208V or 200V column (Table 430.150)

[wasasparky]

When determining amps for 208V motor loads, which column do you use?

200V, correct?

If so, what is the 208V column for?

 

[charlie b]

I would use the 208V column, and be left wondering why the 200V column exists. :-?

 

[wasasparky]

I would use the 208V column, and be left wondering why the 200V column exists. :-?

What about 460V column for 480V systems.
What about 230V column for 240V systems.
etc.

 

[infinity]

What about 460V column for 480V systems.
What about 230V column for 240V systems.
etc.

We use those because there are no 440 and 480 volt or 220 and 240 volt columns. The motor is rated at 208 volts then you should use the 208 volt column. 430.250 tells you at the top of the table that certain voltage ranges can use the voltage in the column, i.e.-110 to 120 use 115. There is no such note for 200 or 208 volts.

 

[wasasparky]

OK, I am left wondering why the 200V column exists...

This affects the answer in the Mike Holt problem in the "Art 430.24 calculation question"
I used the 200V column to get the same answer as Mr. Holt's.
If the 208V column should be used, then his answer is incorrect.

 

[rbalex]

It all starts in Article 220, Branch-Circuit, Feeder, and Service Calculations. Technically, Sections 430.24, 430.25, and 430.26 could not be used as the basis for motor computations if they were not referenced in Section 220.50.

The Scope of Art 220 “…provides requirements for calculating branch-circuit, feeder, and service loads.” Section 220.5 specifies standard nominal voltages “…for purposes of calculating branch-circuit and feeder loads.” The Tables in Art 430, Part XIV are “ratings” and, as Infinity pointed out, may be used for various nominal voltages.

I consider Art 220 the Electrical Safety Factor Article. Calculations that are compliant with it are moderately conservative. When you realize some Civil, Structural, and Mechanical safety factors can be as high as 2500%, you realize we in the electrical trades get off pretty easily.

The Article is based on manual calculations and the need of plan checkers/inspectors to be able to reasonably review them.

As I said in the other topic.I believed your first answer was the closest. I should have added “consistent with Art 220.” I said your second METHOD was accurate because it accounts for the 30 degree phase shift between the 208 three-phase motor (and 120 single-phase) currents and the 208 single-phase currents.

Personally, if I were grading the test, I would accept either answer if you showed your work. I would accept the 204A answer as the “basic” answer and 199A as the “engineered” answer. In the second case, it would have actually permitted a #3/0 AWG conductor (70C column).

 

[wasasparky]

I would accept the 204A answer as the ?basic? answer and 199A as the ?engineered? answer.

You lost me here...time to fall off the fence...
I think there is one answer - 199A

 

[wasasparky]

I said your second METHOD was accurate because it accounts for the 30 degree phase shift between the 208 three-phase motor (and 120 single-phase) currents and the 208 single-phase currents. .[/FONT]

They both did that.
The difference was the columns used.

 

[rbalex]

The "basic" answer would ignore the phase shift and treat all the line currents "as if" they were in phase. You would arrive at Roya?s answer. IF all the loads were converted to VA, added and recomputed to Amps ? you would also get 199A. However that only works with ?balanced? loads; it is not a ?general? solution.

Basically Art 220 likes answers in ?Amps? and doesn?t attempt to resolve phase shifts, voltage regulation, etc. The results may be modestly conservative ? but the NEC doesn?t care.