How to tell single from 3 phase

How do youtell if a building has 3 phase electric service. Do you see anything wrong with this.


The four-conductor service-drop is the indicator. 3-phase.

I thought so. Do you think the mast head is turn to far up? Looks like water will get inside. Also these wires are right upagainst the building, it is a block wall.

Personally, I don’t like the orientation of the “goose-neck,” but I’d be reluctant to say it is not within “compliance.” However, one of the conductors looks to be entering on a downhill slope, which would be improper.

As for the conductors. The one’s that are touching the block are part of the service drop - i.e., property of the utility company - and were likely attached by employees of the utility company. Generally speaking, utility companies “set their own rules” with regards to their equipment/systems.

If there is no visible damage, I would leave it alone. This is just my opinion. Your mileage may vary :wink:

Thanks Jeff that is the way I was leaning. I appreciate your time.

Look for water damage at the panel and along the conduit, I can see where water would go back down that (1) line and into the mast head. The orientation of that mast head is wrong as well.

How about the messenger attachment,as people are always pointing out lack of the ceramic insulator.

It definitely looks like one of the conductors slopes downward, and* at least* 2 of them were swung above the insulator connection (why?!?!?!). I’d recommend calling the power company to re-connect them properly, and of course note any signs of water entry into the system and/or corrosion.

Pictures can always be misleading. But…from this picture, I would definitely have an issue with the service weatherhead. For issues like this, it is always better to err on the side of safety.

You will need to look at the transformer connection to determine whether it is single phase or three phase. You cannot base a determination on the number of conductors because the utility company may not have used the steel cable as the system’s neutral. You also need to look at the transformer to determine whether it is a Delta or a Wye connection.

There are four conductors entering the mast-head. . .

The number or size of conductors entering a building does not provide enough information to determine the Voltage, number of phases, or the capacity of the electrical service. Single-phase services will usually have two to four conductors. Two-phase services will usually have three to five conductors, and three phase services will usually have three to five conductors.

There are at least five distinct arrangements possible with a service drop having four conductors. The service can be Single-phase, Two-phase, or Three-phase. The five possible arrangements of a four wire service entrance are Single-phase, Two-phase, Three-phase center-tapped Delta, Three phase corner-grounded Delta, and Three-phase Wye.

For anyone who is interested, I have posted an excerpt from one of my electrical courses, which goes into much greater detail, on my web site at:

Two phase does not exist


A standard residential, single-phase, 240 volt drop, is three conductors. A two conductor system would imply a 120 volt, single phase system. I can usually confirm this when I look into the service panel.

If I see four conductors, I’m thinking “three-phase.” I can usually confirm this when I look into the service panel.

I have no idea what a “Delta” is, but apparently I don’t really need to know.

George, you are spot on. But most home inspectors will never see some of the more interesting power configurations that are found in commercial and industrial installations around the country. The missing information in the original post is what kind of building this is. If this is an appartment building or condos (most of us are probably assuming so), than we are almost certainly looking at 3 phase. If this is a manufacturing facility, well then who the heck knows.

Richard, 2 phase certainly does exist but most people will never see it. It is 2 phases, 90 degrees out with 4 poles.

Jeff Wye is 3 phases with all the transformers having the secondaries tied to ground on one end, delta has them conected in a ring (actually a triangle) with the ground either at the center tap of one of the secondaries or on a “corner”. Delta is generally 240v between phases, wye is 208.
In a center tapped delta you have 120v to ground on 2 ungrounded coinductors and 208 on the “wild/orange/red” leg. This is very common in light industrial where most loads are 120/240 and they have a small need for 3p.
This usually only uses 2 transformers.
The center tapped transformer is a big one with the wild leg using a smaller one. I have seen 3p, center tapped delta with 3 transformers but it is pretty rare around here. If you are ever at “Hogs Breath” in Key West look at the transformers feeding that building (and several others) from the raw bar. You will see a 3p delta with 3 transformers right over the bandstand.

Thanks Greg more on two phase … Cookie
There was even some experiments on five phase.


Thanks. Kenneth’s original question was “How do you tell if a building has 3 phase electric service?” There simply isn’t enough information provided by the picture to make the determination. If I were a gambler, I’d put my money on it being thee-phase. The service in the photo is more likely to be three-phase four-wire service than either a single-phase four-wire or a two-phase four wire. If the building is small commercial or multifamily, the odds are also in favor of it being Wye, not Delta. Even though the odds are in favor of the service being three-phase, the odds are not good enough that I’d want to put much money on the bet.

It is true that most home inspectors will never see the different installations that exist. There are many inspectors, however, who are interested in doing commercial inspections. I don’t know whether Kenneth merely had a casual curiosity or this is a building he was actually inspecting. Either way, I was only trying to give him a reasonable and accurate answer to his question.

I’ve had customers buy buildings only to get into the building and incur big unforeseen expenses because they did not have a qualified inspector look at the electrical system.

Six-phase is also popular with utility companies.