sid123456
Let me elaborate a little on the Derating deal...
In general, if we installed Conductors with Insulation having a 90ºC rating, we use the Maximum Ampacity listed in Table 310-16 under the 90ºC column as a "Base Figure", then apply adjustment factors accordingly to end up with a "Derated Value".
We still Terminate according to Article 110.14(c)(1) - meaning the highest Over Current Protection Device we will use is based on that shown in 110.14(c)(1).
**** Exception: Motor Circuits: OCPD for Ground fault and Short Circuit is based on Table 430.22(b), and Conductor load is based on 110.14(c)(1)... more on this later.
Here are some simple Derating examples - each having an Ambient Temperature of 30ºC, and using a common Raceway of greater than 24" length:
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Example #1: System = 120/240V 1 Phase 3 Wire.
Four (4) 3 wire Multiwire Circuits in the same Raceway.
Twelve # 12 THHN cu.
Eight Current Carrying Conductors
(if you have questions why 8 CCCs instead of 12, please let me know)
Maximum Amperage for # 12 THHN cu per 310.16 is 30 Amps
Adjustment Factor for 7-9 Current Carrying Conductors ("CCCs") per Article 310.15(b)(2)(a) = 70%
30 Amps x 0.7 = 21 Amps
Maximum Over Current Protection Device (OCPD) = 20 Amps.
If there were Five 3 wire Multiwire Circuits in the same Raceway, we would either Terminate the # 12s to a 15 Amp OCPD, or use #10s on a 20 Amp OCPD, as the adjustment factor would now be 50% for 10 to 20 CCCs in the same Raceway.
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Example #2: System = 208Y/120V 3 Phase 4 Wire.
Two (2) 4 wire Multiwire Circuits in the same Raceway.
Eight # 12 THHN cu.
Eight Current Carrying Conductors
(if you have questions why 8 CCCs instead of 6, please let me know)
Maximum Amperage for # 12 THHN cu per 310.16 is 30 Amps
Adjustment Factor for 7-9 Current Carrying Conductors ("CCCs") per Article 310.15(b)(2)(a) = 70%
30 Amps x 0.7 = 21 Amps
Maximum Over Current Protection Device (OCPD) = 20 Amps.
Here, if we add just a single 2 wire circuit with the multiwire circuits, the total will be 10 CCCs.
Adjustment factors = 50% for 10-20 CCCs, so once again either we Terminate the #12s at 15 Amps, or use #10s Terminated at 20 Amps.
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Example #3: System = 480Y/277V 3 Phase 4 Wire.
Parallel Feeders in common Raceway (Conduit).
400 Amp Panelboard Feeder
Eight #250 Kcmil THHN cu in same raceway.
250 Kcmil @ 90ºC = 290 Amps
290 x 0.7 (7-9 CCCs in raceway) = 203 Amps per Conductor
203 Amps x 2 = 406 total Amps
If we used Eight 4/0 THHN cu in the same raceway, the maximum OCPD would be 350, as the "Derated Value" would be 182 Amps per Conductor, or 364 Amps combined across two Parallel Conductors.
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Example #4: System = 208Y/120V 3 Phase 4 Wire.
Parallel Panelboard Feeders - in separate raceways:
400 Amp Panelboard Feeder, fed with Two (2) 2-1/2" Conduits - each containing Four (4) 4/0 THHN cu + One (1) #3 EGC
Maximum Amperage of 4/0 THHN cu = 260 Amps
4-6 CCCs in same raceway = 80% adjustment factor
260 Amps x 0.8 = 208 Amps Maximum.
Two separate runs = 416 Max Amps.
Maximum OCPD for these Feeders = 400 Amps
(Since the Conductors are larger than #1, &/or are for a Circuit of 100 Amps or larger, then we may Terminate using the 75ºC column of 310-16)
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Motor Circuits:
The total MCA computed for a motor Circuit may be applied to the size of the Motor Circuit's Conductors, yet we may protect that Circuit with an OCPD that is much larger than what is "typically used" per 310.16 and 110.14
For example, a Single Motor might calc out to have a total Amperage value of 16 Amps, so we use #12 THHN cu for the Circuit Conductors.
This Motor will be started Across-The-Line, and so we adjust our Branch Circuit's "Ground Fault & Short Circuit Protection" (the Circuit Breaker in the Panelboard feeding that Branch Circuit) to be 250% of the Minimum Circuit Ampacity we have calculated for the Motor - in this case 16 x 2.5 = 40 Amps, so we Terminate the #12s to a 40 Amp Circuit Breaker.
If we were using Motor Circuit Breaker (Instantaneous Trip Breaker) we could rate that as high as 800% of the MCA - which results in the Breaker being 125 Amp rated.
Nevertheless, the actual Circuit Conductors must be sized to carry the maximum amperage value calculated for the Motor Circuit.
In these cases, the actual Amperage values in Table 310.16 may be used, if no adjustment factors are required to derate the conductors.
Multiple Circuits in the same raceway, &/or higher ambient temperatures will determine what - if any adjustments are needed.
Table 310.15(b)(2)(a) determines the adjustment factors for multiple CCCs in a raceway, and the Ambient Temperature Correction Factors in Table 310.16 (found at the bottom), determine the adjustment for Ambient Temperature.
Both of these factors combined determine the overall Conductor's "Derating Figure", and are applied to the appropriate column of 310.16 for maximum Amperage.
Hope this clears things up.
Let me know if you have additional questions.
Scott
Last edited by Scott35; 03/09/08 12:41 AM. Reason: left out "80%" adjustment in example #4
