Feeder Calculation

[Angel Electric]

What we have is a Feeder 120/208 four wire in a raceway supplying 10000 VA. nonlinear load continuous load. The raceway containing the feeder will pass through an ambient temp of 25? C. What is the allowable ampacity of this feeder? I get 37 amps and the test booklet I'm working from provides 41amps Referencing 215.2(A) (1)/ 310.15(B) (2) ( A) & (B) (3) (A) / Table 310.15 (B) (16). i need to learn what i'm nto doing correctly or understand that there could be Typo on the test Questions and answers. Please Help? I don't like to move past anything test wise I don't Understand.

 

[Dennis Alwon]

IMO there is not enough info. What is the rating of the wire ? 90C or 75C for one. Hopefully someone else can help cause I gtg

 

[david luchini]

What we have is a Feeder 120/208 four wire in a raceway supplying 10000 VA. nonlinear load continuous load. The raceway containing the feeder will pass through an ambient temp of 25? C. What is the allowable ampacity of this feeder? I get 37 amps and the test booklet I'm working from provides 41amps Referencing 215.2(A) (1)/ 310.15(B) (2) ( A) & (B) (3) (A) / Table 310.15 (B) (16). i need to learn what i'm nto doing correctly or understand that there could be Typo on the test Questions and answers. Please Help? I don't like to move past anything test wise I don't Understand.

Neither 41 or 37 is correct. You would need an allowable ampacity of 33.4 before the application of correction or adjustment factors to supply the load. The load is 10000/208/1.732=27.76A. 33.4A adjusted and corrected for 4 ccc's and 25? C would be 33.4 * 0.8 * 1.04 = 27.79, large enough to supply the load. This does not address minimum conductor size, however, which would be at least #8 since you need a min 35A OCPD (assuming its not a motor load, etc.)

The test booklet got its answer by (wrongly) applying the 125% for the continuous load into the allowable ampacity calculation...27.76/0.8/0.8/1.04=41.7

(I assumed 90? C conductors, but 75? C would be close as the correction factor is 1.05 instead of 1.04)

 

[ron]

So the load is 10kVA @ 208/120V or 27.8A. Continuous load circuit brings us to 34.7A

They mention an ambient on 25 deg, and the regular table is 30 deg, so the multiplier is 1.08 for 60 deg termination temperatures (60 deg because we don't know the terminal rating and the feeder is 100A or less). 34.7 / 1.08 = 32.2A

The nonlinear load implies that the grounded (neutral) conductor is current carrying, not just imbalance so the 80% adjustment factor needs to be used, 32.2 / .8 = 40.2A or rounded up to 41A

 

[ron]

Because we can derate against the 90 column ampacity and pick the lower of the two (derated 90 deg or regular 60 deg for the conductor size), makes this question even more complicated.

 

[charlie b]

Please Help? I don't like to move past anything test wise I don't Understand.

You should move past this one. It is not worth your time to figure out how the author of the test question came up with a wrong answer. David made a valiant attempt, but there is no way to be sure that the error David pointed out is the error the author made. Here are some problems with the question:

• It does not give us a wire size.
• It does not give us the instruction to select the smallest wire size that is code-compliant. (Who says I can?t use 500 MCM for this feeder?)
• It does not give us the wire?s insulation system, as Dennis has already pointed out.
• It says the wire passes through a 25C area, but does not say that that is the ambient for the entire length of the run. (How do we know the wire does not also pass through a 50C area for 90% of the circuit length, with the 25C area being only 10% of the circuit length?)

 

[Smart $]

...

The test booklet got its answer by (wrongly) applying the 125% for the continuous load into the allowable ampacity calculation...27.76/0.8/0.8/1.04=41.7

...

FWIW, the 2014 edition changes to 210.19(A)(1) and 215.2(A)(1) have blurred the meaning of allowable ampacity.

In previous editions, it meant the values listed in an ampacity table... without adjustment and correction. Now the (b) portion of each section refers to allowable ampacity after adjustment and correction have been applied.