This has just reopened the old two phase controversy again. Ed has done
a good job of describing the systems in detail, but be careful with the
terms.
Ask an electrical engineer about a 120/240 volt home service and he will
call it a two phase system. Two phases 180 degrees out of phase is
technically correct.
Ask an electrician and he will tell you the only way to get two phases
is to start with three phases and leave one out (and will also tell you
the engineer is nuts). He is talking about two phases 120 degrees apart.
The electrician will use two phases to either save copper or
transformers, and allow for future expansion.
There is a pretty important distinction, since you can regenerate the
third phase (although inefficiently) using transformers on the 120
degree case, but not with the 180 degree case.
The three phase rotation is of course crucial for rotating machinery.
For single phase or the 180 degree case, you need to generate starting
torque using capacitors to delay a phase.
As a result, you should just avoid the use of the term "two phase". Talk
about a 120/240 volt center tapped service, or two phases of a 120/208
volt three phase service.
In answer to your original question, I would say essentially 100% of
homes, and most of the businesses or buildings serving up to about 10
employees will have the 120/240 volt center tapped service (distributing
120 v single phase 3 wire to most prewired locations). Most businesses
larger than that will receive a 120/208 volt three phase service (or a
higher voltage three phase service), again distributing 120 v single
phase 3 wire to most locations (using privately owned step down
transformers if needed). Approximately 40% of homes (the older ones)
and a few older business buildings will have 120 v single phase 2 wire
(no ground) instead of 3 wire wiring in the building.
There are of course some small businesses (e.g. a photoprocessing lab or
small machine shop) which require three phase service and likewise some
rather large businesses (like offices) which can get by on single phase
service. It depends on history and location. I, for example, have three
phase available in my home shop generated by a rotary phase converter,
because it is several miles to commercial three phase service (and also
a more expensive monthly connection fee).
The next question is, if choosing a feed for a product, what should you
pick? My simplified North American recommendation would be:
Up to 2500 W, stick with single phase 120 v. (Up to 16 amps for standard
receptacles, up to 24 amps for dedicated receptacles)
Up to 5 KW try for single phase which will work on either 240 or 208
volts. (allowing connection to either 120/240 V center tapped or 120/208
V three phase services).
Up to 25 KW aim for 120/208 three phase service, but you may want to
offer a 277/480 V option.
Above 25 KW you probably need to offer multiple options or make custom
arrangements with customers based on available service. 277/480 V is the
next common service in use.
For rotating machinery, you probably need to consider three phase as
soon as you go above 1 or 2 horsepower (1 KW)
For home products, consider Class II products to comfortably serve most
customers with 2 wire receptacles.
Bob Johnson
ITE Safety
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Price, Ed
Sent: Thursday, December 13, 2001 8:23 PM
To: 'Barry Esmore'; EMC-PSTC Forum
Subject: RE: 2 Phases in North America
-----Original Message-----
From: Barry Esmore [mailto:[email protected]]
Sent: Thursday, December 13, 2001 2:39 PM
To: EMC-PSTC Forum
Subject: 2 Phases in North America
Hi All,
Can someone provide an estimate of the percentage of homes and
businesses that have 2 phases in Canada and the USA? Also, what is the
most common voltage between phases?
Thanks and regards
Barry Esmore
AUS-TICK
281 Lawrence Rd
Mt Waverley
Vic 3149
Australia
Ph: + 61 3 9886 1345
Fax: + 61 3 9884 7272
Barry:
AFIK, just about zero percent of USA homes have two-phase power. Heavy
industrial will have a 3-phase delta feed, and light industrial and
commercial will have a 3-phase wye feed.
However, homes are usually fed by a three-wire system. A transformer
changes a single-phase, two-wire distribution feed (about 12 kV) to a
center-tapped, 240 Vrms output. One transformer often serves about 10-20
homes. The center-tap is grounded, and the three output wires are routed
pole-to-pole. Each individual customer (home) has a three-wire feeder
cable (called a "drop") connected from a junction on the pole to a
power-panel on the house. At the house power-panel, the neutral wire is
again grounded. Each 240 Vrms line-to-line is routed through a power
meter and then to a bank of circuit breakers.
At this point, you still have a single-phase system. The voltage is 240
Vrms, from one "hot" line to the other. The voltage from each hot line
to neutral (and ground) is 120 Vrms. Small loads (lights, outlets) are
connected from one hot to the neutral (with an attempt by the
electrician to balance the expected power draw). Heavy loads (water
heater, clothes dryer, air conditioning, heating and cooking) are
connected from one hot to the other hot line.
The typical three-wire electrical outlet in a USA home has a "hot", a
"neutral", and a safety ground connection. The hot-to-neutral is 120
Vrms, the hot-to-safety ground is also 120 Vrms, and the
neutral-to-safety ground is supposed to be zero Vrms (but often is a
half-volt or so). Power flows in the hot-to-neutral circuit, and must
not be routed into the safety ground.
Much older homes may have a two-wire outlet, with a safety ground
attached to the outlet box. If you have (typically) an appliance that
needs a safety ground, the consumer is expected to make the ground to
the box with an adapter and a pig-tail wire to the cover-plate screw!
This is usually ignored by the consumer.
Regards,
Ed
Ed Price
[email protected]
Electromagnetic Compatibility Lab
Cubic Defense Systems
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (Fax)
Military & Avionics EMC Services Is Our Specialty
Shake-Bake-Shock - Metrology - Reliability Analysis
