How many kw electricity can you use on a single phase system?

Answer 1

In common everyday usage and in about 99% of the circuits you will work with, KW and KVA are the same value. However, Watts has a very technical definition that does not exist in every circuit, such as motors, Transformers, and capacitors, but every circuit has volt-amps, or VA. The K just stands for thousands in both abbreviations.

More Information:

VA is short for Volt-Amps, i.e. volts times amps, which may seem to be the same as watts. This is true only for resistive loads, where the phase angle of volts is the same as that of amps. In that case KVA is the same as KW.

What happens with non-resistive or reactive loads, however, is that amps are not in phase with volts. In an inductive load, such as a motor, amps lags volts; while in a capacitive load, amps leads volts. In both of these cases, you cannot just multiply volts and amps to get watts, due to a phenomenon known as power factor; power factor being the ratio of apparent power to true power.

To visualize this, you need to draw the power circle. Since WikiAnswers does not presently support graphic images, please take a piece of paper and follow along with me...

Draw a circle. To make it easy to do the math, draw it centered at the origin, and pretend that it has radius of one. This way, the trigonometry is easy.

Consider that the radius of the circle is VA, KVA, or MVA, what ever scaling factor you want. (Do not confuse this with the trigonometry trick where we also consider the radius to be one.)

Now, pick a point on the circle. Twelve O'Clock is a purely resistive load, where volts and amps are in phase. Nine O'Clock is a purely inductive load, where amps lags volts by 90 degrees. Three O'Clock is a purely capacitive load, where amps leads volts by 90 degrees. In "normal" trigonometry, zero degrees is at 3:00 O'Clock, but, by convention, zero degrees when dealing with reactive power is accepted to be 12:00 O'Clock. Just keep the trigonometric identities straight in your mind.

In practice, with normal electric motors and all other things considered, we see a point on the circle at about 10:30 or 11:00 O'Clock. Let's pick 10:30, to make the math easy. So, draw a line from the origin to the upper left at an angle of 45 degrees with respect to the Y-Axis. Label this line KVA.

Notice that KVA is constant, no matter what the phase angle may be.

Now, draw two more lines; one from the point on the circle at 10:30 O'Clock straight down, perpendicular to and stopping at the X-Axis - label this line KW, and one from that same point to the right, perpendicular and stopping at the Y-Axis - label this line KVAR.

Label the angle of the first line with respect to the Y-Axis as phase angle. Positive meaning inductive, and negative meaning capacitive. Notice that, if you had a 45 degree capacitive load, intersecting at 1:30 O'Clock, the magnitude of the KVAR line would be the same, though positive instead of negative, and the KW line would still be the same.

Now, power factor is KW / KVA, the ratio of apparent versus true. In this case, since we picked 45 degrees as the phase angle - to make it easy - the ratio is 0.707, or the cosine of the phase angle.

A typical power meter will register less than the actual power. In the worst case of a purely inductive load, the power meter would register zero, though the KVA is still what it always was. The power meter is "lying", due to the power factor - energy is still being transferred - and the equipment must be sized to handle it - that is why transformers and other things are often rated in KVA instead of KW.

Now, if you are interested, and most power companies, engineers, and electricians are, then look at KVAR. That is kilo volts-amps reactive. There is a KVAR power factor as well, simply the sine of the phase angle. In this case, again with 45 degrees, it is the same as the normal power factor, 0.707, however, we normally would call that -0.707, to differentiate between KVAR (inductive) and KVAR (capacitive).

So, to answer the original question, "How do you convert KW to KVA", you simply divide by power factor, and to obtain that, you need to know the phase angle. It is simple trigonometry from there.

Note that, if your phase angle is more than plus or minus 90 degrees, we are actually talking about a generator, instead of a load.

In practice, the phase angle is more like 20 to 30 degrees, so the power factor would be slightly higher, and the reactive power factor would be slightly lower. Power companies penalize large customers for poor power factors by measuring it and compensating their power meters or accounts to consider the perceived loss in energy or, more correctly, the increase in actual energy use. Also, poor power factor causes degradation of voltage on power lines, so power companies compensate with capacitor banks, shifting the phase angle back closer to zero.

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There is no longer a need to penalize large customers for poor power factors since their billing is based on current, not Watts. This is why you see a very residential-looking meter on the side of very large buildings. There is a current transformer (CT) placed around each service conductor. Each CT is then wired with very small conductors into the current meter. Next time you go through a drive-through, notice the meter location on the back of the building. You will find it in close proximity to a junction box that contains the service conductors.

A current transformer is nothing more than a larger version of a clamp-type ammeter used by electricians.

Answer 2

Depends on wire gauge, fuse/breaker on that line, and type of service.

In most home uses a single phase outlet is limited to either 15A or 25A. Special outlets are available for single phase appliances that take more current (e.g. certain electric heaters) but they require an electrician to install them and special large gauge wire not commonly sold in hardware stores.

Answer 3

as many as the nearest transformer can supply

Answer 4

3ph 1ton=1.5kw