240V or 208V: which service is this?
A condo or a light-commercial unit fed from a shared three-phase service is 120/208, nothing about the outlets looks different, and a 208V job drawn at 240 sizes the strings and the inverter output wrong.
Answers the Service voltage question during setup.
Everything in this article can be read with the enclosure door open and every metal cover left in place. Do not remove a cover, and do not open a meter enclosure. If a number is not visible that way, it is a question for a licensed electrician, not something to go looking for.
“Residential is always 120/240.”
The common misreading. Here is what is actually true.
Detached single-family houses in North America are almost always 120/240 split-phase, so the assumption is right most of the time. It stops being right the moment the building shares a service: condos, townhouse stacks, mixed-use ground floors, and most light commercial run on a three-phase service, and a single unit tapped off it sees 120/208.
Nothing in the unit looks different. The outlets are the same, the lights are the same, the panel looks the same. The panel label does not settle it either: a panel marked 120/240V is listed for use on a 208Y/120 supply, so the marking says what the panel is RATED for, not what is connected to it. That is exactly the case this article exists to catch.
Why it changes the design
A 208V job needs 208V-rated equipment. Inverters and microinverters are sold in separate 208V and 240V versions of the same model, and the design check flags a 240V unit on a 208V service. The AC output current for the same kilowatts changes with the voltage, and the breaker, disconnect and conductor sizing follow from that current. DC string sizing is unaffected, because that comes from the inverter MPPT window rather than the service.
This is not a rounding error. It is the kind of mistake that produces a plan set which is internally consistent and describes a system that will not behave the way the drawing says.
How to get the answer
Order matters here, because the panel label can only prove one of the two answers.
- 1A label reading "120/208V", or any "3PH 4W" marking, is conclusive: the service is three-phase derived. Nothing left to check.
- 2A label reading only "120/240V 1PH 3W" is NOT conclusive. 1PH 3W means one phase on three wires, two hots and a neutral, and that panel is also listed for use on 208. It tells you what the panel can take, not what is connected.
- 3Where the label is not conclusive, a line-to-line measurement across the two hot legs settles it: 240 volts is split-phase, 208 is three-phase derived. That is a meter reading, so it is the electrician on site who takes it.
- 4Where nobody can measure, the utility bill rate schedule and the service configuration answer it, and a bank of meters on one wall is a strong hint the building is fed three-phase.
- 5Detached single-family home with its own meter: expect 240. Condo, apartment, townhouse, or any commercial space: suspect 208 and confirm.
What to photograph
- The panel door sticker showing the voltage marking.
- The meter bank, if the building has one.
This is guidance for reading equipment that is already visible and for describing it on a drawing. It is not instruction for electrical work. Any work on service equipment, panels, or conductors is for a licensed electrician, and local code and utility rules govern.
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