Floyd asked in Science & MathematicsEngineering · 10 months ago

# Three phase or single phase?

I've been using a panel that has 3 hot legs of 120v. It's basically a fuse panel but i don't know how to tell if I'm using single or three phase. Is it the voltage? You can choose to use all three hot lines or two.

Relevance
• Anonymous
10 months ago

Residential or commercial/industrial?

Measure the voltage between each combination of 2 hot "legs". There are 3 possible combinations. What are the voltages?

Measure the voltage between each hot "leg" and neutral. What are the voltages?

I think that you shouldn't be using electricity that you don't understand.

• 10 months ago

Yes that is 3 phase

• Anonymous
10 months ago

Stop messing around with stuff that you have no idea of. You are a danger to yourself and those around you.

• 10 months ago

Chances are you are dealing with a 3 phase wye connected system. If so, then you should have 3 hots and a neutral wire for a 4 wire system. Verify this as I'm just assuming this to be the case. This setup is the most common for commercial enterprises such as a restaurant.

The three phases are labeled phase A, B and C. Each of these phases will measure 120 volts between itself and the neutral wire. Between any of these phases, like between A and B, or between B and C or between A and C there will be 208 volts (this will have to serve as your "240" volts since that's the only voltages there are in the system, 120 volts or 208 volts). Most commercial equipment that runs on 240 volts can also run on 208 volts. But check the nameplate of the equipment to make sure.

In the service panel where the circuit breakers are, the first breaker at the top on the left is phase A. The next one down is phase B and then the next one down is phase C, and then it keeps repeating like that all the way down to the bottom. The same on the right side. So it goes ABCABCABC etc.

Three phase equipment, or a three phase motor for example, would use phase A, B and C. Your normal single phase loads would just use any one phase plus the neutral wire. If you needed 240 volts (I mean 208 volts!) then you would use two adjacent phases, like A and B or B and C.

Have fun.

• Steven
Lv 7
10 months ago

The voltage BETWEEN phases in a single phase system is 2x(120=240). The voltage BETWEEN phases of a 3-phase system is 1.732x(120=208). 1.732 is 2xsin60 = square root of 3

As others have pointed out, if you don't know these things then you are flirting with death.

• 10 months ago

That one is simple. Label the wires 1, 2, and 3. Measure the Voltage between each wire and ground. Is it 120 Volts? Okay, now measure the Voltage between each pair. If it's 208V, then it is a classic 208V/120V three-phase system. If you measure something totally different than 208V between the pairs then it may be one of those older, goofy industrial power supplies. Suffice it to say that if you have to ask this question on Yahoo Answers, you are not qualified to safely connect anything electrical together. For safety's sake, call a qualified electrician.

• 10 months ago

IS IT COMING FROM A FACTORY WHERE USING 3 PHASE POWER ? IF IT IS TRUE, THERE MUST HAVE A VERY BIG STEP DOWN 3 PHASE POWER TRANSFORMER TO CONVERT 600V INTO 120V, THEN THREE HOT LINES EACH HAS 120V IS EXISTED AS YOU SAID. IT IS THREE PHASE POWER SOURCE BUT CAN BE USED AS SINGLE PHASE POWER SOURCE.

TO USE 120V POWER SOURCE, CONNECT APPLIANCE BETWEEN ANY ONE HOT LINE AND NEUTRAL LINE. TO USE 240V TO POWER STOVE AND DRYER CONNECT TO ANY TWO HOT LINES AND NEUTRAL LINE WILL DO. BOTH STOVE AND DRYER NEED BOTH 240V AND 120V TO RUN.

• 10 months ago

If it is in your home then it is 1 or maybe 2 phase power. Only huge commercial plants ave 3 phase power.

If you measure from ground to each of the terminals you will find ~120V AC.

If your panel has a double Main Circuit Breaker It is Two Phase 240VAC.

• 10 months ago

Residence, you can bet on single phase 240V split in two in north America.

2 hot wires and a common.

3 phase is commercial service, 3 hot wires out of phase with a common.

Both have earth grounding.

Don't even mess with your switch box, you know enough to burn your house down or get electrocuted.

• 10 months ago

If you do not know what you are doing, then you shouldn't. However, three phase refers to a system that has the AC oscillation out of synch by 1/3 of a cycle, so at some point in time, one wire is at -120V, one is at neutral (0V), and one is at +120V. Depending on how the system is wired, effective voltage can be 120V or 240V. -120 to 0 is 120V, and 0 to +120 is 120V. -120 to +120 is 240V.

It is called three phase because the AC current has a waveform, and the three lines are not in synch; the voltage variation is staggered, each by 1/3 of a cycle relative to the other. Thus, as the voltage cycles through each line, the voltage difference between each line remains constant (well, cycles at a constant voltage that depends in magnitude on the phase separation, because AC really flips voltage with each cycle).

If you employ only two wires of the three-phase feed, then you will have one cycle (two of the cycled pairs are eliminated by removal of the third cycle; two of the "phases" were with that wire and one or the other of the two you employed). The question is whether that would be a 120V or 240V circuit, though. Depends on which pair of wires you employ as the feed.