-----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
