ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades History of Mains Voltages

I've been curious how the various mains voltages used around the world came into existence for a long time but haven't been able to find any information except "they were arbitrary choices". However, I suddenly realised that all mains voltages that I know of have something in common - they can be divided by 50, 55 or 60 V, which happens to be the standard voltage range of arc lamps, the first common application of electricity. It seems fairly logical to assume that standardised mains voltages such as 100, 110, 120, 150, 200, 220, 230, 240 and 250 V were chosen for connecting multiple arc lamps in series in order to save on copper. The only outlier, 127 V, is derived from a 220 V 4-wire 3-phase network. What do you think about that theory?

I don't know about the rest of the world but line voltage in the US was to support the Edison incandescent light bulb and various motor designs. It seemed to start out at 110v AC at 60 hz and now has climbed to the standard 120v 60 hz (line to neutral) with a combined line to line of 240.
You will also see 115v and 117v referenced.
We use an arrangement with a center tapped 240v transformer, grounding the center tap. (120/240)
Once you get to 3 phase things are different with a center tapped wye (208/120v or 480/277) being the most common. There are also three different delta arrangements. It can be centertapped in one transformer (240/120/208 red leg), corner grounded (240/240/0) or ungrounded/impedance grounded but that is in special situations requiring more monitoring.
The corner grounded 3 phase service can be the most confusing since the equipment looks like single phase. I have only seen it in sewer lift pumps but there may be other applications out there.

My theory is that these voltages pre-date Edison and were chosen because of arc lamps, which were invented and marketed earlier than incandescent lamps. Three-phase and split-phase systems are usually based on one of these voltages, like the 127/220 V I mentioned. 277/480 V in the US also fits in (480/8=60) but might be a more recent invention, I don't know about that.

Split-phase is the term most commonly used in Europe for single-phase three-wire systems as used in the US. They've never been particularly common but some areas of Germany once had 110/220 V split-phase and the British Isles have 230/460 V supplies for remote rural locations like larger farms. Some central European railways also use split-phase for switch point heaters, at the extremely odd nominal voltage of 231/462 V at 16.7 Hz, derived from the 15 kV overhead wire. I suspect these systems might have been created in order to convert Edison DC supplies to AC without running additional cables. In the UK they were a cost-saving method because these remote places then only required single-phase medium voltage but that might not have been the original reason behind the system as such.

Single phase medium voltage is very common here, for the same reason. They can only run 2 conductors for distribution. The problem is voltage drop on the neutral that gets imposed on the earth. I have the best ground electrode on my street and I see up to 3a of current coming in my neutral conductor from the PoCo with the main breaker off. There are similar currents seen on the pole grounding wires.

Is that single wire earth return? I only see one conductor up there and the other connection of the primary seems to go down the pole.

Voltage drop on the neutral is a common issue in rural areas, which is one reason why many countries historically favoured TT supplies in those areas and sometimes required double-pole fusing because the neutral could assume dangerous potentials.

There is also a neutral that connects all the way back to the 3 phase distribution but that neutral is over a mile long on that spur with 21 transformers hanging on it so there will be voltage drop that shows up in the dirt from the pole grounding conductor and the customers GES. I have 3 amps on my neutral with the main breaker off and the pole grounds very from about 0.8a and almost 3 depending on which one you look at.

Ah, I see! In Europe medium voltage is pretty much exclusively ungrounded delta, so no neutral anywhere in sight, "single phase" is commonly used to refer to two out of three phases.