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We're building a new house and I'm not confident with the skills of the electrician who installed the receptacles. In our country, residential structures are supplied with 240 V, 60 Hz (single phase).

Here's what I currently know: 1. Hot and neutral conductors may have been switched (he used the same color of wires for both so there's really no way of knowing which is which when installing a receptacle) 2. Grounding conductor is connected to a ground rod. 2. Grounding conductor is not yet bonded with the neutral conductor 3. At the receptacle, voltage across the hot and neutral conductors are 240V, as expected. 4. However, voltage across either hot and neutral and ground hovers around half of the line-to-neutral voltage, at about 120V.

What could cause this? It's my understanding that since the neutral conductor is already grounded at the transformer on the utility pole, even if the neutral conductor at the load side is not yet bonded with a grounding conductor, measuring the voltage across neutral and a grounding conductor should read a very small voltage (e.g. < 3V).

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    \$\begingroup\$ What country do you live in? This matters a lot for determining building codes such as the correct coloring for your wiring, and for determining whether your power supply may be split phase or single phase as you state. \$\endgroup\$
    – marcelm
    Feb 26, 2017 at 19:17
  • \$\begingroup\$ Hi, I live in the Philippines, and our power supply is single phase as far as I know. \$\endgroup\$
    – Darwin Bautista
    Feb 27, 2017 at 3:25

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One possibility is that your power supply is not single phase as you state, but rather split-phase.

If that is the case, you don't have a hot and a neutral, but two hot wires. The two hot wires are 120V to ground each, and 180° out of phase, resulting in 240V across them.

This would explain your 240V line voltage, your 120V hot-earth and "neutral"-earth measurements, and the identical wire colors for your hot and "neutral" wires.

To confirm or deny this, you could (carefully!) measure the voltage between your hot or "neutral" and a known ground reference such as a stake in moist soil, or a tap water pipe - assuming you have metal piping.

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  • \$\begingroup\$ Ok that was weird. All along I thought that our supply was single phase. But I measured both lines against ground (one probe of multimeter connected to a metal pipe driven through ground) and I'm still getting the same reading! That is, 120V either line to ground. With these measurements, should I conclude that this is really a split-phase supply? \$\endgroup\$
    – Darwin Bautista
    Feb 27, 2017 at 3:43
  • \$\begingroup\$ @DarwinBautista It certainly looks like it, yes. I searched a little for split phase in the Philippenes, but came up inconclusive. I'd say: call the electricity company if you want to be sure :) \$\endgroup\$
    – marcelm
    Feb 27, 2017 at 13:00
  • \$\begingroup\$ Knowing that our electrical system is indeed split-phase, I tested all other receptacles to check if all are properly grounded. There were two receptacles which behaved differently: line-to-line voltage: 240 V (as expected) line-to-ground (both phases): 60 V What could be happening here? I'm sure that the ground prong is not disconnected (because I'm getting a stable reading). \$\endgroup\$
    – Darwin Bautista
    Mar 1, 2017 at 13:14

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