Determining Voltage as well as Amperage of a Service

Hi all,

I did a search for this but did not find anything.

I was told that I need to record the amperage and voltage ratings of services I inspect. I’m clear on amperage but not on reporting on voltage.

Thank you in advance for any help.


Where did you get this information I would love to read a copy … Thanks … Roy

For the US in most dwellings it will be single phase, 120/240 volts. In a dwelling unit in a large building you might see single phase 120/208 volts. Some old houses around here still have single phase 120 volts services. There are other systems. Easiest way to tell is with a volt meter.

Most SOP’s do require an inspector to report on service amps, as well as voltage (NACHI’s SOP also indirectly requires reporting voltage), if it can be determined by visual means. Measuring current or voltage is NOT required by the standards.

Look at the number of wires on the service drop, or the number coming into the main service disconnect. Two (2) wires is an older 120V single phase service, and three (3) wires is a 120V/240V or 120V/208V single phase service. Commercial services may have more wires and/or higher voltages.

Most residential services are 3-wire 120V/240V, but in some areas (e.g around NYC) residences may have 120V/208V services. Check the panel label to help figure that out.

P.S. On the utility side there is no “ground” wire, and the “neutral” is typically the bare wire.

Some nice graphics there. I looked at a few of them and the fourth graphic (using the NEXT button) is incorrect. The CB in the center is a 2 pole not a single pole breaker. I would have added directly to the post but I do not have permission.

Nominal Voltages in the United States for most residences are 120V and 240V, They are 60 Hz, single-split-phase. They are usually derived from a single-phase transformer with a 7,200V primary.

Other secondary Voltages that you may encounter are 208Y/120 and 480Y/277, three-phase, with 12,470V and 13,200V being the most common primary Voltages. Most light to moderately sized commercial systems are three-phase Wye. Three-phase Delta is more common in industrial facilities. Most are solidly grounded.

If you are in the Eastern US, you may also see some two-phase systems. Two-phase systems are rare West of the Mississippi and almost completely non-existent West of the Rocky Mountains. Two-phase systems are commonly mistaken for three-phase.

Here’s the graphic


Yes that was the one.

I don’t know all that much about how the message board works. You might want to ask Chris about being able to post pictures and graphics. The first time I tried to post the breaker graphic, a message popped up saying that I wasn’t authorized to post a picture. It worked a few minutes a few minutes later though.

I was able to copy the url but got a warning about using those graphics without permission. Seems to be pretty standard message board stuff. Even Mike Holt allows you to use his stuff if you get permission first.

I’m a little unclear as to the best way to determine amperage myself. I was taught to use the lowest capacity device to estimate amperage. For instance, if the panel is rated for 100amps and the meter is rated for 200 amps, then the amperage is 100. Of course, the SE cables have to be considered too. My question is how to determine the amperage of the SE cables and the main disconnect. Typically the main will be labeled but what happens when the main disconnect is not labeled? How do we estimate that?