Ampacity Derating for Depth

[ejohnson]

NEC Article 310.60(C)(2)(b) states that ??an ampacity derating factor of 6 percent per 300-mm (1-ft) increase in depth for all values of rho shall be permitted.?. Does the derating factor not decrease with an increase in depth to a point at which it becomes zero? If not, using this derating method, there is a point at which the conductor will have negative ampacity.
Example: 15kV 1/0 AWG shielded single conductor copper cable, no neutral (select smallest neutral size in table)

Using table in IEEE Std. 835-1994 (pp. 349):
75% load factor, 90 Rho, 90?C max. conductor temp. from table, 105?C desired conductor temp. rating, 25?C ambient temp. used in table, 21?C desired ambient temp., 0 for temp. rise due to dielectric loss, 0 for new temp. rise due to dielectric loss, 234.5 inferred temp. of zero electrical resistance, 48? burial depth to top of duct bank

Using formula from IEEE Std. 835-1994, Section 3.4.1 (pp. Intro-10): Adjusting for ambient temperature, the multiplier obtained is 1.03.

Using formula from IEEE Std. 835-1994, Section 3.4.2 (pp. Intro-10): Adjusting for maximum conductor temperature, the multiplier obtained is 1.08.

Derating for depth: 1.00 ? 1.5(.06) = .91

Ampacity rating for 1/0 cable with 1/6 neutral: 139A

I = 139*1.03*1.08*.91 = 140A (rounding down)
Same example with 240? (20-ft) burial depth to top of duct bank:

Derating for depth: 1-(20*12-30)/12*.06 = -.05

I = 139*1.03*1.08*-.05 = -7A (rounding down)
How can this be possible? Also, is there no reason to account for the difference in ampacity between conductors at the top of a duct bank and conductors at the bottom of the same duct bank?

 

[charlie b]

First of all, ampacities can be either read directly from a table (perhaps with a temperature correction or some other adjustment factor applied) or determined “under engineering supervision.” The NEC authors go no further than to presume that if the later method is used, the engineer in responsible charge of the calculation (i.e., the PE who will seal and sign it) knows how to handle such issues as temperature, RHO values, and burial depth. However, since you will someday be that person, I will take it a step further.

Your error is that you are attempting to use a derating factor in the middle of an engineering calculation. Take a closer look at the language of 310.60(C)(2)(b). That derating factor only applies to ampacity values that are presented in a table or in a figure. That is, it applies to values obtained from the NEC itself, not from anything the IEEE standards have to offer on the subject. If you are using an IEEE methodology, then you are doing the ampacity determination “under engineering supervision,” and you are going to have to know (independently of the NEC rules) how to handle the issue of burial depth.

How then, you may ask, would I deal with burial depth, in an engineering calculation? The short answer is by using the Neher McGrath method. This is a non-linear calculation with a vast great number of variables, and is far more complex that I would want to undertake alone. So I turn to any one of a number of available software packages that have that methodology built in. I create the system model, and let the software figure out the ampacities.

I can say that as you make the ductbank deeper and deeper, the ampacity values reported by the software get smaller. That makes sense, because it is more difficult to reject heat if you have to pass that heat through more and more dirt. However, in apparent contradiction to that trend, the ampacity of a conductor at the top of the ductbank will be lower than that of an identical conductor in the same circuit at the bottom of the ductbank. The reason for this anomaly is that internal to the ductbank you are not yet rejecting heat via dirt, and that the heat from the lower conductors rises and surrounds the higher conductors.

Regarding the derating factor of 6 percent per foot of burial depth, I interpret that in the following way. To start with, the assumed depth of the top of the ductbank is 30 inches (2.5 feet). It is a bit tricky to find that in the book, and I will leave that as a homework assignment. So let us assume that a conductor would have an ampacity of 100 amps, given a 2.5 ft. burial depth (to the top of the ductbank). Every extra foot of depth reduces the ampacity by a factor of (1.0 – 0.06), or a factor of 0.94. Then,
? If you make the depth 3.5 feet, the ampacity becomes 100 times (.94), or 94 amps.
? If you make the depth 4.5 feet, the ampacity becomes 94 times (.94), or 88.4 amps.
? If you make the depth 5.5 feet, the ampacity becomes 88.4 times (.94), or 83.1 amps.
? If you make the depth 6.5 feet, the ampacity becomes 83.1 times (.94), or 78.1 amps.
? Go deep enough, and the ampacity shrinks towards zero. But it never reaches zero.

 

[ggunn]

Regarding the derating factor of 6 percent per foot of burial depth, I interpret that in the following way. To start with, the assumed depth of the top of the ductbank is 30 inches (2.5 feet). It is a bit tricky to find that in the book, and I will leave that as a homework assignment. So let us assume that a conductor would have an ampacity of 100 amps, given a 2.5 ft. burial depth (to the top of the ductbank). Every extra foot of depth reduces the ampacity by a factor of (1.0 – 0.06), or a factor of 0.94. Then,
? If you make the depth 3.5 feet, the ampacity becomes 100 times (.94), or 94 amps.
? If you make the depth 3.5 feet, the ampacity becomes 94 times (.94), or 88.4 amps.
? If you make the depth 3.5 feet, the ampacity becomes 88.4 times (.94), or 83.1 amps.
? If you make the depth 3.5 feet, the ampacity becomes 83.1 times (.94), or 78.1 amps.
? Go deep enough, and the ampacity shrinks towards zero. But it never reaches zero.

Is that what you meant to say (all the depths 3.5 feet and the ampacity changing)? From context, it appears to me that you meant to add a foot on each line, i.e., 3.5 feet, 4.5 feet, etc. Cut and paste error, maybe?

 

[charlie b]

Is that what you meant to say (all the depths 3.5 feet and the ampacity changing)?

You are right. I have gone back and corrected my post. It was a cut-and-paste-and-forget-to-edit error on my part.

 

[jghrist]

I agree with Charlie but would add that if you get anywhere close to 20 feet, you should be using "enginnering supervision" instead of trying to apply the tables with corrections.

Of course, the ampacity might get negative if the depth approaches that of Hell where the ambient temperature exceeds the insulation rating.

 

[steve66]

Regarding the derating factor of 6 percent per foot of burial depth, I interpret that in the following way. To start with, the assumed depth of the top of the ductbank is 30 inches (2.5 feet). It is a bit tricky to find that in the book, and I will leave that as a homework assignment. So let us assume that a conductor would have an ampacity of 100 amps, given a 2.5 ft. burial depth (to the top of the ductbank). Every extra foot of depth reduces the ampacity by a factor of (1.0 ? 0.06), or a factor of 0.94. Then,
? If you make the depth 3.5 feet, the ampacity becomes 100 times (.94), or 94 amps.
? If you make the depth 4.5 feet, the ampacity becomes 94 times (.94), or 88.4 amps.
? If you make the depth 5.5 feet, the ampacity becomes 88.4 times (.94), or 83.1 amps.
? If you make the depth 6.5 feet, the ampacity becomes 83.1 times (.94), or 78.1 amps.
? Go deep enough, and the ampacity shrinks towards zero. But it never reaches zero.

Charlie:
I basically derate the same way you show it, but I don't think the NEC is specific about what we are derating by 6% .

Are we derating the basic cable ampacity? If that the case, the ampacity does go to zero. (A negative ampacity wouldn't make any sense, so anything negative would just be considered zero.)

Or are we derating the ampacity the wire has at a 1' higher burial depth? That gives the same result you described. That's the method I've used, because it doesn't make any sense for the ampacity to go clear down to zero. The deeper we go, the less difference another foot should make.

But I think its open to interpertation by the AHJ. (Although I've never had an AHJ question if a conduit was burried too deep.)

So for a 20.5' depth, some people might call it 18x6 or 108% derating which would leave us 0 ampacity.

But I think most people would agree with you and say its 0.94^18 or about 33% of the ampacity at a 2.5' depth.

 

[charlie b]

I agree, Steve, that the sentence can be interpreted two ways. And I can cite no code language that backs up my interpretation.

I no longer have access to any of the software packages that perform such calculations. I would be curious to learn what results they give. If anyone has the ability and the time, can you put a single cable in a single concrete-encased conduit, and vary the depth to the top of concrete, and tell us what happens to the calculated ampacity?

 

[jumper]

I agree, Steve, that the sentence can be interpreted two ways. And I can cite no code language that backs up my interpretation.

I no longer have access to any of the software packages that perform such calculations. I would be curious to learn what results they give. If anyone has the ability and the time, can you put a single cable in a single concrete-encased conduit, and vary the depth to the top of concrete, and tell us what happens to the calculated ampacity?

Free trial demo software, offered by the company who sells it. I have no clue as to its accuracy or anything else.

http://www.usi-power.com/Products & Services/USAmp/USAmp.htm