If I have three single phase connections with each having its own single phase energy meters And I connect 1kw load on each of these phase continuosly 24/7 for a month.


Now next month instead of three single phase connections I opt for a single three phase connection with a 3 phase energy meter and I connect the same single phase loads (1kw) on each of the 3 phases 24/7 for a month.

At the end of the month will by electricity bill in Case 2 be lower than that of Case 1 and why?

P.S. This is not a homework question, I'm trying to understand 3 phase power vs 1 phase power.


So single phase loads don't seem to have an advantage in a 3 phase power supply system.

Now how about a single phase 2kW motor running 24/7 on a single phase supply for a month and then the next month i use a 2kw 3phase motor and run it 24/7 on a three phase supply. In this case surely the 3phase system will have an advantage of lower consumption (lower electricity bill). Isn't it?

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    \$\begingroup\$ That will depend on the rates charged for the electricity : companies decide the rates based on their customer base and many other factors... \$\endgroup\$ – Solar Mike May 5 '18 at 10:06

The bills will normally be the same - unless the 3 phase connection is charged on an industrial tariff while the single phase is a domestic tariff. That's a matter for negotiation with the supplier.

But consider the cost of installing the supplies.

Each single phase supply requires two wires from the substation to your house. That's potentially a total of 6 wires, whereas the 3 phase supply only requires 3 (in Delta).

So in practice those three single phase supplies will be the three phases of a 3-phase supply to save copper. (Normally, with a fourth Neutral to handle any imbalance between them)

  • \$\begingroup\$ Thanks for the answer ,please see my edited question. \$\endgroup\$ – Miguel Sanchez May 5 '18 at 14:53
  • \$\begingroup\$ What makes you think the edit changes anything? \$\endgroup\$ – Brian Drummond May 5 '18 at 14:55
  • \$\begingroup\$ I thought a 3 phase load draws less current from each wire (phase) than a single phase load would. \$\endgroup\$ – Miguel Sanchez May 5 '18 at 15:07
  • \$\begingroup\$ But it draws current from more wires, so ...? \$\endgroup\$ – Brian Drummond May 5 '18 at 15:10

The daily energy usage measured would be the same

But, most consecutive months are not the same length :) and suppliers may charge different tarriffs etc.


Your electricity bill will be the same. You still need the same amount of power from your provider. Altough if you connect 1 kW to 1 phase it could be that a fuse goes off or you get a unbalanced phase if you have other power-eating devices on that phase (like dish washer, electric cooker). I would take case2. Here is a good explaination to this topic

  • \$\begingroup\$ So when does a 3 phase system really perform better vs a single phase system? \$\endgroup\$ – Miguel Sanchez May 5 '18 at 14:54
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    \$\begingroup\$ If you need high power you should take the 3 phase system, like you said 2kW Motor, for low power devices you should take the single phase because you have lower costs for converting it into dc for example. Your power supplier looks on all 3 phases and the current (Power) you need on them. So you will have three times more current on one phase than on a 3 phase system on one phase, but you still have the same amount of power drawn \$\endgroup\$ – Ribisl May 5 '18 at 15:33

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