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We have a 480V 3-phase connection for a borehole pump, and we need to get a temporary 240V connection for lighting, no more than a few hundred watts.

We are in South Africa which operates at 240V, and when measuring each phase on the 3-phase line we get 480V RMS.
To get 240V, we measured one of the phases and neutral, which gives exactly 240V RMS as we want, however this goes against my research online that it should be 277V RMS.

Is it safe to use this? Is this normal, or are the phases at a different angle perhaps? All three phases give 240V in reference to neutral.

The three phase line has the following colours, the same as the UK:
3-phase colours

Edit: To clarify, I'm 100% certain that any connection between the three phases is always 480V and at least one connection between a phase and neutral is 240V, I'll do some more testing with the other phases to see if they also give 240V when measured against neutral.

Edit 2: The supply turned out to be a High Leg Delta type, as mentioned in the answer, and I am getting 240V successfully, working great.

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  • \$\begingroup\$ U.K. 230 volts is really 240 but pretends to be 230. Europe is 220 and pretends to be 230, thus we are harmonised. Mains voltages usually have quite wide tolerances so 480 is probably just a generous 400v 3 phase supply. Bet it drops when you turn on the pump. Single phase supplies in the U.K. are from a phase to ground. That's probably what you're looking for \$\endgroup\$ – Will Jan 3 '15 at 20:28
  • \$\begingroup\$ So how it ended ? Hope you are safe there after all connections done ! \$\endgroup\$ – x4mer Jan 9 '15 at 18:05
  • \$\begingroup\$ @x4mer Thanks, it's working perfectly, we have light :D Decided to test a 2kW load - no harm trying, and it worked fine, so it shouldn't be a problem with the <500W of lighting. \$\endgroup\$ – baharini Jan 10 '15 at 19:54
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You might be having a High Leg Delta type of supply like on the picture High Leg Delta 480V

Which might explain why you see 240V between the neutral and phases. Please recheck voltage again to make sure that this is the case.

In other case you should have either 277V line voltage or 415V phase voltage

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  • \$\begingroup\$ Thanks! I will recheck the voltages tomorrow. What I am 100% certain of is that any connection between phases is 480V and I can get a 240V connection by connecting neutral to at least one of the phases, I'll double check what the situation is when tried with the rest in case I made a mistake there. Either way, since I do get 240V, is it safe to use? \$\endgroup\$ – baharini Jan 3 '15 at 20:04
  • \$\begingroup\$ To be 100% sure, you should check the source power capacity so that you don't overload it. However if you do have this type of connection I don't think few hundred watts on 240V legs will make a significant issue. Actually this type of connection is intended to provide lightning power - exactly what you are looking for. \$\endgroup\$ – x4mer Jan 3 '15 at 20:16
  • \$\begingroup\$ The power supply is a transformer for driving two very large water pumps and my source is a smaller pump's power box (with the trip switches, timer etc.) located about 100m away but it uses the same transformer. Since the transformer is so large it's interesting that it may well be a High Leg Delta power supply, since for a dedicated application like large induction motors it would seem unnecessary to provide power at a lower voltage too. \$\endgroup\$ – baharini Jan 3 '15 at 20:29
  • \$\begingroup\$ @baharini If you have a High Leg Delta supply then P-N will be 240V on two phases and lower (about 208V) for the third phase to neutral. It seems unlikely, but you may have a split-phase system. See en.wikipedia.org/wiki/Split-phase_electric_power \$\endgroup\$ – Russell McMahon Jan 4 '15 at 10:08
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    \$\begingroup\$ Thanks, yes it turns out it was a High Leg Delta supply. It's working at 240v fine, no problems. \$\endgroup\$ – baharini Jan 10 '15 at 12:00
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South African mains is 230 V phase to neutral, and 400 V phase to phase.

You're using the right formula, sqrt(3), but the wrong input figures.

If you have 480 V phase to phase then you have a different, higher voltage supply, and you can't get 230 V without a transformer.

An auto-transformer for a few hundred watts wouldn't be very big or expensive. Make sure it is rated for the full voltage, or more, you can't use a transformer above its rated V.

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  • \$\begingroup\$ Thanks! The transformer which is used to step down the grid power to three-phase is designed for some very large pumps and I'm using a smaller secondary pump's power box but it still gets its power from the main transformer, it's likely the pumps require the higher voltage. What I am 100% certain of is that any connection between phases is 480V and I can get a 240V connection by connecting neutral to at least one of the phases, I'll double check what the situation is when tried with the rest in case I made a mistake there. Either way, since I do get 240V, is it safe to use? \$\endgroup\$ – baharini Jan 3 '15 at 20:09
  • \$\begingroup\$ It's safe to use phase to ground like that. Every house in the uk does. You still need to wire in a fuse box etc. if you aren't sure get an electrician \$\endgroup\$ – Will Jan 3 '15 at 20:31
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    \$\begingroup\$ @baharini Just one concern: Check that the neutral side of the 240 is close to earth, by measuring it against the real ground. It would be unsafe to use a 240 V supply if both live and neutral were 400V above real earth! That's not impossible given the slightly unknown wiring. \$\endgroup\$ – tomnexus Jan 4 '15 at 16:55

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