NEC 220 has many was to calculate demand factors on various loads for feeders and services for dwellings, restaurants, schools and other occupancies. To the best of my knowledge these calculate demand factors are always used to calculate acceptable feeder, service conductor and overcurrent size. Recently an engineer pointed out that NEC 215.2 requires the higher of either the calculated load or 125%of continuous and 100% of non-continuous load. Plainly the 125%of continuous and 100% of non-continuous load will always be larger than a calculated load employing demand factors. What am I missing!
NEC 215.2
- Thread starter wyboy
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luckylerado
Senior Member
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- Lake of the Ozarks
Services vs Feeders vs Branch Circuits
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
To luckylerado, I say it’s not a simple as that. To wyboy, I say that your engineer stated it incorrectly. Here is an example.
From 215.2(A)(1)(a)
From 215.2(A)(1)(b)
Looking at both methods, I know that I have to select the larger of the two, and I select 4/0.
On the other hand, if I were to limit the current-carrying conductors to 3, then 215.2(A)(1)b) would have me selecting the 2/0. But the larger of the two methods would force me to select the 3/0.
From 215.2(A)(1)(a)
- Load is calculated as follows: 100 amps continuous (counted as 125 amps) and 70 amps non-continuous.
- Total calculated load is therefore 195 amps.
- This would lead me to select a 3/0 copper THHN.
From 215.2(A)(1)(b)
- Maximum load to be served (NOT “calculated load,” and this is where the engineer got it wrong) is 170 amps.
- Knowing nothing else, I might select a 2/0 copper THHN.
- But the plan is to put 9 current-carrying conductors in the same conduit. That means I need to derate by 70%.
- Minimum ampacity is therefore 170/.7, or 243 amps.
- This would lead me to select a 4/0.
Looking at both methods, I know that I have to select the larger of the two, and I select 4/0.
On the other hand, if I were to limit the current-carrying conductors to 3, then 215.2(A)(1)b) would have me selecting the 2/0. But the larger of the two methods would force me to select the 3/0.
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