Feeder cable sizing

[hhsting]

Nec 2017 section 210 says to size feeders based on noncontinuous plus 125% continuous load.

What i dont understand is panel schedules dont show sum of noncontinuous plus 125% continuous load. Instead panel schedule shows demand load cal based on NEC 2017 section 220 and demand factors which is not the same as sum of noncontinuous plus 125% continuous load. Is this discrepancy between two sections? How does one size feeder feeding panelboard? Based on calculated demand amps?

Question;

1. I am now confused on how to size feeders. Should the feeder cable be sized based on demand factors and demand amps shown panel schedule or sum of noncontinuous plus 125% continuous load? What nec 2017 code section?

 

[bwat]

Nec 2017 section 210 says to size feeders based on noncontinuous plus 125% continuous load.

Well first off, it’s 215 that is for feeders. 210 is for branch circuits.

And then 220, as you stated, is for calculations of various circuit types.

Hint: if you know all the loads, both continuous and non-continuous, then 220 doesn’t do much for you.

 

[hhsting]

Well first off, it’s 215 that is for feeders. 210 is for branch circuits.

And then 220, as you stated, is for calculations of various circuit types.

Hint: if you know all the loads, both continuous and non-continuous, then 220 doesn’t do much for you.

I don’t its commercial kitchen, dwelling units. Demand factors are applied to size feeders but then what we reapply on top of demand load noncontinous or continuous 125% to size feeders? How exactly does this work?

 

[augie47]

If it's a commercial kitchen, follow 220.56 for overall feeder demand. As noted in the associated Table, its takes diversity factor into consideration whereas the individual circuits do not.

 

[hhsting]

If it's a commercial kitchen, follow 220.56 for overall feeder demand. As noted in the associated Table, its takes diversity factor into consideration whereas the individual circuits do not.

Ok but thats not the question

 

[kwired]

Any values that come from art 220 calculations are taken at 100% for feeder/service loading.

 

[Fred B]

Nec 2017 section 210 says to size feeders based on noncontinuous plus 125% continuous load.

What i dont understand is panel schedules dont show sum of noncontinuous plus 125% continuous load. Instead panel schedule shows demand load cal based on NEC 2017 section 220 and demand factors which is not the same as sum of noncontinuous plus 125% continuous load. Is this discrepancy between two sections? How does one size feeder feeding panelboard? Based on calculated demand amps?

Question;

1. I am now confused on how to size feeders. Should the feeder cable be sized based on demand factors and demand amps shown panel schedule or sum of noncontinuous plus 125% continuous load? What nec 2017 code section?

As already stated 210 is not for feeder calculation, is only applied to the extent of determining branch circuit sizes, and how that may effect feeder size.
215.2(A)(1) gives you two options for calculation of feeder sizes. Both would need to be calculated then apply the largest load calculation to feeder size.
For 215.2(A)(1)(a) you need to know both continuous loads (125%) and non continuous loads (100%) then calculate total loads. (I believe this is sum of all loads at respective percentages).
For 215.2(A)(1)(b) you need to know maximum load to be supplied after adjustment factors applied. (I believe this is your demand load)
Your feeder loads are independently calculated by 220 applying 220(Part I) and 220(Part III), ( Part IV), or (Part V) requirements.
Now which calculation is larger. That is what size load you need to apply to feeder size.
Also See Informational Note 1 for 215.2(A)(1)(b) it references to see Annex D for examples of the various methods to arrive at part (b) size.

 

[wwhitney]

I think the OP's question may be the following: you go through the Article 220 load calculation for a feeder and get a single load in amps. Now you look at 215.2(A) and it wants that load split into a continuous portion and a non-continuous portion. So how do you split the result of the Article 220 calculation?

Cheers, Wayne

 

[hhsting]

I think the OP's question may be the following: you go through the Article 220 load calculation for a feeder and get a single load in amps. Now you look at 215.2(A) and it wants that load split into a continuous portion and a non-continuous portion. So how do you split the result of the Article 220 calculation?

Cheers, Wayne

Right thats the question


Sent from my iPhone using Tapatalk

 

[xptpcrewx]

I think the OP's question may be the following: you go through the Article 220 load calculation for a feeder and get a single load in amps. Now you look at 215.2(A) and it wants that load split into a continuous portion and a non-continuous portion. So how do you split the result of the Article 220 calculation?

Cheers, Wayne

It can be done at the same time with the Article 220 calculation, before, or after if you kept track of the continuous/non-continuous load. It all gives you the same answer.

 

[kwired]

As already stated 210 is not for feeder calculation, is only applied to the extent of determining branch circuit sizes, and how that may effect feeder size.
215.2(A)(1) gives you two options for calculation of feeder sizes. Both would need to be calculated then apply the largest load calculation to feeder size.
For 215.2(A)(1)(a) you need to know both continuous loads (125%) and non continuous loads (100%) then calculate total loads. (I believe this is sum of all loads at respective percentages).
For 215.2(A)(1)(b) you need to know maximum load to be supplied after adjustment factors applied. (I believe this is your demand load)
Your feeder loads are independently calculated by 220 applying 220(Part I) and 220(Part III), ( Part IV), or (Part V) requirements.
Now which calculation is larger. That is what size load you need to apply to feeder size.
Also See Informational Note 1 for 215.2(A)(1)(b) it references to see Annex D for examples of the various methods to arrive at part (b) size.

(A)(1)(b) is not about art 220 demand factors it is about ampacity adjustments for number of conductors in raceway and/or ambient temperature.

If you have a feeder load that you calculated with art 220, it is taken at 100%. No way are you separating continuous and non continuous out of that figure that has demand factors applied to portions of it, they have already accounted for what is needed in their methods there.

 

[xptpcrewx]

If you have a feeder load that you calculated with art 220, it is taken at 100%. No way are you separating continuous and non continuous out of that figure that has demand factors applied to portions of it, they have already accounted for what is needed in their methods there.

What about 125% of the largest motor? (220.50)

 

[wwhitney]

If you have a feeder load that you calculated with art 220, it is taken at 100%. No way are you separating continuous and non continuous out of that figure that has demand factors applied to portions of it, they have already accounted for what is needed in their methods there.

I don't believe that is correct. Article 220 doesn't discuss continuous loads. You might have a calculation where all the input loads to the Article 220 calculation are continuous, and then the result is obviously continuous. So it would get a 125% factor for 215.2(A)(1). Conversely all the input loads to the calculation might be non-continuous, and then the result is obviously non-continuous.

My first thought is that when doing an Article 220 calculation, all the inputs should be labeled continuous or non-continuous, and those labels should be tracked through the calculation and demand factors, so that the result is a continuous amount plus a non-continuous amount (the simple sum of which would match the unlabeled computation result).

My second thought is that you do the Article 220 calculation twice, once looking at all the loads, and then again looking at only the continuous loads. The latter computation would tell you what part of the first computation should be considered continuous; the difference is the non-continuous part.

I'm not sufficiently familiar with the various algorithms in Article 220 to know if the above two options would always give the same result, or if there is an example where they would differ.

Cheers, Wayne

 

[augie47]

In example D3(a)in Annex D they do size the feeder at 125% of the continuous and 100% of the non-continuous.

 

[Fred B]

(A)(1)(b) is not about art 220 demand factors it is about ampacity adjustments for number of conductors in raceway and/or ambient temperature.

If you have a feeder load that you calculated with art 220, it is taken at 100%. No way are you separating continuous and non continuous out of that figure that has demand factors applied to portions of it, they have already accounted for what is needed in their methods there.

The adjustment factors are only applied after the calculation of the raw loads.

NEC 2017 215.2(A)(1)(b) "The minimum feeder conductor size shall have an allowable capacity not less than the maximum load to be served after the application of any adjustment or correction factors."
215.2(A)(1) "General. 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......."
Part III includes demand factors in the adjustments for conductor size look at 220.42. Also informational note for 215.2(A)(1)(b) includes adjustments for voltage drop, with reference to 210.19(A)
When using 215.2(A)(1)(a) you would use Article 220 calculation before adjustment factors and then apply the 125% continuous and 100% of non continuous loads. Various loads are recognized as continuous or non continuous within the NEC, one means established by the definition in Article 100. As you are doing the calculations you will see specific loads meeting either cont. or non cont.

Use for example an overly simplified calculations for lighting load. If feeder is only serving the lighting load, in a dwelling unit load is at 3VA per sqft before any adjustment if you are dealing with 2500sqft you have before adjustment 7500VA. Lighting loads supplied with 120/240V and you have 31.25 amps as a starting point. If none of that load is continuous then for 215.2(A)(1)(A) you use only 100% or 31.25A. Lighting in a commercial space could fit definition of continuous load and then would be calculated at 125%
Now if you use 215.2(A)(1)(b) you apply adjustments to that for simplicity say your only adjustment is that found in 220.42. 1st 3000VA at 100% 3000/240 is 12.5A, leaving 4500VA at a factor of 35% leaving 1575VA/240V is 6.56A so now (b) provides 19.06A adjusted. If this is applied to feeder size general rule you would size feeder to the larger of either methods. This illustration feeder would be sized to 31.25a.

 

[wwhitney]

NEC 2017 215.2(A)(1)(b) "The minimum feeder conductor size shall have an allowable capacity not less than the maximum load to be served after the application of any adjustment or correction factors."

To be clear, this is meant as "The minimum feeder conductor size shall have an allowable capacity, after the application of any adjustment or correction factors, not less than the maximum load to be served." The adjustment and correction factors have nothing to do with Article 220. They refer only to (2017) 310.15(B)(2) and (3).

Cheers, Wayne

 

[hhsting]

The adjustment factors are only applied after the calculation of the raw loads.

NEC 2017 215.2(A)(1)(b) "The minimum feeder conductor size shall have an allowable capacity not less than the maximum load to be served after the application of any adjustment or correction factors."
215.2(A)(1) "General. 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......."
Part III includes demand factors in the adjustments for conductor size look at 220.42. Also informational note for 215.2(A)(1)(b) includes adjustments for voltage drop, with reference to 210.19(A)
When using 215.2(A)(1)(a) you would use Article 220 calculation before adjustment factors and then apply the 125% continuous and 100% of non continuous loads. Various loads are recognized as continuous or non continuous within the NEC, one means established by the definition in Article 100. As you are doing the calculations you will see specific loads meeting either cont. or non cont.

Use for example an overly simplified calculations for lighting load. If feeder is only serving the lighting load, in a dwelling unit load is at 3VA per sqft before any adjustment if you are dealing with 2500sqft you have before adjustment 7500VA. Lighting loads supplied with 120/240V and you have 31.25 amps as a starting point. If none of that load is continuous then for 215.2(A)(1)(A) you use only 100% or 31.25A. Lighting in a commercial space could fit definition of continuous load and then would be calculated at 125%
Now if you use 215.2(A)(1)(b) you apply adjustments to that for simplicity say your only adjustment is that found in 220.42. 1st 3000VA at 100% 3000/240 is 12.5A, leaving 4500VA at a factor of 35% leaving 1575VA/240V is 6.56A so now (b) provides 19.06A adjusted. If this is applied to feeder size general rule you would size feeder to the larger of either methods. This illustration feeder would be sized to 31.25a.

To be clear, this is meant as "The minimum feeder conductor size shall have an allowable capacity, after the application of any adjustment or correction factors, not less than the maximum load to be served." The adjustment and correction factors have nothing to do with Article 220. They refer only to (2017) 310.15(B)(2) and (3).

Cheers, Wayne

Then in Fred B post uses article 220 in place of the adjustment and correction factors 310.15(b)(2)(3) and that would be not correct? Then what is correct?

 

[Fred B]

To be clear, this is meant as "The minimum feeder conductor size shall have an allowable capacity, after the application of any adjustment or correction factors, not less than the maximum load to be served." The adjustment and correction factors have nothing to do with Article 220. They refer only to (2017) 310.15(B)(2) and (3).

Cheers, Wayne

I've had to read this a few times, so you saying in escense, after finding load amount, that number stays static, and as you apply the adjustment or correction factors to the wire size, you keep adjusting that wire size until after the adjustments (per 310) the load capacity of conductors now match your predetermined load? Thus the seaming oversized conductors found by my illustration for 215.2(A)(1)(a) when compared to that of (b), (a) was not calculated correctly, the factors found in 220 need to be applied to both prior to the adjustments found in part (b) from your statement.

It seem that the code may acknowledge there maybe a difference in results between the 2 methods when it says to use the largest of the 2.

So see if this is a better understanding, Example: Start with a XXAmp load (L), by itself says Z for conductor size, but after applying adjustments (c) that conductor no longer has an XXA capacity but now is L-c, (with adjustment) and L not=(L-c). So now using an educated guess you recalculate using a larger value for L call it L1, in the end L1-c must equal L. L=(L1-c), and then you use L1 ampacity for determining conductor size
It also seems that 220 calculations would come before that of the adjustments referenced in 215.2(A)(1)(a) or (b) in order to get to the value of (L)?

Article 220 is "Branch-Circuit, Feeders, and Service Load Calculations." From that it would appear Article 220 would definitely have something to do with getting to the conductor size, in that it gets you to the load requirements. 215 does directly makes reference to 220 requirements in determining feeder size. However I don't see any mention of 310 in Article 215. Or is this a case of "all other relevant code" requirements seen in many code articles?

Article 220 applying to both 215.2(A)(1)(a) and (b). Let's see if this looks better:

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If feeder is only serving the lighting load, in a dwelling unit load is at 3VA per sqft before any adjustment if you are dealing with 2500sqft you have before adjustment 7500VA. 220.42 application also applies. 1st 3000VA at 100% 3000/240 is 12.5A, leaving 4500VA at a factor of 35% leaving 1575VA/240V is 6.56A so now (a) provides 19.06A non continuous load at 100%. Lighting loads supplied with 120/240V and you have 31.25 amps as a starting point. If none of that load is continuous then for 215.2(A)(1)(A) you use only 100% or 31.25A. Lighting in a commercial space could fit definition of continuous load and then would be calculated at 125% 23.825A
Now if you use 215.2(A)(1)(b) you apply adjustments to that for simplicity say your only adjustment is that found in 220.42. 1st 3000VA at 100% 3000/240 is 12.5A, leaving 4500VA at a factor of 35% leaving 1575VA/240V is 6.56A so and now (b) provides 19.06A adjusted. Then apply adjustment from 310 If this is applied to feeder size general rule you would size feeder to the larger of either methods. This illustration feeder would be sized to 31.25a. 19.06A, In a residential unit. If an adjustment made per 310 for temp 310.15(B)(2)(a) 75° column, ambient temp 87-95° give adjustment of .94 it is now 17.916A. That is less than minimum allowable ampacity of 19.06A, so must size up, if use conductors rated to start with rated 20.4A by the adjustment of .94 you now have 19.176A which meets minimum ampacity required.

---

Does this look better?

 

[wwhitney]

Let's try this:

If a feeder is only serving the lighting load, in a dwelling unit that's 3VA per sqft before any load calculation, so if you are dealing with 2500sqft you have gross 7500VA. 220.42 now applies. 1st 3000VA at 100% 3000/240 is 12.5A, leaving 4500VA at a factor of 35% leaving 1575VA/240V is 6.56A so the load is 19.06A. If none of that load is continuous then for 215.2(A)(1)(a) you use only 100% or 19.06A. Lighting in a commercial space could fit the definition of a continuous load and then would be calculated at 125%, or 23.825A for 215.2(A)(1)(a).

Now for 215.2(A)(1)(b) you apply adjustment and correction factors to the table ampacities to determine the conductor size, but only at 100% of the load, even if the load is continuous. If an adjustment needs to be made per 310 for temp 310.15(B)(2)(a) 75° column, ambient temp 87-95° of 0.94, you need a conductor whose table ampacity is 19.06/0.94 = 20.28A to end up with an ampacity of 19.06A.

So if this 19.06A load is a continuous load, (a) controls and you will need a conductor with a table ampacity of 23.83A. While if this 19.06A load is a non-continuous load, then (b) controls and you will need a conductor with a table ampacity of 20.28A.

Cheers, Wayne

 

[Fred B]

Let's try this:

If a feeder is only serving the lighting load, in a dwelling unit that's 3VA per sqft before any load calculation, so if you are dealing with 2500sqft you have gross 7500VA. 220.42 now applies. 1st 3000VA at 100% 3000/240 is 12.5A, leaving 4500VA at a factor of 35% leaving 1575VA/240V is 6.56A so the load is 19.06A. If none of that load is continuous then for 215.2(A)(1)(a) you use only 100% or 19.06A. Lighting in a commercial space could fit the definition of a continuous load and then would be calculated at 125%, or 23.825A for 215.2(A)(1)(a).

Now for 215.2(A)(1)(b) you apply adjustment and correction factors to the table ampacities to determine the conductor size, but only at 100% of the load, even if the load is continuous. If an adjustment needs to be made per 310 for temp 310.15(B)(2)(a) 75° column, ambient temp 87-95° of 0.94, you need a conductor whose table ampacity is 19.06/0.94 = 20.28A to end up with an ampacity of 19.06A.

So if this 19.06A load is a continuous load, (a) controls and you will need a conductor with a table ampacity of 23.83A. While if this 19.06A load is a non-continuous load, then (b) controls and you will need a conductor with a table ampacity of 20.28A.

Cheers, Wayne

Tables 310.15(B)(2)(a) says to multiply the allowable ampacity by the correction factor shown in table not divid.
But when I threw some numbers in it seemed while technically inaccurate mathematically, it seemed to get it close enough to size a conductor by the tables. Even at 1000A the difference is only .17A