Phase/voltage Q

Old 04-28-03, 06:20 AM
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Phase/voltage Q

If I have two-phase power and each leg is 120v, I can get 240.

But how does 3-phase 208 play in this scene? The reason I ask is, a poster to an e-mail forum recently purchased a former TV studio that has 600-amp service and says it's 3-phase 208.

The service is then divided into forty 20-amp breakers to feed the lighting grid.

How does the 3-phase 208 translate into forty 20a 110-volt circuits? Is 208 standard, or could he be mistaken?
Old 04-28-03, 06:29 AM
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Normally, we refer to the residential service that you have as single-phase, not two-phase. We say that you have two legs of power that are 180 degrees out of phase, but that's not regarded as "two-phase power." It's semantics.

208 volts is the voltage difference between two of the three phases. Many appliance are designed to run on 208 volts, but most are not three-phase appliances. They are simply single-phase appliances getting power from a three-phase system. Things that are actually using three-phase power (i.e., are using all three phases) are almost always big motors.

Three-phase systems usually also provide the standard 120-volt service too, from any one phase to the neutral.

So this studio is just using single-phase power derived from a three-phase supply.
Old 04-28-03, 07:49 AM
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In addition to the information from John:
The (40) 20A 1 pole circuit breaker each get power from one of the three hot phases (legs) and references to neutral (grounded conductor) measuring 120V phase to neutral. So, generally (13) of the total breakers are connected to Phase 'A', (13) to Phase 'B' and (14) to Phase 'C' or some balanced combination totally (40). Many three panelboards have (42) single pole breakers resulting in (14) single pole circuits phase.

If you used a two pole breaker then the voltage between hots is 208V. If you used a three pole breaker, then the measurement between any two hots is 208V. In this situation, the phases (hots) are 120 degrees (on an ossiliscope) apart, and on your residential service, the two hots are 180 degrees apart.
Old 04-29-03, 06:03 AM
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Thank you very much.
-- RJ

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