Continuous load calculation

[Smart $]

...

Hope this clears things up
Clear as mud

Good attempt and an "A" for effort... but I believe there should be 4 steps not including verification:

1. 110.14(C) termination temperature limitations
2. 210.19 or 215.2 minimum circuit conductor size
3. 210.20 or 215.3 coordination of conductor size through 240.4 and 240.6 with minimum OCPD
4. 310.15(B) ampacity adjustment

You mixed #1 and #2 hereinabove into one step. Additionally, there should be no reason to verify... the largest of the 4 minimum size determinations is the one.

 

[Dennis Alwon]

Table 310.16 #1 THHN @ 90? = 150 amps
150 x 80% = 120 amps

So if the minimum branch circuit must be 125 amps then how can #1 satisfy that need if after derating it is only good for 120 amps?

 

[kevin]

Although this is my first post to this thread, I voted in the poll for 1/0 with a 125A OCPD. I have subsequently read Jim Pauleys excellent article and request that my vote be changed to #1 with a 125A OCPD.

 

[Smart $]

So if the minimum branch circuit must be 125 amps then how can #1 satisfy that need if after derating it is only good for 120 amps?

Minimum branch-circuit conductor size is non-continuous plus 125% continuous... before any adjustment. Derating is an adjustment.

 

[Smart $]

PS: After derating, the adjusted allowable ampacity must be equal to or greater than the load served. In this thread's example, the derated ampacity must only be 100A or greater.

 

[Dennis Alwon]

I agree with you but that makes no sense. If I have conductors that have no derating involved they must be good for 125 amps however if I have derating the conductor only needs to be as good as 100 amps. Thus the actual ampacity of the conductors is less than if there is no derating.

In this case say there are only 3 ccc in the conduit. The ampacity needs to be 125 amps, which I believe we all agree. No derating so a #1 is needed since we use the 75C rating.

Now add 3 more ccc's in this conduit and then #1 is still good for the load. The ampacity of the branch circuit that is supposed to be 125% of the load is now almost equal to the load and not the load + 125%.

No need to explain- I do get it but it feels wrong.

 

[Smart $]

I agree with you but that makes no sense. If I have conductors that have no derating involved they must be good for 125 amps however if I have derating the conductor only needs to be as good as 100 amps. Thus the actual ampacity of the conductors is less than if there is no derating.

In this case say there are only 3 ccc in the conduit. The ampacity needs to be 125 amps, which I believe we all agree. No derating so a #1 is needed since we use the 75C rating.

Actually the determining factor in this case, even with only 3 ccc in conduit, is the minimum ocpd rating of 125A. Since you cannot go to a lesser rated ocpd, the minimum conductor ampacity a 125A rated ocpd can protect is 111A (110A is the next lower standard rating). Because this ampacity rating is an adjusted allowable ampacity, it includes derating. Since there is no derating for 3 ccc, the minimum permitted is a #2 THHN at 130A (>111A). Verifying the other requirements:

110.14 ampacity of #2 at 75?C is 115A (>100A)
210.19/215.2 ampacity before adjustment is 130A (>100A?125%)
210.19/215.2 ampacity after adjustment is 130A (>100A)​

Now add 3 more ccc's in this conduit and then #1 is still good for the load. The ampacity of the branch circuit that is supposed to be 125% of the load is now almost equal to the load and not the load + 125%.

Read the first sentence of 210.19(A)(1) and 215.2(A)(1). A #2 THHN was good for 3 ccc... but now you have 6.

210.20/240.4/240.6 125A ocpd, 111A minimum circuit ampacity req'd, #2 THHN 130A derated to 80% is 104A... too small; #1 THHN 150A derated to 80% is 120A (>111A)

110.14 ampacity of #1 at 75?C is 130A (>100A)
210.19/215.2 ampacity before adjustment is 150A (>100A?125%)
210.19/215.2 ampacity after adjustment is 120A (>100A)​

No need to explain- I do get it but it feels wrong.

Sorry, sorta did anyway :roll:

...but I understand that it feels wrong. It's because the generally accepted understanding is not quite right

 

[Dennis Alwon]

You lost me smart. I did read the first sentence but that doesn't change the fact that the circuit must be 125 amps. #2 can be protected with a 125 amp breaker but it I don't see how it can carry the continuous and non continuous load which is 125 amps. You are always a step ahead but you are now blowing my mind.:grin:

 

[Smart $]

You lost me smart. I did read the first sentence but that doesn't change the fact that the circuit must be 125 amps. #2 can be protected with a 125 amp breaker but it I don't see how it can carry the continuous and non continuous load which is 125 amps. You are always a step ahead but you are now blowing my mind.:grin:

The first requirement is NOT the circuit ampacity must be non-continuous plus 125% continuous. The first requirement IS that the conductor's ampacity (i.e. the circuit ampacity if less than the ocpd) must carry the maximum load. In this case, the maximum load is 100A.

An additional requirement is imposed that the conductor's ampacity, before adjustment, must not be less than the non-continuous plus 125% continuous.

Everyone wants to roll these two requirements up into one.

 

[yired29]

You lost me smart. I did read the first sentence but that doesn't change the fact that the circuit must be 125 amps. #2 can be protected with a 125 amp breaker but it I don't see how it can carry the continuous and non continuous load which is 125 amps. You are always a step ahead but you are now blowing my mind.:grin:

Dennis did you check out the links in posts 14, 15, and 34? If not please take a look at them.

 

[yired29]

Here is a Email I got from Jim Pauley form CMP 2 today.

Hi Chris:



I am pretty familiar with this issue. The text is saying that you basically apply the 125% factor to determine the minimum conductor size (taking the terminations in 110.14 into account). The application of adjustment factors can take into account the higher insulation ratings, etc if applicable.



Quick example:



200A continuous load ? I need a conductor that has an ampacity of 250A minimum (210.19(A) ? also need a 250A OCPD by 210.20(A)), . So a 250 kcmil would meet that requirement at 75C. In no case with the conductor be smaller than a 250 kcmil.



Now if I need to adjust for conduit fill, ambient, etc. ? then I can do that from the 90C column if applicable. That may mean I need conductor larger than 250 kcmil, but it can never get smaller. When I do apply the factors, I am looking at meeting two criteria ? can I carry the load (which is 200A)? and Am I protected by the overcurrent device (which is a 250A device)? So if I derate and get to say, 230A, then I am OK ? I can carry 200A and I can round up to a 250A OCP. If I derate and came up with 220A, then I need a larger conductor because I can only go to 225A on the OCP.



The mistake folks seem to make is attempting to derate and land at the 250A number rather than looking at the OCP and the load.



When I wrote the rule originally, I used the words ?without the application of any adjustment or correction factors?. During the next code cycle, there was a couple of proposals that changed it to ?before the application?? because they read ?without? to say you could skip the adjustments completely. I argued that folks would read ?before? and attempt to apply all of the factors in sequence. I lost the battle at the panel (but appear to have predicted correctly that it would be read wrong?J)



Jim Pauley