Equivalent 3 phase vs single phase power draw

Close. 1.732 = square root(3)

Ned Simmons

thank you...

i

If I understand your question correctly:

You are asking for an answer in amps and giving conditions in power and/or amps. For a resistive load.

So for a single phase resistive load and a 3 phase resistive load with the same power consumption, the total amount of amps would be the same. The three phase load would simply be 3 loads, each a third of the single phase load.

For example with a single phase load of 1, the three phase load would be three loads of 3. The same power would be dissapated(sp?) but the current through each of the 3 phase loads would be 1/3 of the single phase load.

With a resistive load any current shifts would not be a factor. Things get interesting when the load is inductive and/or capacitive.

Bob AZ

I do not think that 3 times the single phase load is the correct answer. It would involve double counting of current. I am not claiming to be a big expert in this, but 3 times cannot be the right answer.

i

If line-to-line voltage is the same in both cases, singlephase line current will be 1.732 x threephase line current for same total power

-- like Ned sed!

thank you...

i

But we are dealing with a resistive load. No inductance or capacitance. A resistive load will not concern itself with phasing. So the 3 phase thing is moot.

If we had a 3 phase device that has an ouput function, motor, transformer or the like then we would be dealing with other things like is the 3 phase Delta or WYE? And is the loading on the 3 phase, phase-to-phase or phase to neutral/ground. Bob AZ

Bob, The premise under discussion here is multiple phase transmission of power. You might be confusing that with the concept of power factor. Correct me if I'm wrong.

In article , Bob AZ wrote: :But we are dealing with a resistive load. No inductance or capacitance. :A resistive load will not concern itself with phasing. So the 3 phase :thing is moot. : : If we had a 3 phase device that has an ouput function, motor, :transformer or the like then we would be dealing with other things like :is the 3 phase Delta or WYE? And is the loading on the 3 phase, :phase-to-phase or phase to neutral/ground. :Bob AZ

Whether the load is connected delta or WYE does not matter. What _does_ matter is how the voltage is measured, and the article you responded to (and neglected to quote!!) explicitly stated, "If the line-to-line voltage is the same ... ." A WYE connected load will see a line-to-neutral voltage that is (1 / sqrt(3)) times the line-to-line voltage, so the total power for 3 such identical resistive loads is V*I*sqrt(3), where V is the line-to-line voltage and I is the current in any one line.

capacitance.

phase

OK, for ease of calculations, letrs say we have a 2400 watt 240 volt single phase heater. The line current is 2400/240, or 10 amps.

Now, a 2400 watt 3 phase heater is 800 watts per phase. The three 800 watt elements are wired in delta. The load current for each element is

800/240, or 3.33 amps.

By drawing the vector diagram of the 3 load currrents and and some trig, the line currrent for each leg is 3.33 * square root of 3, or

5.77 amps

And 5.77 * square root of 3=10

Resistance can be used if you prefer. Say we have 3 heater elements that measure 15 ohms. If one was connected across a 240 volt single phase line, the power dissipated would be 240 * 240/15, or 3840 watts, with a current of 16 amps. Three of those in parallel would be 11,520 watts total, with a current draw of 48 amps.

Now, wire those three elements in delta. Each element is still across

240 volts, so each one still dissipates 3840 watts (for a 11520 watt total) and draws 16 amps. But since it's three phase, the line current is 16 * square root of 3, or 27.7 amps.

And 48 /square root of 3 also =27.7...

It's still early-I hope all those numbers are right, lol...