Single/three phase

Old 03-25-03, 07:24 AM
Harry I
Visiting Guest
Posts: n/a
Question Single/three phase

What are the advantages/disadvantages of single phase vs. three phase?
Old 03-25-03, 07:36 AM
Join Date: Nov 2001
Location: Taylors, SC
Posts: 9,261
Upvotes: 0
Received 0 Upvotes on 0 Posts
With three phase, you can run higher horsepower equipment. Generally, this is a commercial installation and it is not cheap.
Old 03-25-03, 07:53 AM
Join Date: Feb 2003
Posts: 475
Upvotes: 0
Received 0 Upvotes on 0 Posts
Another advantage of 3-phase power is that you have three "hot" phases instead of two. In other words a 200 amp service will yield 600 amps of 120V power instead of 400 amps @ 120V. Most 3-phase systems use a "Y" configuration for the transformer which will supply 208V (nominal) on the output phase to phase instead of the 240V (nominal) phase to phase on a single phase system. Therefore, all 2 and 3 pole equipment must be rated for 208 volts.

Three phase power is found commonly in commercial and almost always in industrial settings - rarely if ever in residential settings (I've never seen it in a dwelling). In larger commercial and industrial settings, the power is delivered in higher voltages (i.e. 480/277) and transformers are set in the facility to obtain 208/120 volts. Lighting circuits in such facilities are usually rated at 277 volts instead of 120 volts because you can put over twice as many fixtures on a 20 amp circuit.
Old 03-25-03, 08:02 AM
Join Date: Dec 2001
Location: Calgary Canada
Posts: 690
Upvotes: 0
Received 0 Upvotes on 0 Posts
Just a clarification the voltage system in Canada is 347/600 not 277/480.
Old 03-25-03, 11:43 AM
Visiting Guest
Posts: n/a
It essentially comes down to efficiencies.
Three systems are designed around motors, the largest application for electric power in U.S.
A three induction motor does not require a capacitor for starting or running-inherently more efficient and simpler in construction and maintenance. The tradeoff is in materials and assembly cost-higher for 3 motors. Typical 1 applications are not constant duty- blowers, sump pumps, etc. Very small 1 motors (vacuum cleaner motors, exhaust fans,kitchen appliances) are an even cheaper and less efficient design-shaded pole.
Motor efficiencies increase with size, thus you rarely see a multi phase motor smaller than about 2 HP. And I don't think I've seen a 1 motor bigger than 10HP.
Old 03-25-03, 03:54 PM
Visiting Guest
Posts: n/a
Another advantage of a three phase motor is the ease by which its rotation can be reversed. Just switch two of the phase wires. Electric utilities often serve smaller three phase loads with 120/240 volt, 4 wire, 3 phase delta because it only requires two transformers, whereas 120/208 (or 277/480) volt, 4 wire, 3 phase requires three transformers. And if only two transformers are required, only two phases (and a neutral) are required on the utility's part. One oddity of the former (120/240 volt, 4 wire, 3 pase) is that two of the three phases supplied to the user have a voltage to ground of 120 volts (nominal) while the third phase is somewhere around 208 volts to ground. Depending on where you are, this phase is referred to as the "wild" leg or the "freak" leg and the service is called "open delta". However, the phase to phase voltages are all equal (240 volts). In a wye voltage, all phases have the same voltage to ground. For some deep well applications (and a few other situations), sometimes a 3 wire, 3 phase ungrounded delta service is desirable. For this situation, one phase can become grounded and the system (and therefore the pump motor) will continue to operate without blowing a fuse.

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: