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[Mike Sokol]

I'm sensing some confusion on the part of our international visitors when we discuss single-phase vs. 3-phase distros. I've always hated that while we call typical USA 120/240-volt power "single phase", it is in fact TWO phases of power, 180 degrees apart. That's how all the math works for current, voltage, etc... When I've discussed this conundrum with my power station buddies they laugh and tell me I'm nuts, that 120/240 volt power only needs a single phase. I think that's because when they say "phase", they're referring to a single "wire phase" on a transmission line from the substation, which we then convert into two hot wires at 180 degrees of phase using a center-tapped transformer feeding a typical house or office. Of course, industrial buildings and theaters use all three of the power line "phases" that are 120 degrees apart, and thus that's obviously 3-phase. I usually end up referring this this single-phase 120/240-volt stuff as "Split Phase" when discussing wiring with my sound teams, but is there a better way? Or should we stand up for our right to call it what it is, 2-phase, and let the electricians mumble to themselves about our sanity?

Just trying to clarify, not obfuscate...

 

[David Buckley]

I'm a big fan of the term "split phase", which is the nearest thing to a reasonable description of USA 110/220 eleccy.

Sometimes on here I get confused between a single phase distro, and a single phase distro, one having just one hot, the other having two hots.  I did see, in the UK, some years ago, a big single phase 110V distro intended to run of a genset for a visiting band, that was really was single phase with cams, just one hot.

Two phase is just wrong; there really are (or, more accurately, were) two phase systems and they were something else again.

[Geoff Doane]

... Or should we stand up for our right to call it what it is, 2-phase, and let the electricians mumble to themselves about our sanity?

Just trying to clarify, not obfuscate...

 

An electrician once pointed out to me that it's not "2 phase" unless you're just using 2 phases of a 3-phase system.

It's probably best to call it what the electrician will understand, so that we get what we need when specifying a service for our sound or lighting gig.  Maybe even giving some extra or redundant information so that nobody makes ASSumptions.

For my typical needs, I find that asking for a "120/240V, 100A, 4-wire service.  2 hots, neutral and ground", gets me what I want and need.  Or alternately, a "range receptacle, breakered at 50A*, NEMA 14-50 type" for smaller services.  Don't confuse the issue (as I've seen in some recent threads here) by saying you need "100A of power" when you're talking about the 14-50 receptacle.  You may wind up with five 5-20 receptacles, each on its own breaker.  No good at all if you have some actual 208 or 240V loads, and you lose the benefit of having everything on a known common ground (your distro).

GTD

* In Canada and maybe parts of the US too, it's important to mention the 50A breaker.  Most of these are installed in homes, for electric ranges (stoves), and in that particular case they are allowed to use 8/3 (plus ground) wire and breaker it at 40A.  But if you want to make full use of the distro, spec 50A, and they'll have to use larger wire, which even if you're not using it to full capacity, will give you a stiffer supply, which is usually a good thing.

[TJ (Tom) Cornish]

In my opinion, "two-phase" is an understandable and reasonable way to describe the load side.  How it's derived from the power company isn't particularly relevant to end users.

I seriously doubt anyone would confuse a modern usage of "two-phase" with a system that's been obsolete for 100 years.

[Jonathan Johnson]

For my typical needs, I find that asking for a "120/240V, 100A, 4-wire service.  2 hots, neutral and ground", gets me what I want and need.  Or alternately, a "range receptacle, breakered at 50A*, NEMA 14-50 type" for smaller services.  Don't confuse the issue (as I've seen in some recent threads here) by saying you need "100A of power" when you're talking about the 14-50 receptacle.  You may wind up with five 5-20 receptacles, each on its own breaker.  No good at all if you have some actual 208 or 240V loads, and you lose the benefit of having everything on a known common ground (your distro).

It's good to specify the voltage (120/240V), as the NEMA 14-50 range plug *may* be used on two phases of three-phase wye service, providing 120/208V. Most ranges in North America are rated for both configurations. If you don't specify, you could end up with 120/208V in some commercial installations.

[Jeff Bankston]

I'v always called it 2 phase. I have been made fun of. I dont care. I'm out of phase most of the time anyway. I cant hep it , I'm a drummer.

[Mike Sokol]

An electrician once pointed out to me that it's not "2 phase" unless you're just using 2 phases of a 3-phase system.

So is connecting a 50-amp "stove plug" to two legs of 3-phase, with the resultant 120/208-volt output, "Two Phase", while hooking it into two legs of 120/240-volt center-tapped transformer "Single Phase"? Seems like splitting hairs.

 Since I studied Electrical Engineering before I had any "Electrician" training, I tend to think of this as a different in phase shift (120 vs. 180 degrees) rather than how it's derived from the power company.

And yes, I know historically about 2-phase/90-degree power systems which are apparently still in use in some old electric railroads up north. From Wikipedia "As of 21st century, two-phase power was superseded with three phases and is not used in the industry. There remains, however, a two-phase commercial distribution system in Philadelphia, Pennsylvania; many buildings in Center City are permanently wired for two-phase[citation needed] and PECO (the local electric utility company) has continued the service. This type of service happens to exist in Hartford, Connecticut. It does serve a few buildings in that city."

[Steve M Smith]

I think I understand it now.  It could be confusing to us as all of our power distribution is three phase, so a single phase supply is just one leg of the three phase.

When I first heard the term two phase, I assumed it was two phases at ninety degrees rather than a centre tap on a single phase.

What I don't know though is how it is derived in the US.  Is it still from a transformer on part of the three phase distribution further down the line or do you run both single/two phase and three phase systems?


Steve.

[Mike Sokol]

What I don't know though is how it is derived in the US.  Is it still from a transformer on part of the three phase distribution further down the line or do you run both single/two phase and three phase systems?

In the US we only distribute 3-phase over any distance. Then at the point of use we either send in all three phases or a single phase. That single phase is typically 240 volts which is then split down to two hot legs with 120 volts each by a center-tapped transformer. So essentially, all small and medium size buildings in the US get either 3-phase 120/208 or single phase 120/240 service. We also use 3-phase 277/480 volt service for a lot of industrial plants to run motors, and many industries receive even higher voltages for industrial processes such as glass making, etc... I'm sure others here can chime in on special voltages needed by specific industries.

[Jeff Bankston]

I think I understand it now.  It could be confusing to us as all of our power distribution is three phase, so a single phase supply is just one leg of the three phase.

When I first heard the term two phase, I assumed it was two phases at ninety degrees rather than a centre tap on a single phase.

What I don't know though is how it is derived in the US.  Is it still from a transformer on part of the three phase distribution further down the line or do you run both single/two phase and three phase systems?


Steve.

single phase is one hot wire and a ground connected to a transformer. the transformer splits one hot wire into 2 hot wires. one phase goes in the transformer and two phases come out. single phase is used in the rural countryside. its cheap and is all thats needed. you can run any 240v equipment on it. my cousins farm is fed with one hot wire and a ground. thats all you see in the mississippi and california country side and most of rural america. heres a foto.

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