Feeder calc. w/cont. & derating

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charlie b said:
Not true, though I am not yet sure if it makes a difference. It does not say that this is the miinimum ampacity. What it says is that the smallest conductor you can use is one that has that ampacity.
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Charlie
You are correct. My post was" The way I am reading 210.19A1 is as follows:
Before the application of any adjustment or correction factors, the minimum
ampacity of a branch circuit that supplies continuous load and non-continuous
shall have an allowable ampacity of 125% of the cont load + 100% of the
non-cont load."

It should have read "Before the application of any adjustment or correction factors, the minimum size of a branch circuit that supplies continuous load and non-continuous shall have an allowable ampacity of not less than 125% of the cont load + 100% of the non-cont load."
I don't think it makes any difference.
 
I'd like to point out that the only reason that there will be differences between the two approaches being discussed is the 'round up rule' for breakers protecting conductors.

I am claiming that the 100% + 125% rule only applies to _conductor_ ampacity part of the time. But we both agree that this rule _always_ applies to the OCPD, and that a further requirement is that the OCPD must protect the conductor.

There are a limited number of specific and _small_ 'windows' given by 240.6 where conductors of a particular ampacity will be considered protected by OCPD of a larger trip rating. If the result given by 210.19(A)(1) falls into one of these windows under the interpretation that I am promoting, then I get a different answer from Bob. But outside of these 'windows' the OCPD requirement forces the conductor sizes to agree.

-Jon
 
This was taken from Mike Holts Web Site:

"With so many different Code rules that modify the general requirements, it does become overwhelming to a circuits conductor and overcurrent protection device. However the following steps and examples should help you understand the basic rules of conductor sizing and protection.

Step 1 - Size the overcurrent protection device in accordance with Sections 210-20(a), 215-3, and 384-16(d). These three NEC rules required the overcurrent protection device (breaker or fuse) be sized no less than 100% of the noncontinuous load, plus 125% of the continuous load. Section 240-6(a) contains the list of standard size overcurrent protection devices.

Step 2 - Select the conductor to comply with Sections 110-14(c), 210-19(a), 215-2, and 230-42(a). Sections 210-19(a), 215-2 and 230-42(a) required the conductor to be sized no less than 100% of the noncontinuous load, plus 125% of the continuous load. In addition, Section 110-14(c) requires a consideration of the temperature rating of the equipment terminals when sizing conductors. Section 110-14(c) requires the circuit conductors to be sized according to the 60?C column of Table 310-16 for equipment rated 100 amperes and less, unless marked otherwise, and equipment rated over 100 amperes must be sized to the 75?C column of Table 310-16.

Author’s Comment. These important Code rules were added to the 1993 and 1996 NEC and are often overlooked. The purpose of these new rules are to insure that the heat generated on the equipment terminals can be properly dissipated without damaging the conductors. For all practical purposes, most electrical equipment is design to accept conductors sized to the 75?C column of Table 310-16.

Step 3 - The selected conductor must be protected against overcurrent in accordance with Section 240-3. Section 240-3. This requires the branch circuit, feeder, and service conductors be protected against overcurrent in accordance with their ampacities as specified in Table 310-16. However, Section 240-3(b) permits "the next size up device" if the conductors are not part of a multioutlet branch circuit supplying receptacles, and the ampacity of the conductors does not correspond with the standard ampere rating of a overcurrent protection fuse or a circuit breaker as listed in Section 240-6(a), and the next higher standard rating selected does not exceed 800 amperes.

Author’s Comment. The ampacity of a conductor is it’s current rating in amperes that it can carry continuously, after applying conductor ampacity reduction factors for conductor bundling and ambient temperature. In addition, the 1996 clarified in Section 110-14(c), that the ampacity reduction of THHN (90?C) conductor is based on the conductors ampacity as listed in the 90?C column of Table 310-16 and not on the terminal temperature rating.

Branch Circuit Continuous Load Example: What size branch-circuit overcurrent protection device and conductor (THHN) is required for a 19 kVA of nonlinear loads (75?C terminals). The branch-circuit is supplied by a 208/120 volt, 4-wire, 3-phase, Wye connected system.

Step 1 - Size the overcurrent protection device in accordance with Sections 210-20(a) and 384-16(d). The first thing that we must do convert the nonlinear load from kVA to amperes:

Amperes = VA/(Volts x 1.732), Amperes = 19,000/(208 volts x 1.732), Amperes = 52.74 amperes, rounded to 53 amperes

The branch-circuit overcurrent protection device must be sized not less than 125% of 53 amperes, 53 amperes x 125% = 66 amperes. According to Section 240-6(a) we must select a minimum 70 ampere overcurrent protection device.

Step 2 - Select the conductor to comply with Sections 110-14(c) and 210-19(a). Section 210-19(a) also requires the branch-circuit conductor to be sized no less than 125% of the continuous load, 53 amperes x 125% = 66 amperes. We must select the conductor according to the 75?C terminals temperature rating of the equipment terminals. No. 6 THHN has a rating of 65 amperes at 75?C and can not be used, therefore we must select a No. 4 which has a rating of 85 amperes at 75?C.

Step 3 - The No. 4 THHN conductor must be protected against overcurrent in accordance with Section 240-3. We must verify that the No. 4 THHN is properly protected against overcurrent by the 70 ampere overcurrent protection device. Since we have more than three current-carrying conductors in the same raceway, we must correct the No. 4 THHN conductors ampacity as listed in the 90?C column of Table 310-16. Corrected Ampacity No. 4 THHN = Ampacity x Note 8(a) Adjustment Factor Corrected Ampacity No. 4 THHN = 95 amperes x 80% Corrected Ampacity No. 4 THHN = 76 amperes

The No. 4 THHN which is rated 76 amperes after ampacity correction is properly protected by a 70 ampere overcurrent protection device in compliance with the general requirements of Section 240-3.

Feeder Continuous Load Example: What size feeder overcurrent protection device and conductor (THHN) is required for a 184 ampere continuous load on a panelboard (75?C terminals) that supplies nonlinear loads. The feeder is supplied by a 4-wire, 3-phase, wye connected system.

Step 1 - Size the overcurrent protection device in accordance with Sections 215-3 and 384-16(d). The feeder overcurrent protection device must be sized not less than 125% of 184 amperes, 184 amperes x 125% = 230amperes. According to Section 240-6(a) we must select a minimum 250 ampere overcurrent protection device.

Step 2 - Select the conductor to comply with Sections 110-14(c) and 215-2. Section 215-2 also requires the feeder conductor to be sized no less than 125% of the continuous load, 184 amperes x 125% = 230 amperes. We must select the conductor according to the 75?C temperature rating of the panelboards terminals. No. 4/0 THHN has a rating of 230 amperes at 75?C.

Step 3 - The No. 4/0 conductor must be protected against overcurrent in accordance with Section 240-3. We must verify that the No. 4/0 THHN conductor is properly protected against overcurrent by the 250 ampere overcurrent protection device. Since we have more than three current-carrying conductors in the same raceway, we must correct the No. 4/0 THHN conductors ampacity as listed in the 90?C column of Table 310-16. Corrected Ampacity No. 4/0 THHN = Ampacity x Note 8(a) Adjustment Factor Corrected Ampacity No. 4/0 THHN = 260 amperes x 80% Corrected Ampacity No. 4/0 THHN = 208 amperes

The No. 4/0 THHN which is rated 208 amperes after ampacity correction is not considered protected by a 250 ampere overcurrent protection device. This is because "the next size up rule" in Section 240-3(b) would only permit a 225 ampere protection device on the 208 ampere conductor [240-6(a)]. Therefor we must increase the conductor size to 250 kcmil in order to comply with the overcurrent protection rules of Section 240-3
 
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Sorry, I didn't see the private messages.

There really isn't much that I can add to the above post; as I mentioned in the discussion it all hinges on the interpretation of "before the application of derating or correction factors". This can be read two different ways, and from those two methods one can derive the necessary method to apply the interpretation.

Above is quoted a _well respected_ authority which provided the correct method for one of the interpretations. The method derived from the selected interpretation is correct, so really this should be read as a very well informed and respected interpretation of the words.

I can try to formulate an analysis of why I believe the Mike Holt is wrong, but that is a pretty steep hill to climb :)

A rough outline of my thoughts on the topic would be:
1) The 125% rule for continuous load is clearly to protect the breaker, not the conductors (since we are permitted to bypass the 125% rule if we use 100% rated _breakers_)
2) Breakers of trip rating X may protect conductors of ampacity Y where Y is less than X, as long as we meet the rules for 'next size up standard sized breakers' and 'load less than Y'
3) The interpretation that I believe to be correct always meets the general requirement of ampacity greater than load and conductor protected by OCPD.

-Jon
 
Sizing Conductors, Jon and I against the world!

Sizing Conductors, Jon and I against the world!

Charlie, Bob, Winnie

A few months behind but I'm with Winnie. I think the key to the proof of accurate interpetation is the difference in "Minimum Ampacity" and "Minimum Size" as Charlie pointed out.

110.14(C) Temperature Limitations, requires me to use the temperture rating associated with the ampacity of a conductor based on the temperature rating of the connection(s). Also, I am permitted to use higher temperature rated conductors only for adjustment, correction, or both.

Paraphrasing even further, To me this means that anytime I want the ampacity of a conductor I use the 60 or 75 degree temperature column of Table 310.16 but if I have 90 degree wire then I can use that column only for the purposes of adjustment(>3 conductors) or correction (Ambient Temp.)

Back to 210.18(A), if we go down the Branch-Circuit road, The first sentence dictates the minimum ampacity can not be less than the max load. (100% continuous or not) To find the ampacity I should use the 75 degree column for the previous examples. If I need to adjust or correct and I have 90 degree wire I can use the 90 degree column. Based on my read of 110.14(C) I have two minimum ampacities, always, one at 75 degrees protecting the circuit breaker from overheating because of the connection and sometimes, two protecting the condutor from overheating because of more than 3 CCC's or higher Ambient Temp. For the the second min ampacity I can use the 90 degrees column if that matches the wire.

Now moving on to the "Minimum ... Conductor Size" mentioned in the second sentence of 210.19(A)(1) Now an additional minimal requirement is that the ampacity must not be less than 125% cont. + 100% non cont. before adj. or corr. factors. I believe the code uses the terminology "min ... cond size" in a failed attempt to be clear/accurate/consistent. "Before the application of any adjustment or correction factors" is almost the same thing as saying don't use the 90 deg column and don't adjust the numbers in the table for raceway fill or ambient temperature. (Remember you can only use the 90 deg column for adjusting and correcting.) This second minimal requirement is also designed to protect the circuit breaker. (or other component) By requiring the conductor to be larger the temperature of the conductor at the breaker lug under the same load will be less. Circuit breakers are normally tested at 80% of their rated load for continuous operation. Sometimes they are tested and listed to operate continuous at 100% rated load. In this case the Exception lets you out of the 125% requirment.

In this case it seems to me that if you read the sections mentioned here and take them at face value word for word, especially emphasising the two seperate requirements of minimum ampacity and minimum size then you will agree with Winnie.

I hope that the next code cycle will clarify this. The problem is to write the code to be accurate and to avoid requiring overdesign and at the same time avoid requiring to many steps just to size a conductor and to make it plain. It did't take long searching the internet and this site to see that a lot of pretty smart and code educated people see it different ways.


Ed Jackson, PE

Why can't we all just get along? ;)
 
After reading the iaei article and then reading the 2005 NEC example D3(a) I got to agree with winnie. Further Mike Holts process uses the derating for confirming the circuit breaker size and not the conductor. The way I read the code, I agree with Bob. However, the examples in the code book makes me go with Winnie. Sizing a conductor is one of the most important issues of an electrical design. I can't believe that we still get different interpretations on how to do this.
 
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