Some of our accounts have 3-phase services, which include a peak demand surcharge on top of the charge for actual kilowatt-hour consumption.

I've also seen a sites with both single and 3-phase installations. the 3-phase services supply (and meter) only that equipment which runs on 3-phase power. All other equipment and lighting is routed to single-phase panels.

Given that some equipment can be purchased either single or 3-phase nowadays, The question is (site capacitity notwithstanding) whether a given amount of "work", horsepower or whatever, would be best run on single-phase lines up to a certain point, before acceding to 3-phase operation and its accompanying demand surcharges. Given a choice, is it economically equivalent to run area lights on 3-phase circuits if single phase panels area available?

Would a new installation be smart to have both kinds of services installed, if 3-phase was also needed?

In a residential envirmonet, as far as I know Three Phase is not available. It is not needed. Homes don't have heavy-draw HVAC's, Xenon power supplies in access of 3000 watts, 3 phase blower motors, 3 phase popcorn machines, 3 phase Icee Machines, - you name it.

Call for 3 phase.... You will never regret it. Not only that, but all your 120 volt single phase stuff can be balanced on the load much easier. Remember, 208 volt open delta 3 phase is measured from leg to leg. Each leg to neutral is still 120 volts. there is a 120 degree rotation of 3 phase power, and that is why 208 is measured across each leg.

There are some other 3 phase configurations, but most of them are obsolete. One of them is closed delta. It has a wild leg with is useless for 120 volt devices. Puget Power is not installing those systems anymore. All new construction is 3 phase open delta.

As an example, Southsound Cinemas is strictly 3 phase, with no single phase power coming into the building. Another example, Old Cinema 5 in Mount Vernon (Now defunct and dead) was strictly three phase, with no single phase entering the building.

By the way, welcome aboard! Good to have you with us.

And the answer of course is, "it depends" (also known as "I don't personally know enough about this to give you a good answer"). It is the case that large motors run notably more efficiently on 3-phase power than single-phase. Googling around, Action Machinery has a chart that asserts that a 1/2 hp motor is 33% efficient in single-phase, and 50% efficient in 3-phase. But at 10hp, it's 64% and 67%.

To answer Jack's question definitively, we'd need to know what the 3-phase rates and demand surcharges were, etc.

I can't really speak to how efficient 3-phase rectifiers are vs. single-phase rectifiers, but I imagine the win is even less than with motors.

I think the only way you end up with both 3-phase and single-phase service is if the 3-phase service was added in to an existing single-phase installation.

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No, I think you have a misconception. In the US, 3-phase Y power always produces 120V hot-to-ground, and 208V hot-to-hot (well, for low-voltage. There's also 277V/480V, and higher voltages...)

Single-phase power produces 240V. Commonly "220V" is used to refer to all sorts of things, including a) devices that take either 208V or 240V b) Only 208V devices c) Europoean (and other) power standards d) only 240V devices. We could probably go on at length here, to what end I am not so sure...

--jhawk

Single phase:

After full-wave rectification:

Note how the voltage drops to zero each half cycle.

Three phase:

After full-wave rectification:

The yellow across the top represents the rectifier output voltage. Without any filtration or regulation it's almost DC with some ripple.

BTW, Euro household voltage used to be 220 but is now nomimally 230 just as we used to talk about 115 but it's now mostly about 120.

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Matthew, two legs of single phase power have a 180 degree phase shift, so it comes out to 240 volts.

Three phase open delta power (or some call it "Y" power) has a 120 degree phase shift between each leg, and that's why it comes out to 208 volts between legs in the normal industrial market in the USA. Special applications might require a higher voltage - as Adam pointed out.

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Switching rectifiers often do NOT need 3-phase power... and there is no particular penalty for it on the final output...the larger rectifiers often will be 3-phase only to keep the current draw per leg down.

As for projector motors...3-phase motors should be run via a controller of some sort to keep them from "banging" on. For cinema use, there are many motor controllers to choose from that will take 120VAC single phase and generate 230VAC 3-phase.

Next I'm waiting for one of you guys to go through the math of WHY when you have 208 VAC when you have each leg starting with 120VAC (and a 120 degree phase shift).

Steve