Procedure for sizing general conductors and OCPDs

A

alej27

Member
Location
Venezuela
Occupation
Electrical Engineer
I’d like to know if the following step-by-step procedure is correct to size the conductors and protections of feeders and branch-circuits, based on the 2020 NEC. I want to make sure I understand the NEC correctly regarding this.

Procedure only for current-carrying conductors; no ground wires nor balanced neutral wires. And only overload protection is considered, no short-circuit protection is considered. The load is assumed as given.
1. Data
  • Feeder or branch-circuit.
  • Continuous load: 30 A.
  • Non-continuous load: 20 A.
  • Current-carrying conductors (CCC) in same raceway: 4.
  • OCPD rated 80%.
  • Ambient temperature: 40°C.
  • Terminal temperature rating (OCPD): 75°C.
  • Conductor insulation temperature rating: 90°C.
  • Not all conditions of sec. 240.4(B) are satisfied, so such exception can't be applied.
  • Voltage drop isn't a concern. And the NEC doesn't even require voltage drop consideration. So it's omitted.
  • Short-circuit protection (OCPD interrupting rating) isn't considered for simplicity, neither for the OCPD nor for the conductor size.
2. OCPD selection
  • Minimum OCPD size [sec. 215.3]: 30 A * 1.25 + 20 A * 1.00 = 57.5 A.
  • Minimum OCPD standard size [table. 240.6(A)]: 60 A.
3. Conductor selection by ampacity without considering OCPD
  • Case a) (sizing by ampacity by continuous load multiplier):
    • Minimum conductor ampacity [sec. 215.2(A)(1)(a)]: 30 A * 1.25 + 20 A * 1.00 = 57.5 A.
    • Minimum conductor gauge at an insulation temperature rating equal to the terminal temperature rating (75°C) [table 310.16 and sec. 110.14(C)(1)(a) item (3)]: #6 AWG (ampacity of 65 A at 75°C insulation).
  • Case b) (sizing by ampacity by ambient temperature and number of CCCs in same raceway):
    • Total load: 30 A + 20 A = 50 A.
    • Conductor ampacity correction factor for more than 3 CCC in same raceway [table 310.15(B)(1)]: 0.91.
    • Conductor ampacity correction factor for ambient temperature different from 30°C [table 310.15(C)(1)]: 0.80.
    • Minimum conductor ampacity [sec. 215.2(A)(1)(b)]: (50 A)/(0.91 * 0.80) = 68.68 A.
    • Minimum conductor gauge at an insulation temperature rating equal to the conductor insulation rating (90°C) [table 310.16 and sec. 310.15(A) second paragraph]: #6 AWG (ampacity of 65 A at 75°C insulation).
  • Final selection by ampacity without considering OCPD: the one that satisfies both cases [sec. 215.2(A)(1)], that is the thicker wire or equivalently the one with greater ampacity: both cases yield the same, so #6 AWG.
4. Conductor selection by ampacity considering OCPD
  • Selection of allowed ampacity of conductors:
    • Conductor ampacity at an insulation temperature rating equal to the terminal temperature rating (75°C) [table 310.16]: 65 A.
    • Conductor ampacity at an insulation temperature rating equal to the conductor temperature rating (90°C), and derrated by temperature and CCCs: 75 A * (0.91 * 0.80) = 54.6 A.
    • The lowest of the ampacities is chosen [sec. 310.14(A)(2)]: 54.6 A. This is the allowed ampacity of the conductors. And the OCPDs must be sized according to this allowed ampacity [sec. 240.4].
  • Is the previous allowed ampacity equal to or greater than the OCPD size? If yes, then the calculated final conductor gauge remains (#6 AWG at 90°C insulation). If no, but all conditions of sec. 240.4(B) are satisfied, then the calculated final conductor gauge remains (#6 AWG at 90°C insulation). If no, and not all conditions are satisfied, then the conductor gauge must be increased.
    54.6 A ≥ 60 A? No. And since it is assumed not all conditions of sec. 240.4(B) are satisfied, it's necessary to recalculate the minimum conductor gauge, this time based on the chosen OCPD standard size and not the load. It's not permitted to instead reduce the OCPD from 60 A to 50 A (the previous standard size) because then sec. 215.3 is violatted in that the minimum size must be 57.5 A.

  • Minimum conductor ampacity based on OCPD: (60 A)/(0.91 * 0.80) = 82.42 A.
  • Minimum conductor gauge at an insulation temperature rating equal to the conductor terminal rating (90°C) [tabla 310.16]: #4 AWG (ampacity of 85 A at 90°C insulation). This is the gauge by ampacity considering the OCPD.
I’d like to know if the formulas are correct, if the logic in the text is correct, if I’ve added an extra step or an extra factor in a formula, if I’ve missed a step, if I’ve done a step that’s actually not required by the Code, etc.
 
Correction 1: I accidentally swapped the tables for correction factor for CCCs and for ambient temperature. But the factors are correct.
Correction 2: In the last line, between the parentheses, the ampacity of the #4 AWG wire at 90°C insulation is not 85 A, but 95 A. But the gauge selection (#4 AWG) remains the same.

I'm still open to comments in my computations.
 
seemingly a perfect thread for wwhitney

my world is more simplistic... calculate the load (100% non continuous + 125% continuous);, select the conductor to carry that load (taking ambient & fill adjustments into account) and select the over-current device to protect that conductor per 240.4
 
As a final step: It is confirmed the chosen conductor (with an ampacity at the terminals temperature rating derrated for continuous use of 85 A/1.25 = 85 A * 0.80 = 68 A, and with an ampacity corrected for the conditions of use of 95 A * (0.91 * 0.80) = 69.16 A) is protected by the OCPD (with a current rating of 60 A).
 
Jraef said:
That is nowhere to be found in the NEC by the way...
Click to expand...
It’s not explicit in the NEC, but all sections regarding overcurrent protection (branch-circuits, feeders, PV systems, etc.) say something in the lines of “if the OCPD is listed for operation at 100% of its rating, the minimum ampacity of the conductors does not require the 125% factor for continuous loads”, which for me is to say that OCPDs not listed 100% are therefore “listed” 80% (the inverse of 125%.)

IF, as per augie47's more simplistic approach, you size the conductors at 125% of the the load, then size the OCPD for the conductor, you END UP with the OCPD rated at 80%, but only because 80% is the inverse of 125%...
Click to expand...
But that’s half the story, it does not consider ampacity derrating for conductor bundling and ambient temperature, don’t you think?
 
alej27 said:
It’s not explicit in the NEC, but all sections regarding overcurrent protection (branch-circuits, feeders, PV systems, etc.) say something in the lines of “if the OCPD is listed for operation at 100% of its rating, the minimum ampacity of the conductors does not require the 125% factor for continuous loads”, which for me is to say that OCPDs not listed 100% are therefore “listed” 80% (the inverse of 125%.)
Click to expand...
That is merely because the tolerance of a breaker is 20% unless otherwise listed. So a 20A breaker may start its long process of tripping on thermal at 16 amps, or may never trip before reaching 24 amps.

Also, why do you have 4 CCC's in a conduit. In very few cases is the feeder load heavy enough to require the neutral to be considered a current carrying conductor.
 
alej27 said:
It’s not explicit in the NEC, but all sections regarding overcurrent protection (branch-circuits, feeders, PV systems, etc.) say something in the lines of “if the OCPD is listed for operation at 100% of its rating, the minimum ampacity of the conductors does not require the 125% factor for continuous loads”, which for me is to say that OCPDs not listed 100% are therefore “listed” 80% (the inverse of 125%.)


But that’s half the story, it does not consider ampacity derrating for conductor bundling and ambient temperature, don’t you think?
Click to expand...
In full context it does:

my world is more simplistic... calculate the load (100% non continuous + 125% continuous);, select the conductor to carry that load (taking ambient & fill adjustments into account) and select the over-current device to protect that conductor per 240.4
 
Strathead said:
That is merely because the tolerance of a breaker is 20% unless otherwise listed. So a 20A breaker may start its long process of tripping on thermal at 16 amps, or may never trip before reaching 24 amps.

Also, why do you have 4 CCC's in a conduit. In very few cases is the feeder load heavy enough to require the neutral to be considered a current carrying conductor.
Click to expand...
Conductor bundling doesn't have anything to do with the OCP selection. And it is only the ambient temperature at or around the breaker that affects the trip time of the breaker.
 
augie47 said:
In full context it does:

my world is more simplistic... calculate the load (100% non continuous + 125% continuous);, select the conductor to carry that load (taking ambient & fill adjustments into account) and select the over-current device to protect that conductor per 240.4
Click to expand...
It isn't as if the code isn't complex enough on its own.
 
augie47 said:
seemingly a perfect thread for wwhitney
Click to expand...
I think if you had put an @ sign in front of my username, I would have gotten a notification about this thread. At least most forums work that way.

augie47 said:
my world is more simplistic... calculate the load (100% non continuous + 125% continuous);, select the conductor to carry that load (taking ambient & fill adjustments into account) and select the over-current device to protect that conductor per 240.4
Click to expand...
That is sometimes overly conservative--the termination temperature tabular ampacity has to be at least 100% non-continuous load + 125% continuous load, but the adjusted and corrected ampacity only has to be 100% of the load, with no 125% continuous use factor.

Also, it sometimes violates 215.3/210.20, because in the last step, if 240.4(B) does not apply, the necessary OCPD may be smaller than the calculated minimum ampacity (100% non-continuous load + 125% continuous load).

Cheers, Wayne
 
alej27 said:
I’d like to know if the following step-by-step procedure is correct to size the conductors and protections of feeders and branch-circuits, based on the 2020 NEC. I want to make sure I understand the NEC correctly regarding this.
Click to expand...
Your procedure is correct and thorough.

A minor terminological note: the NEC definition of ampacity always refers to a value after ampacity correction and adjustment. Unfortunately a couple of the NEC sections relevant to this procedure misuse the term ampacity to mean "ampacity without correction and adjustment" or "tabular ampacity (directly from the table)." I recommend not making this confusion and always using different terms for ampacity and for values without correction and adjustment.

Cheers, Wayne
 
wwhitney said:
(Article 310.15 derating results & 240.4 OCPD)
may be smaller than the (Article 220) calculated minimum ampacity (100% non-continuous load + 125% continuous load).
Click to expand...
In that case doesn't 125% rule prevail, for both wire size & OCP?
 
So I would summarize the procedure as follows, for a given choice of OCPD type (100% rated or non-100% rated) and conductor insulation temperature rating:

1) Determine the minimum OCPD size per 215.3/210.20. Based on that, determine a minimum conductor ampacity based on 240.4.
2) A further minimum conductor ampacity is 100% of the total load.
3) The minimum tabular conductor ampacity (no ampacity adjustment or correction) at the termination temperature rating is 125% of the continuous load plus 100% of the non-continuous load. (The justification for a 125% factor here is weak, but that is what the NEC requires).
4) Pick the OCPD size from step 1 and the smallest wire gauge that satisfies the limits in steps 1, 2, and 3.

Cheers, Wayne
 
ramsy said:
In that case doesn't 125% rule prevail, for both wire size & OCP?
Click to expand...
You edited my quote sufficiently that I'm not sure what scenario you are referring to.

But I was saying that if you size the wire based on both a termination ampacity and final ampacity calculated from the "125% rule," that doesn't necessarily ensure the allowable OCPD size will be large enough. When 240.4(B) does not apply, the maximum OCPD size will be the next lower standard size. Which might not satisfy the "125% rule," in which case you'd have to upsize the wire until you can bump up the OCPD one size.

Cheers, Wayne
 
ramsy said:
The last sentence in Appendix D3(a) shows Table 250.122 also prevails over the derating schemes in Article 310.15.
Click to expand...
The OP is about sizing the ungrounded conductors, rules for sizing the grounded conductor and the EGC are indeed different.

Cheers, Wayne
 
Strathead said:
Conductor bundling doesn't have anything to do with the OCP selection. And it is only the ambient temperature at or around the breaker that affects the trip time of the breaker.
Click to expand...
Not entirely.

If you have to adjust a conductor's ampacity because of bundling, you may still end up needing say a 20 amp OCPD on a conductor larger than 12 AWG - because you adjusted it's ampacity in that application.
 
You must log in or register to reply here.
Top