Electrical System Voltage Overview

Home Inspectors in the State of Washington are required to follow the Standards of Practice as described in the applicable Washington Administrative Code – Chapter 308-408C.  Electrical system inspection is described in section 110… but it poses a conundrum.  Inspectors are required to describe the system voltage, yet we are not required to insert any tool, probe or testing device into the main or sub-panels… but the only way I can think of to determine the system voltage is by inserting probes into the panel!

Why would that be necessary?  After all, everyone knows that Puget Sound Energy delivers 120/240 volts to houses, right?  (If you’re old enough, you’ll remember the standard use to be 110/220, but that’s for a different post.)  Well… no.  The vast majority of homes do have three wires coming in to the home and 120/240 volts available in the panel – 120 volts between one of the hot legs and the neutral, or 240 volts between two hot legs.  (For the purpose of this post, we are ignoring the ground wire.)  This diagram represents this common single phase three wire system:







But power is sometimes delivered differently.  Commercial buildings, but also many condominiums and town-homes, have four wires at the entrance with 120/208 volts available in the panel.  Why?  With commercial buildings, it is economical to bring one four wire service into the building and then break it into three distinct 120 volt sections.  With condos and town-homes, it is again an economical way to bring power into a group of living units and break that service into discreet 120 volt sections.  This diagram represents this three phase four wire Wye system:







One housing unit – or commercial unit – will get 120 volts using phase A, another will get 120 volts using phase B, etc.  Combining power from two of those legs generates not 240 volts, but 208 volts.  Why?  It gets complicated and basically has to do with the phasing differences between two and three hot wire systems … between the two hot wires in the three wire system the voltage is multiplied by 2 and with the three hot wires in a four wire Wye system the voltage is multiplied by 1.73  (the square root of 3).  Like I said it gets complicated and understanding the why is less important than understanding why you should care.

And why should you care?  Any condominium or town home with 120/208 volts can and will run any standard electrical equipment, so the issue is relatively transparent to most people.  But those items that require two hot legs delivering either 208 volts or 240 volts will take longer to do their work when provided with 208 volts.  Electric clothes dryers will take longer to dry clothes.  Electric water heaters will take longer to bring water to the temperature set point.  Electric furnaces will take longer to get a space up to temperature.  I was recently explaining this to a client at a condo inspection, and the tenant, who was there listening, said “Well that explains that.  When I moved in my dryer was taking longer to dry clothes than it had in my last place, and I didn’t know why.”

So if you see us or other inspectors going above and beyond by sticking probes into electrical service equipment, you’ll now understand why!

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