# Dumb Question #1



## BamaBino (Jun 8, 2011)

34.1 3phase with Y-load (R only)

A three-phase 208 V rms system supplies heating element connected in wye.

What is the Resistance of each element if the total balanced load is 3 kW?


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## Flyer_PE (Jun 8, 2011)

14.4 ohms

VPhase for a 208 system is 120 Vac.

It's balanced so you have 1kW per phase.

P=V2/R

R=V2/P = 1202/1000 = 14.4


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## BamaBino (Jun 8, 2011)

Flyer_PE said:


> 14.4 ohms
> VPhase for a 208 system is 120 Vac.
> 
> It's balanced so you have 1kW per phase.
> ...


Flyer, Thanks for working the problem.

But I didn't ask the question that I was asking. I cut/pasted incorrectly.

In Camera's Practice Problems 7th-Ed has the

Problem 34.1

"A three-phase 208 V rms system supplies heating element connected in wye.

What is the Resistance of each element if the total balanced load is 3 kW?"

How do we know that the 208V is line-voltage and not phase-voltage?

Similarly, Problem 34.5

"The balanced wye load consist of the three 3+j4 impedances. The system voltage is 110 V rms."

How do we know that the 110V is line-voltage and not phase-voltage?

Thanks


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## BamaBino (Jun 8, 2011)

Does the word "system" mean line-voltage?


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## Flyer_PE (Jun 8, 2011)

^Unless you are told otherwise in a problem, the voltages given will typically be line voltages.


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## Complex Imaginary (Jun 28, 2011)

I agree with Flyer-PE (as usual ... he's awesome). Unless otherwise told for 3-phase problems, the given voltage is the line voltage. However, on the test, unless it can be deduced from what's given in the problem, it's not really fair to make the test-taker assume what the voltage is supposed to indicate. Just because we're familiar with the lingo by being in the industry and knowing how people talk about it doesn't make it fair. Just like when talk about transformers, they say "That's a 480 volt transformer"...but what they're really saying is "That's a 480/277V transformer".

Another way to think about it is by being familiar with common voltages. If they give you 208 V, and you are wondering if it's line or phase, it would help to consider the options:

If it's phase voltage, that means the line voltage is 360. 360/208 is a very weird combo in the real world. On the other hand, if you assume 208 is the line, then 120 is the phase voltage, and 208/120 is very common combination. This is just a helpful way to think about it. The test or question shouldn't make you assume these things, and I on the actual PE, they review the questions for this sort of thing.

Josh

Complex Imaginary

The best Power PE Exam available

http://www.compleximaginary.com


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