Agree with everything above, except for the incorrect assumption that the two legs of split-phase are out of phase with each other. (180° is a common misunderstanding of split-phase)
The batteries in series analogy is correct, and shows that they can't be out of phase.
Sorry for the following rant, I won't budge on this and I won't try to argue further. For me this is like trying to convince you that when It's daytime here, it is night time on the other side of the earth. I know it is true and if you don't get that, I am not going to bother arguing.
I am not going to continue an argument past this one post. Believe what you want, but I will never change my mind. This is important for safety and proper understanding of split phase power. I work with trained licensed electricians and electrical engineers all the time, and they all agree that split phase power L1 and L2 are 180 degrees out of phase and that is never in question. We ALWAYS measure with reference from the ground bonded neutral.
As for me personally, I am an audio engineer first. My work with power follows from there. Are you familiar with balanced line connections and bridged amplifiers? No one ever questions that those paired signals are 180 degrees out of phase. When talking about audio, a bridged amplifier as well as a balanced audio interconnects both look just like split phase power. And we ALWAYS say the two signals are 180 degrees out of phase from each other. If you connect a dual trace oscilloscope with it's common lead to the audio ground and then trace A to the + signal and trace B to the - signal, they will appear on the screen 180 degrees out of phase. They are constantly moving in the opposite direction. When one lead goes to a positive voltage, the other lead is going to a negative voltage. That IS 180 degrees out of phase, and it is exactly what you see if you measured split phase power with the scope common terminal at the neutral ground bonded point. So YES, the two hot legs of 120/240 volt power ARE 180 degrees out of phase. Using a scope that can measure line voltages, you will find exactly the same thing. The traces for L1 and L2 will be 180 degrees out oif phase when referenced from the ground bonded neutral point. And the A, B, and C phases of 3 phase power are all 120 degrees out of phase from each other.
The battery voltage adding example is different. If they are said to be "In Phase" or of the same polarity, that would only be true if you measure each battery with the common terminal always going to the most negative post of the battery system. In my example above, I specifically stated "GROUND THE CONNECTION BETWEEN THE 2 BATTERIES". This is placing the two batteries (Out of Phase) because one battery has it's positive post grounded while the other battery has it's negative post grounded. One is connected backwards when compared to the other. If you connect your digital DC volt meter black lead to this middle ground connection, measuring the voltage of each battery with the meter's red lead, one of them will measure +12 volts, and the other will measure -12 volts. They are going in opposite directions in reference to ground.
When two 12 volt batteries are connected in series to make a normal 24 volt system, then they would be "in phase" with then negative of battery A tied to the bonded ground connection, while the battery B negative terminal is connected to +12 volts of battery A. With your meter black lead at the grounded potential, battery A will measure +12 volts, and battery B will measure +24 volts. So yes, this system adds with the batteries "in phase" because we are looking at it from a different reference point. But this is ABSOLUTELY NOT how USA 120/240 volt works. This is more like measuring the 230 volt output of a single phase European system. Here we don't ground bond the L1 phase. We ground bond the neutral mid point. And this is also why people have had issues trying to use a European spec single phase 230 volt inverter in the USA. Some of them can have the ground bond lifted, and then use an auto transformer to create a "neutral" and ground bonding this new floating neutral. The big problem here is the PC board in the inverter was laid out expecting the real neutral to be at ground potential like the chassis of the inverter. This "conversion" forces what should be neutral, zero volts from ground to now be swinging at 115 volts from ground. This is a bad idea. Split phase is 2 outputs that ARE 180 degrees out of phase in reference to a grounded neutral.
End of rant.
Copied directly word for word from Google AI search.
Yes, split-phase power, also known as dual phase or two-phase power, is 180 degrees out of phase. Split-phase power is a type of single-phase electric power distribution that uses two 120 V AC lines that are 180 degrees out of phase with each other, along with a common neutral. This creates a voltage difference between the two hot wires, which typically each carry 120 volts, while the voltage between the two hot wires is 240 volts.
