-phase generator, you need two or three transfer switches. If you get a three-phase generator, you need a three-phase transfer switch.

If you have a generator that is too small to power all of the loads that can potentially be connected, you must either provide a means to prevent connecting them or a means for disconnecting the generator if the connected loads are too high. Fuses will do that, but you need to be careful to coordinate the fuse characteristics with the generator capability. In addition, blowing fuses will be a nuisance and possibly result in problems due to shutting off loads unexpectedly.

If you are going to size the generator just to supply critical loads, it might be convenient to connect those loads to one phase and size the generator to power everything on that phase.

Depending on your exact distribution configuration and intentions, there may be other details to consider.

• What's the difference between a transfer switch and a changeover switch?
– VM7
Jul 30, 2020 at 19:04
• @VM7 Same thing, different names. Jul 30, 2020 at 21:00
• @Simon B, thank you.
– user80875
Jul 30, 2020 at 21:02

If you have a 3-phase generator, you need a 4-pole changeover switch to switch between the grid and the generator, switching all 3 phases and neutral. Never allow the generator and grid to be connected at the same time.

If you have a single phase generator that is large enough to drive all the loads, then set up a 4-pole changeover switch to change between the grid supply and the generator. Again, this should switch all the phases and the neutral.

Then connect the single phase of the generator to all three phases on the generator inputs of the changeover switch. So long as you only have single phase loads, the loads won't notice the difference.

Note: 3 phase systems often use an under-sized neutral, on the basis that the maximum current in the neutral can never be more than that of one phase. This won't apply if you're using a single phase generator. Make sure your neutral wiring can carry the full current from the generator.

• For 25 kVA coming in from the mains how would you size the singe phase generator? Assuming all appliances are single phase which one would be preferred?
– VM7
Jul 30, 2020 at 19:00
• @VM7 Add up all the loads you need during the power cut. 25kVA is 100A if it is single phase. That's a big single phase generator. If you need that much power, a 3 phase one may be more practical. The total kVA works out the same if it's single phase or 3 phase. Jul 30, 2020 at 21:04

I would say, 'Go for the 3 phase generator' for the following reasons:

1. Absolutely no changes would be required to be made in the existing wiring.
2. Back-up power would also be ensured for 3 phase loads which may be installed in the future.
• You'd have to balance loads there, You can't just run 1 AC, you need to run 3 and make sure each is on a different phase.
– VM7
Jul 29, 2020 at 21:12
• Hi VM7, It was presumed that a balanced load is already being presented to the grid. If it is not, then it would have to be done now and that would call for some rewiring. Aug 1, 2020 at 18:15