# In an AC supply why does two single phase 110V wires connected to hot and neutral produces 220V

I have seen people connect 110V to the hot end and 110V to the neutral end of the socket to give a 220V to appliances like refrigerators.

My understanding is that current flows, whenever there is a voltage difference between wires like a 110V hot wire and 0V neutral wire, giving 110V supply to a device.

1. But how is it possible where two 110V wires are supplying being at the same voltage able to run a device?

2. Also, how does 110V + 110V produce 220V mathematically?

• The signals are out of phase. When one is at +110V the other is at -110V, with 220V between them. Apr 4, 2021 at 19:58
• The difference between $110 \,[\text{V}]\cdot\sqrt{2}\cos(\omega t)$ and $110 \,[\text{V}]\cdot\sqrt{2}\cos(\omega t + \pi)$ (a 180 degree phase shift) is very much not zero. Apr 4, 2021 at 19:59
• If there's 110 volts on the wire, it's definitely not neutral. It's hot and cold, as they're called, with neutral being the midpoint between them. Or, less ambiguously, L1 and L2, or X and Y, as opposed to N (aka W). ...There really are a lot of different names for them, aren't there? Apr 4, 2021 at 20:09
• "220V to appliances like refrigerators" It's a big refrigerator that needs 240V. All the refrigerators I've ever been that close to are designed for 120V. In a typical North American wiring scheme, the only 240V appliances are the stove, the dryer, and the furnace (if it's electric). And while a 3-prong 240V socket looks like an oversized 120V socket, it's got ground and two hot wires in opposing phase (so, 240V between them). Apr 4, 2021 at 21:43
• Please be aware that different parts of the world have different electrical systems. Yours seem to be two phase household mains. In other parts of the world you can have three phase household mains, where your observation is no longer true. Apr 4, 2021 at 22:20 simulate this circuit – Schematic created using CircuitLab

Figure 1. On a split-phase supply L1 and L2 are 180° out of phase.

• L1 - N = 110 V.
• L2 - N = 110 V.
• L1 - L2 = 220 V.

You saw something other than what you thought you saw.

• You may want to add something that comes to my mind, reading the OP's first paragraph. There are Multi-wire Branch Circuits that feed kitchen countertops, for example, where the wiring to the outlets look like this. These are permitted under NEC, though there are special requirements at the entrance panel breakers (both L1 and L2 are required to simultaneously "break" in the event of an over-current or a dual-AFCI or GFCI event on either leg.) It might be that the OP is referring to one of these.
– jonk
Apr 4, 2021 at 22:10
• Go ahead, @jonk, if you want to edit or post a supplementary answer. We don't have them this site of the Atlantic. Apr 4, 2021 at 22:35
• I might, if and only if I see additions from the OP. Otherwise, it's mostly a waste of time, I think. The OP needs to clarify a few things based upon what's already been added here.
– jonk
Apr 5, 2021 at 1:04

Voltage is always measured relative to a reference point, generally the reference point in a mains supply system is the "neutral" wire which we tie to the general mass of the earth.

Relative to the reference point the voltage at any instant can be either positive or negative, lets think of a DC system first, imagine we have one wire at +120V and another one at -120V. Now we have 240V between the +120V wire and the -120V wire.

Ok, what about AC? on AC the voltage is continuously changing in a (hopefully) sinusoidal pattern. There are various ways to characterise the voltage of an AC system, but the most common is RMS, the RMS voltage of an AC system is the equivalent DC voltage that would deliver the same average power into a resistive heating load.

Now some might say AC voltages don't have polarity and that is true when we look at a single hot wire in isolation, but when we look at more than one such supply there is something analogous to polarity, but rather than being 1D AC voltages (in a single frequency system) are 2D, they have a magnitude and a phase angle.

Just as we have to define a zero level for measuring any voltage, for measuring single-frequency AC voltages we have to define a zero phase angle. Usually we would define one of our phase conductors as being 0°, if the other supply is equal and opposite we consider it to be 180° out of phase.

This is the situation you have with the typical split single phase supplies used in north America. The transformer secondary is configured such that the two hots are equal and opposite.

Three phase wye supplies on the on the other hand have three hot conductors that are 120 degrees out of phase. Since the phases are only partially opposed the voltage between phases is only √3 times the phase to neutral voltage rather than double it.