Have you read any of the links I provided?
Continuous load calculation
- Thread starter yired29
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Have you read any of the links I provided?
Rick Mack
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- Fort Mill, SC
Comments based on 2008
On the derate for # of conductors in raceway, you would start with the ampacity for
#1 THHN = 150A, then reduce ampacity to 80%. 150A * .80 = 120A
This is adequate for the load to be served (100A), and can be used with a 125A OCP
[240.4(B)] which has already been selected based on 125% of cont. load.
So #1 THHN with 125A OCP is the answer.
The key to getting this right, and where most people seemingly get it wrong, is where 210.19(A)(1) and 215.2(A)(1) state,"the minimum branch- [or 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."
Application of derating per Table 310.15(B)(2)(a) is an adjustment. For this 100A continuous load, the minimum size conductor must be rated 100A continuous ? 125% = 125A or greater, before adjustment, in the appropriate ampacity table, 310.16 here. For THHN, that would be #2.
Termination provisions of 110.14(C) requires the conductor size to be not less than #3 for 100A@75?C. We use actual load current here, not non-continuous plus 125% continuous loads.
For derating, the allowable ampacity is 80% of the table value. Reverse calculating, we need the conductor to have an ampacity equal to or greater than 100A ? 80%, which is the same as 100A ? 125%, or 125A. For THHN that is #2 at 130A.
So working through the three sizing requirements—minimum conductor size, termination temperature, and derating—what is the minimum size permitted under the conditions stated?
Are you saying a #2 is allowed? #2 at 90? = 130 amps. 130 x .8 = 104 amps. 104 amps would serve the load but would not be allowed on a 125 amp OCPD. 125 amp OCPD is the minimum for this load 210.20 (A) 100 x 125% = 125 amp OCPD.
No. Not for this question the majority of breakers are only rated 80%.
As I thought would happen the answers to this poll are all over. That was the point of this question. I would like to know what the minimum size conductor for this installation is based on NEC. This just shows the confusion of the language of before the application of adjustment and correction factors.
I do have another thought that maybe someone can answer. I see that some people say a # 2 "THHN" conductor with a 125 amp breaker. My question is the # 2 under the 75 "C" column is 115 amps are you allowed to use this wire and round up to the next standard size overcurrent device which is the required 125 amps? The wire being a # 2 "THHN" is rated under the 90 (C) column as 130 amps. I would not be comfortable with that myself.
No, I am not saying #2 is allowed. I asked what the minimum size conductor (though I inadvertently left out conductor in my last sentence) was per the conductor sizing rules stated. You are correct, the conductor size must additionally be coordinated with the OCPD rating per 210.20 for branch circuits and 215.3 for feeders, and thus 240.4 for conductor protection. With a required 125A minimum OCPD, per 240.4(B) and 240.6(A) the 310.15 conductor ampacity (including adjustment) must be greater than 110A.
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No a #2 would not be allowed on a 100 amp continuous load. For a 100 amp continuous load we need a conductor that has a minimum ampacity of 100 x 125% = 125 amps.
In the question it was stated 75? terminations.
A #2 has an ampacity of 115 amps at 75? this conductor is too small. This is without the adjustment for CCC's.
I say #1 THHN
I do not have the link. If you want to see a example of what is being discussed you can go to: ECM magazine march 1, 1998 article "The middle of the wire isn't its end". I can not say with certainty that what applied then apply's now. Hope this helps clear up some confusion.
The question was considering direct conductor sizing requirements only: 110.14(C), 210.19(A)(1) or 215.2(A)(1), and 310.15(B)(2)(a). Under these requirements, the minimum conductor size is #2 THHN.
Taking the question out of context as you have, it is only because of the minimum required OCPD rating and conductor protection rules the conductors must be sized at #1 THHN.
Rick Mack
Member
- Location
- Fort Mill, SC
I believe it is misleading to size the conductor at #2 THHN and not consider the other
requirements for a branch circuit which include the selecting of the OCP at 125% of
continuous. If we were only looking for the allowable ampacity of a particular conductor
under specified conditions of "no. of conductors/ambient temperature" to protect the
insulation from overheating, we could ignore the OCP requirement.
Sizing a conductor for a branch circuit has to consider the OCP.
That means the conductor in this case should be #1 THHN.
I do not believe anything has been taken out of context. The question asked What minimum size OCPD and THHN conductors for a 100 amp continuous load.
210.19 (A) (1) Would require a #1 we need 100 x 125% = 125 amps.
#1 has an ampacity of 130 amps @ 75?
#2 has an ampacity of 115 amps @ 75?
A #2 for a 100 amp continuous load would never be large enough unless 100%rated assembly and OCPD.
In the poll I left out the 6 CCC's and 75? terms. by accident but it was in the first post.
I'm not in disagreement overall. I was just trying to demonstrate how to determine minimum conductor sizing per conductor sizing rules as intended.. for all situations, not just the one posed in the OP.
The next step was going to point out that conductor protection rules may override that determination... but you guys can't answer a simple question without reading more into it than there actually is.
I was referring to your taking my question out of context, not your own.
Not per 210.19(A)(1) alone. Any table value of 125A or greater would satisfy its requirement. Your OP conditions limited the determination to THHN Cu, so #2 rated at 130A would meet the requirement.
Here I'm not absolutely certain what you are saying, but it appears you conglomerated OCPD requirements with conductor protection and termination temperature limitations. It is this type of determination that gets everyone confused. Take it one step, one rule at a time and the [final] determination is very clear.
100 amp continuous load THHN copper conductors 6 CCC's in a raceway 75? terminations.
Determine the minimum size OCPD and conductors.
Step 1 Minimum size OCPD
210.20 Minimum size OCPD = continuous load x 125%
100 x 125% = 125 amps
240.6 Standard size OCPD
125 amp OCPD is a standard size so this is the minimum size OCPD
Step 2 Minimum size conductors
210.19 (A) (1) The minimum size branch circuit conductors, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125% of the continuous load.
100 x 125% = 125 amps.
Table 310.16
We can't use 90? column based on 110.14 (C) (1) (b)
So the smallest conductor would be a #1 THHN 310.16 75? #1 = 130 amps.
210.19 (A) (1) has been satisfied along with 110. 14 (C) (1) (b) and 240.4 (B)
Step 3 Adjust for more than 3 CCC?s in a raceway
110.14 (C) Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
In step 1 we determined that a #1 was the smallest size conductor allowed.
Now there are 6 CCC's in a raceway Table 310.15 (B) (2) (a) 4-6 conductors adjust conductor ampacity to 80% of Table 310.16 ampacity.
Table 310.16 #1 THHN @ 90? = 150 amps
150 x 80% = 120 amps
Step 4 Verify conductor is sized properly
#1 THHN with an adjusted ampacity of 120 amps
Can this conductor terminate on OCPD determined in step 1. YES
Does this conductor comply with the minimum size conductor determined in step 2. YES
Does the conductor have a final ampacity not less than the maximum load to be served. YES
Does the conductor comply with the rules in 110.14 (C) YES
Remember the definition of ampacity
Ampacity. The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.
This question was to find out what everyone thought about adjustment and correction factors for continuous loads. Do the conductors have to have an ampacity of 125% of the continuous load after corrections and adjustments? (This is the way I had been taught.)
Or, do they need to have an ampacity not less than the maximum load to be served after adjustments and corrections along with following the rules in 240.4 (B), 110.14 (C) (1), and 210.19 (A) (1)? (The way I believe now.)
Hope this clears things up
Clear as mud
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