Ampacity adjustment confusion

[cppoly]

210.19(A)(1) says conductors must be sized for 100% non continuous load + 125% continuous load before ampacity adjustment. Why is this before and not after? Let's say there's an 18A non continuous load on 60 degree terminals. #12's satisfy this requirement. What if there's ampacity adjustment of 50% that is required. This puts #12's at 15A ampacity based on THHN conductor. The overcurrent device will be sized in accordance with conductor ampacity, but it doesn't make sense that the conductors aren't sized to meet the load after ampacity adjustment.

 

[david luchini]

but it doesn't make sense that the conductors aren't sized to meet the load after ampacity adjustment.

You are ignoring the first sentence of 210.19(A)(1)..."Branch-circuit conductors shall have an ampacity no less than the maximum load to be served."

This would preclude your using a #12 THHN with a 50% ampacity adjustment for an 18A load.

 

[charlie b]

210.19(A)(1) says conductors must be sized for 100% non continuous load + 125% continuous load before ampacity adjustment.

That is not what it says :happynoaccording to "Charlie's Rule" :happyyes: ). It says that the minimum conductor size (before adjustments and corrections) must have that ampacity. That is not the entire story. Let's take a non-continuous load of 13 amps and a continuous load of 4 amps. Accounting for the 125% factor, that gives us a calculated load of 18 amps. So no matter what else has to be taken into account in the next design steps, be it an "adjustment" (i.e., for the number of current-carrying conductors in a conduit), or be it a "correction" (i.e., for ambient temperature), the conductor has to end up with an ampacity of at least 18 amps.

This is how I read the meaning of the word "before" in the context under discussion: Before you start dealing with the other issues (i.e., the adjustments and the corrections), you need to calculate the required ampacity. That will tell you the answer you need to arrive at, after you do the adjustments and corrections.

 

[david luchini]

That is not what it says :happynoaccording to "Charlie's Rule" :happyyes: ). It says that the minimum conductor size (before adjustments and corrections) must have that ampacity. That is not the entire story. Let's take a non-continuous load of 13 amps and a continuous load of 4 amps. Accounting for the 125% factor, that gives us a calculated load of 18 amps. So no matter what else has to be taken into account in the next design steps, be it an "adjustment" (i.e., for the number of current-carrying conductors in a conduit), or be it a "correction" (i.e., for ambient temperature), the conductor has to end up with an ampacity of at least 18 amps.

This is how I read the meaning of the word "before" in the context under discussion: Before you start dealing with the other issues (i.e., the adjustments and the corrections), you need to calculate the required ampacity. That will tell you the answer you need to arrive at, after you do the adjustments and corrections.

I don't think that is quite right, Charlie. The load in your example is 17 amps, so the conductor has to end up with (after corrections/adjustments) an ampacity of at least 17 amps. The minimum conductor size, according to 210.19(A)(1), would be #14 (75deg), and the minimum OCPD size would be 20A per 210.20(A)...but 240.4(D) would require a minimum conductor size of #12 to be protected by a 20A OCPD.

 

[cppoly]

I don't think that is quite right, Charlie. The load in your example is 17 amps, so the conductor has to end up with (after corrections/adjustments) an ampacity of at least 17 amps. The minimum conductor size, according to 210.19(A)(1), would be #14 (75deg), and the minimum OCPD size would be 20A per 210.20(A)...but 240.4(D) would require a minimum conductor size of #12 to be protected by a 20A OCPD.

Notice how you used the word "after". That is my confusion in original post. The word "before" is used in 210.19(A)(1).

 

[david luchini]

Notice how you used the word "after". That is my confusion in original post. The word "before" is used in 210.19(A)(1).

Yes, there are two things that 210.19(A)(1) tells you.

First, the conductors must have an ampacity not less than the load to be served. The ampacity of the conductor would be AFTER any corrections/adjustments are made.

Second, the MINIMUM SIZE of the conductor, BEFORE any corrections/adjustments, shall have an allowable ampacity not less than the non-continuous load plus 125% of the continuous load.

The second part doesn't tell you what the ampacity of the conductor needs to be, it tells you what the minimum size of the conductor can be.

Example D3(a) in Annex D gives a good example of this (using 215.2(A)(1) for feeders, but the approach is the same)...
In the example there is a load of 51A non-continuous and 68A continuous, for a total load of 119A.

The MINIMUM CONDUCTOR size (before any adjustments) is based on 51A + 68A*1.25 = 136A, so 1/0 would be the minimum conductor size.

The example then shows an adjustment factor of 70% for number of CCCs, and a correction factor of 0.96 for ambient temperature.

Using a 2/0 90deg conductor, the ampacity of the conductor will be 195*0.7*0.96=131A. Since the corrected ampacity of 131 is large enough to carry the load (119A), the 2/0 is acceptable for the circuit.

 

[ggunn]

Stupid comment deleted.

 

[cppoly]

Yes, there are two things that 210.19(A)(1) tells you.

First, the conductors must have an ampacity not less than the load to be served. The ampacity of the conductor would be AFTER any corrections/adjustments are made.

Second, the MINIMUM SIZE of the conductor, BEFORE any corrections/adjustments, shall have an allowable ampacity not less than the non-continuous load plus 125% of the continuous load.

The second part doesn't tell you what the ampacity of the conductor needs to be, it tells you what the minimum size of the conductor can be.

Example D3(a) in Annex D gives a good example of this (using 215.2(A)(1) for feeders, but the approach is the same)...
In the example there is a load of 51A non-continuous and 68A continuous, for a total load of 119A.

The MINIMUM CONDUCTOR size (before any adjustments) is based on 51A + 68A*1.25 = 136A, so 1/0 would be the minimum conductor size.

The example then shows an adjustment factor of 70% for number of CCCs, and a correction factor of 0.96 for ambient temperature.

Using a 2/0 90deg conductor, the ampacity of the conductor will be 195*0.7*0.96=131A. Since the corrected ampacity of 131 is large enough to carry the load (119A), the 2/0 is acceptable for the circuit.

Good example. I see the application of how this is used. I think I am fighting over the language of the 210.19(A)(1) section. To me, when I see the word minimum I think "the least quantity or amount possible, assignable, allowable, or the like," as described by the definition. This means, if you say minimum, then it is acceptable to use at the very least. Part of 210.19(A)(1) says "the minimum branch circuit conductor size, before the application of any ampacity adjustment or correction factors, shall have an allowable ampacity of not less than the noncontinuous load plus 125% of the continuous load." However, this is not entirely true, since of course there is the stipulation of ampacity adjustment after the fact. So while this sentence is used to get a baseline ampacity, it is really is not THE minimum ampacity, as this is finally determined by the ampacity adjustment / correction factors as the final step of this process.

I have another question.

Check out the example in 220.20(A) after the exception. Check out Step 3 which calculates conductor adjusted ampacity. There's a 25A continuous load in the example, and to calculate the required ampacity of the conductor using the adjustment factor of 80%, it takes the load 25A and divides by 80% to get 31.25A. I don't get this. I thought ampacity adjustment factors are applied to the conductor's rated ampacity based on insulation. For instance, in your example from D3(A) in your post you took 2/0 conductor and applied the ampacity adjustment of .7*.96 to the conductor's rated 90 degree ampacity of 195A, which turned out to be 131A. However, according to this example in 220.20(A), it is taking the load and dividing by the ampacity adjustment. If we did this in your D3(A) example, of the load of 119A / (.7 * .96) = 177A! This would require 3/0 conductor. Something doesn't make sense.

 

[david luchini]

Good example. I see the application of how this is used. I think I am fighting over the language of the 210.19(A)(1) section. To me, when I see the word minimum I think "the least quantity or amount possible, assignable, allowable, or the like," as described by the definition. This means, if you say minimum, then it is acceptable to use at the very least. Part of 210.19(A)(1) says "the minimum branch circuit conductor size, before the application of any ampacity adjustment or correction factors, shall have an allowable ampacity of not less than the noncontinuous load plus 125% of the continuous load." However, this is not entirely true, since of course there is the stipulation of ampacity adjustment after the fact. So while this sentence is used to get a baseline ampacity, it is really is not THE minimum ampacity, as this is finally determined by the ampacity adjustment / correction factors as the final step of this process.

On the contrary, the statement is entirely true. What you seem to still be missing is that the second sentence in 210.19(A)(1) establishes a minimum conductor SIZE, not a minimum conductor ampacity.

the minimum branch circuit conductor size, before the application of any ampacity adjustment or correction factors

For example, if you had a 100A load which consisted of 80A non-continuous load and 20A continuous load, and you wanted to use a #3 THHN in a 100degF ambient, the conductor would have a corrected ampacity of 115*0.91 =105A, so #3 THHN has a sufficient ampacity for the load. HOWEVER, the second sentence in 210.19(A)(1) requires a minimum conductor size, before correction, that has an ampacity that is not less than the non-continuous load + 125% of the continuous load. This would be 80A + 1.25*20A = 105A. #3 (at 75deg) has an allowable ampacity of 100A which is less than 105. That would make the minumum conductor size for the circuit #2. You can't use the #3 THHN, even though its corrected ampacity is sufficient for the load, because it is smaller than the minimum allowable conductor size.

The application of the adjustment/correction factors do not change the minimum allowable conductor size.

I have another question.

Check out the example in 220.20(A) after the exception. Check out Step 3 which calculates conductor adjusted ampacity. There's a 25A continuous load in the example, and to calculate the required ampacity of the conductor using the adjustment factor of 80%, it takes the load 25A and divides by 80% to get 31.25A. I don't get this. I thought ampacity adjustment factors are applied to the conductor's rated ampacity based on insulation. For instance, in your example from D3(A) in your post you took 2/0 conductor and applied the ampacity adjustment of .7*.96 to the conductor's rated 90 degree ampacity of 195A, which turned out to be 131A. However, according to this example in 220.20(A), it is taking the load and dividing by the ampacity adjustment.

Multiplying a specific conductor ampacity by correction factors to find the final conductor ampacity, or dividing the required circuit ampacity by the correction factors to find the a required conductor size are just two sides of the same coin.

For instance, in D3(a) the ampacity required for the load is 119A. I could check 1/0 THHN and see that the corrected/adjusted ampacity would be 170A * 0.7 * 0.96 = 114A. That would be too small for the 119A load. Checking 2/0 THHN, I see that the corrected/adjusted ampacity would be 195 * 0.7 * 0.96 = 131A, which is large enough for the load.

The other side of the coin would be to take the required ampacity for the load and divide it by the adjustment/correction factors: 119A / (0.7 * 0.96) = 177A. I would then see what conductor has an ampacity of at least 177A. Looking at 1/0 THHN I see 170A which is smaller than 177, so 1/0 THHN is too small. Looking at 2/0 THHN I see 195A which is larger than 177, so 2/0 THHN is acceptable.

If we did this in your D3(A) example, of the load of 119A / (.7 * .96) = 177A! This would require 3/0 conductor. Something doesn't make sense.

As noted above, the ampacity of 2/0 THHN is 195. So only a 2/0 conductor would be required.

 

[cppoly]

Thanks david. That cleared things up.

My mistake was that I was mixing ampacity and minimum conductor size terminology and also their application. Also something I just picked up when following your example, when determining ampacity of a conductor, this does not involve sticking to the terminal rating of equipment. So in the example of 177A required ampacity, I was looking at the 75 degree column and said 3/0 are required. Ampacity is strictly based off the conductor insulation and applying derating factors.

However, determining minimum conductor size takes into account the terminal rating of the equipment. I think I was mixing these two.

Minimum conductor size calculations(100 + 125%) is used for making sure the terminals of equipment can dissipate heat fast enough.

 

[david luchini]

Thanks david. That cleared things up.

My mistake was that I was mixing ampacity and minimum conductor size terminology and also their application. Also something I just picked up when following your example, when determining ampacity of a conductor, this does not involve sticking to the terminal rating of equipment. So in the example of 177A required ampacity, I was looking at the 75 degree column and said 3/0 are required. Ampacity is strictly based off the conductor insulation and applying derating factors.

Yes, that is correct. You will find this in 110.14(C) where it says "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both."

But you also must remember that you with the adjusted ampacity of the higher temperature rated conductor, you can't carry a load higher than 75deg rated ampacity.

For instance, 2/0 is rated for 175A at 75 deg. If you used a 2/0 THHN corrected for 90 deg F, the ampacity of the conductor be 195 * 0.96 = 187A. Even though the conductor has an ampacity of 187A, you are limited to a 175A load per 110.14(C).

(And as an aside, you must also comply with 240.4 for the protection of the conductor with the adjusted/corrected ampacity. For example, if you had a 115A continuous load on a feeder, then 215.2(A)(1) would require a 1/0 minimum conductor, and 215.3 would require a 150A minimum circuit breaker. If you wanted to use 1/0 THHN, and the feeder had to be adjusted for 6 ccc's, and corrected for 100degF ambient, then the ampacity of the 1/0 would be 170 * 0.8 * 0.91 = 124A. In this case, 1/0 is not smaller than the minimum conductor size, and the corrected/adjusted ampacity is large enough to carry the load, so the 1/0 complies with 215.2(A)(1). HOWEVER, the 1/0 with an ampacity of 124A would NOT be properly protected by the 150A c/b per 240.4(B).)