Feeder Calculations

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erickench

erickench

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Brooklyn, NY
Hi, I'm a new member and I'm a little confused regarding the NEC. Section 215.2 states that the following:

Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV and V of Article 220. The minimum feeder-circuit conductor size before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

The clause seems to be contradicting itself. The methods of article 220 seems to give you a lower number than the %125 x continuous load plus noncontinuous load rule because of the application of the demand factors. And in other websites the methods of article 220 are emphasized. Which is the correct method for sizing a feeder?
 
Feeder Calculation

Feeder Calculation

Okay, sorry for this post. I think the correct method is to apply the demand factors i.e. %35 to first 3000 VA, and then multiply the General Lighting VA and the largest motor VA by %125 and then divide by the service voltage.
 
After further study I've come to the conclusion that the %125xContinuous load + noncontinuous load rule applies to non-dwelling occupancies.:D
 
erickench said:
After further study I've come to the conclusion that the %125xContinuous load + noncontinuous load rule applies to non-dwelling occupancies.:D
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Do not confuse LOAD with AMPACITY.

Article 220 deals with LOAD calculations only.
 
Non-Listed Occupancies

Non-Listed Occupancies

Okay, I'm going to put this to rest right now. There are occupancies such as factories(not industrial lofts), subways and other places like outdoor lighting, i.e.roadways, parking lots etc. that are not listed in NEC table 220.12 nor do they have any demand factor or calculation method listed anywhere in the NEC.
I worked for the New York City Transit Authority for 23 years and we always used the 125% continuous plus noncontinuous rule for things like tunnel lighting, pump rooms and fan chambers, etc. It is in these unlisted occupancies that the 125% continuous plus noncontinuous rule should be applied. So if anybody from the NEC panel is listening then please take this into consideration when making the next set of changes to the NEC. Section 215.2 needs to be clarified for these unlisted occupancies.:cool:
 
Smart $ said:
Do not confuse LOAD with AMPACITY.

Article 220 deals with LOAD calculations only.
Click to expand...

Yes and feeder conductors shall have an ampacity not less than required to supply the load as calculated in Article 220. So this load calculation does determine the minimum conductor sizes for feeders, services and distribution.:)
 
erickench said:
?

Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV and V of Article 220. The minimum feeder-circuit conductor size before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

The clause seems to be contradicting itself. The methods of article 220 seems to give you a lower number than the %125 x continuous load plus noncontinuous load rule because of the application of the demand factors. And in other websites the methods of article 220 are emphasized. Which is the correct method for sizing a feeder?
Click to expand...

Article 220 applies to all occupancies dwelling and non-dwelling. Check out example D3 in Annex D, notice the end result of the calculation is for the ?minimum? size feeder or service conductors.

I believe the ?before the application of any adjustment or correction factors? applies to 310.15(b)(2) & Table 310.16 (bottom).
 
tryinghard said:
Yes and feeder conductors shall have an ampacity not less than required to supply the load as calculated in Article 220. So this load calculation does determine the minimum conductor sizes for feeders, services and distribution.:)
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Your statement, highlighted in red, is the very reason why people find the subject confusing.

Article 220 calculations only determine **load**, not the minimum conductor sizes (i.e. ampacity). Yes, they play a major role in determining the required minimum ampacity, but that is another calculation... and not an Article 220 calculation. In doing the Article 220 calculations, one has to keep continuous and non-continuous loads separate. One can then perform the additional calculation to determine the minimum ampacity for service and feeder conductors.
 
Smart $ said:
Your statement, highlighted in red, is the very reason why people find the subject confusing.

Article 220 calculations only determine **load**, not the minimum conductor sizes (i.e. ampacity). Yes, they play a major role in determining the required minimum ampacity, but that is another calculation... and not an Article 220 calculation. In doing the Article 220 calculations, one has to keep continuous and non-continuous loads separate. One can then perform the additional calculation to determine the minimum ampacity for service and feeder conductors.
Click to expand...

Major enough that you cannot install a compliant feeder without an Article 220 calculation. :cool:

These are integral, where else would you define noncontiguous loads besides the opposite of ?continuous? in definitions.

Keeping these separate is required for a correct outcome/result and the conductor must be sized at least to handle this result.
 
Example D3

Example D3

That's because the occupancy of example D3 does not have a demand factor or any other calculation method listed anywhere in the NEC. Okay it's listed in NEC Table 220.12 for calculating the lighting load. But the examples pertaining to dwellings don't apply the 125% continuous plus noncontinuous rule. I think the reason is because in the general lighting calculation for dwellings the receptacle loads are included. Receptacles are considered noncontinuous. How would you separate the receptacle loads from the lighting loads if you calculate them using NEC Table 220.12? And even if you were to take them off a drawing you would still have to apply the demand factor and not apply the 125% continuous plus noncontinuous rule.
 
Example D-3

Example D-3

Okay it has a demand factor for receptacle loads. But because it's a nonresidential occupancy the lighting and receptacle loads are calculated separately. So you could easily apply the 125% continuous plus noncontinuous rule. Boy is this subject confusing.:confused:
 
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