120/208V or 277/480V.... huh?

Those are the voltages the panel or device is rated to handle.

I take it you are talking about a service panel.
 
Did you know that in some parts of the mining industry they used 7 phase power. I was told that when running big motors with a lot of cable, the extra phases reduce the total amount of copper required.

Think motors 2-3 times the size of house and running miles of copper.

Martin
 
I have a question.

I have an air compressor that calls for 208 delta or 360Y both three phase on the data plate of the motor. The compressor itself calls for 208/3 phase 155 amp.

I have 240/3 phase and my local compressor guy and two electricians say it will work fine. one electrician says I need two BUCK transformers to knock the power down.

I went with the majority and it does not work when connected to three phase 240. the contactor clangs back and forth. I replaced the contactor and coil and get the same clanging. The motor ohms good and not grounded.

ANy thoughts.
 
Kbob... the 240Vac source, represents only a 5% over-voltage above typical design values! The motor, when connected in delta should tolerate it!

More significant, however, is your observation about the contactor "clanging!" It suggests a starter problem.

How is the motor started, DOL, D-Y, Part-winding, or Soft-Start?

Regards, Phil Corso ([email protected])
 
H
> what does it mean when something is rated 120/208V or 277/480V? What do the two numbers mean?

Electrical supplies can be Single phase, such as 120V or in Much of the world, 230V relative to Neutral. However in the States, supplies are usually 2-wire + Neutral, that will give you 120V to Neutral on two wires and 240V between the two Hot wires, as wired to most houses. However larger sites may well have 3-phase supplies, particularly for motors. 120/208V is obviously a 3-phase supply which usually has 3 Hot wires, each one 120V from Neutral, but only 208V between any two Hot wires. Higher power 3-phase supplies are typically 277V each Hot wire to Neutral with 480V between each of the 3 phase wires.

If you already have a 3-phase supply, you may well want some of that power to provide 120/240V for Office buildings etc. Look at http://dl.dropbox.com/u/88752751/Transformer for 120-240V from 3-phase - 30Jun2012.doc
 
> you probably have done some investigation of branch-circuit losses. Some advocate the use of Power Factor Correction (PFC) devices, for energy-use reduction.
>
> My study, to the Florida EPA, showed that increasing branch-circuit wiring for major appliances, by one-size, will achieve the same savings at a fraction of the cost!
>
> I am talking of new house wiring, of course!

Since when is a personal computer considered a major appliance?
 
> Only have 125 coming from one wire in electrical panel. Other wire is dead.
> Tried to check if there was a fuse box or something and did not find any.

> What to do?

...Reset the circuit breaker, its possible you have a tie broken between your two breakers. Double check that both breakers are in the on position and hold there. If this does not work then denergize the circuit, ALL OF IT AND CONFIRM WITH A VOLTAGE METER THAT IT IS DE-ENERGIZED, at the device tie the phase to the ground then at the panel check to see if you have continuity between phase and ground, if not then your wire has been compromised and must be spliced or replaced.
 
>> Can you get 208v to supply something
>(as per the nameplate requirement) from
>a 240/120 household system?
>
>if the load is resistive it is possible

Remember the rule that your conductors gauge or thickness directly correlates with its maximum current carrying capacity (amps). A resistive load could be powered on 208v even though it's rated for 240v but this lower voltage of 32 less volts going to this 'heater' (I'll call it) now needs 'a little' more amps to keep its wattage rating where it is (for example, a plug-in heater at 1kW).

1 kW heater rated for 240 volts will draw 4.2 amps; if this same heater is then loaded up with 208 volts the amps now become 4.8 (0.6 amp increase, slight increase, but something worth noting when using the other voltage). 208 volt will now draw more amps in its supply conductors.

If these conductors (wires) are too thin for this additional ampacity, a fire could happen. I'm sure that most of the time it's ok because in North America this is such a common problem and I'm sure the manufacturers safeguard against this theoretical risk. But this risk is a real risk trust me.

Likewise say, you have the opposite problem, where you have a 208v rating and want to hook up with the higher 240v. Overvoltage to an appliance can be just as bad as overamperage talked about above. I've had a hot water tank rated for 208v or 240v (dual rating) but it was hooked up to single-phase 480v, this is an extreme example of overvoltage because the person wiring it in didn't know what he was doing, and what happens? The bottom of the tank blows out and leaks everywhere below (there was a restaurant below).

Overvoltage can damage equipment. Just like overamperage, the more you deviate from your rating (10s of volts different, then worse yet 100s of volts different, and so on), the more likely damage will occur. Keep your appliances voltage rating exactly as labeled or as close to as possible.

As for induction motors, they sometimes can be dual-tapped for both voltage choices. If they are only one of the other, then I wouldn't recommend using the unlabeled voltage. For the same reason as the resistive load, you could overamp or overvolt, causing overheating or damage to motor. If the voltage is too low, the motor might not start, or spin slower and likely overheat from trying to get to its rated r.p.m. (this needs to suck more amps now to spin at its speed).
 
S
Excellent answer, if by "heater" you mean "motor". Actual "heaters" draw LESS current at lower supply voltage, thus conveniently enabling them to obey the P = E^2/R rule.
 
Heavier motors love 3-phase power and its dumb powering them on single-phase. Motors work better on 3-phase power because of their revolving magnetic field characteristics. 480volt is a typical 3-phase power source and because of that it is used to power heavy motors only, not plugs, not lights. Other 3-phase sources include 208volts and 600volts, both are also good for motors. It's good for them! This is why high-rises and industrial sites have 3-phase power because they have heavy motors and machinery and these loads need 3-phase.

Single-phase power examples are anything in a home whether it be 120 or 240 volts (furnace motor included, this motor is not powerful enough to warrant a 3-phase supply to homes). 208volts can also be single-phase as well as 3-phase.

In a commercial environment, which is now 3-phase source, the single-phase loads here are convenience receptacles or 'plugs' and lights. Convenience receptacles are almost always 120 volts, like in a home. Lights, on the other hand, may not be 120 volts, but rather 277 or 347 volts depending on your 3-phase source whether it be 480 or 600 volts.

Lights, heaters or other resisitive loads can benefit from the higher voltage associated with 3-phase power but they don't require it because they don't have a revolving magnetic field like motors so they are almost always single-phase power in a high-rise. This is all part of 'splitting out' your 3-phase supply.

120 (single phase aspect) of 208 (3-phase aspect) that's essentially what 120/208 means. But there's so much more to know really, and I haven't read all of these responses but a lot of people probably have answered a lot of this on here.
 
> b). 120/240 Volts split phase, which is two legs of 120 volts, one going positive exactly when the other goes negative - a phase shift of 180º.

This is correct. But why if you measured this with an oscilloscope would you get 240 volt sine wave? Shouldn't you get 0 volts if 120 - 120 = 0? Why does it double the voltage? I think this has to do with the fact that the pole pig transformer outside is center tapped neutral.

> c). One leg of 120 V. and a second leg of 88 V., at 180º phase shift. This provides some 120 V. output and some 208 V. output: 88 + 120 = 208.

I've never heard of one leg being 88 volts. I think you're referring to single-phase 208volt, this is 120volt each leg with respect to neutral, not 88 volts left over in leg 2. It's generally not uneven like that.
 
S
> This is correct. But why if you measured this with an oscilloscope would you get 240 volt sine wave?

Yes, a scope or meter (or load) will see 240V. If it weren't 240V, you couldn't hook larger household loads (range, water heater, dryer, AC condenser) across those two lines and have them do anything.

> Shouldn't you get 0 volts if 120 - 120 = 0? Why does it double the voltage?

Because when leg A is at +120, leg B is at -120 which is a difference of 240. Arithmetically, 120 - (-120) = 120 + 120 = 240
 
> Excellent answer, if by "heater" you mean "motor". Actual "heaters" draw
> LESS current at lower supply voltage, thus conveniently enabling them to obey
> the P = E^2/R rule.

You are correct ONLY if it is a pure resistive load... P = E * I and for this case you use this version of Ohms Law: I = E/R but again, you are correct only if this is a pure resistive load...
 
S
"You are correct ONLY if it is a pure resistive load... P = E * I and for this case you use this version of Ohms Law: I = E/R but again, you are correct only if this is a pure resistive load..."

Which is more correct than the guy who's talking about heaters and then tries to apply rules about inductive loads to them. Heaters are just resistance elements and you feed them power. Very simple. Ohms law applies just fine.
 
> I would like to see your calculation for power consumption if you believe 240V is cheaper than
> 120V to the same load. Power (watts) is P=IE (current times voltage). If you draw 10 amps
> at 120V, you will draw 5 amps at 240V so...

> P=10x120V = 1200 watts
> P=5x240V = 1200 watts
Smaller gauge copper costs less...
 
> I have a question.

> I have an air compressor that calls for 208 delta or 360Y both three phase on the data plate of the motor. The
> compressor itself calls for 208/3 phase 155 amp.

> I have 240/3 phase and my local compressor guy and two electricians say it will work fine. one electrician says
> I need two BUCK transformers to knock the power down.

> I went with the majority and it does not work when connected to three phase 240. the contactor clangs back and
> forth. I replaced the contactor and coil and get the same clanging. The motor ohms good and not grounded.

> ANy thoughts.

you aren't able to carry the motor's starting current; that's why the contactor chatters. You need to look up the starting current and select wire gauge per NEC for your application and probably delayed trip over current protection.

Probably best to hire somebody or go ask a local electrician so you don't run your motor backwards and ruin your compressor. (no kidding)

BTW- three phase power isnt 120/240, it is 120/208 or 277/480 etc. THREE phase or hot wires, neutral and ground. in the UK it's 240/415 and in germany it's 220/380. (that's in most homes over there)
 
> you can run 12/2 to a 240 and your computer will use less amps and that
> means less your electric bill will be.

Wrong.

Stuff uses less amps at higher voltage this is correct but it uses the same amount of power, which the power companies measure in kw/h, and it is exactly the same amount regardless of if you run your computers xwatt power supply at 120 or 240V.
 
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