Confusing equation not making sense

[StevodaSparky]

so here is a question I originally got wrong on this test and then I got it right but based on the NEC it doesn't make sense. As far as I know the final decision should be on the weakest link in the circuit or in this case the lowest rated device or termination which is 75 degrees Celsius.

The answer is a. 97.5 Amps but if you calculate based on 75 degrees it comes out to b. 87.5 Amps

Can someone please explain?

Plus I have more equations that fit this same style if anyone wants to take a crack at it.


Based on the information below, what is the calculated allowable ampacity for the following electrical circuit installation?:

A raceway is installed inside a commercial building.
The raceway contains twelve (12) current-carrying, size 2/0 AWG copper conductors, with type THHN / THWN-2 insulation.
The conductors are supplying four (4), 3-phase loads.
The loads are electric kilns in a pottery shop with nameplates that specify “MCA” (Minimum Circuit Ampacity) ratings of 136 Amps.
Each kiln has termination ratings of 90°C.
All other electrical terminations in these circuits are rated @ 75°C.
Question 3Select one:
a.
97.5 Amps
b.
87.5 Amps
c.
72.5 Amps
d.
116 Amps

 

[wwhitney]

Let's see, Table 310.16 says that the ampacity of 2/0 Cu is 175A @ 75C and 195A @ 90C. So there are 3 different limitations:

(1) Source end termination limit. The problem specifies that this is a 75C termination. So this has a 175A ampacity.
(2) Run of the conductors. The conductor insulation temperature is 90C, so we start with 195A. There are 12 CCCs in the raceway, so the adjustment factor is 0.5. There is no mention of ambient temperature, so we will assume no correction is required. That makes an ampacity of 97.5A here.
(3) Load end termination. The problem specifies that this is a 90C termination. So this has an ampacity of 195A.

Calculation (2) is the lowest result, so 97.5A is the circuit ampacity. Note that the 0.5 adjustment factor in calculation (2) does not apply to the termination calculation, as at the terminations the conductors are no longer in a common raceway.

It is also worth noting that install is not NEC compliant, as the kilns require a minimum circuit ampacity of 136A, and 97.5A < 136A.

Cheers, Wayne

 

[StevodaSparky]

Let's see, Table 310.16 says that the ampacity of 2/0 Cu is 175A @ 75C and 195A @ 90C. So there are 3 different limitations:

(1) Source end termination limit. The problem specifies that this is a 75C termination. So this has a 175A ampacity.
(2) Run of the conductors. The conductor insulation temperature is 90C, so we start with 195A. There are 12 CCCs in the raceway, so the adjustment factor is 0.5. There is no mention of ambient temperature, so we will assume no correction is required. That makes an ampacity of 97.5A here.
(3) Load end termination. The problem specifies that this is a 90C termination. So this has an ampacity of 195A.

Calculation (2) is the lowest result, so 97.5A is the circuit ampacity. Note that the 0.5 adjustment factor in calculation (2) does not apply to the termination calculation, as at the terminations the conductors are no longer in a common raceway.

It is also worth noting that install is not NEC compliant, as the kilns require a minimum circuit ampacity of 136A, and 97.5A < 136A.

Cheers, Wayne

so I guess my confusion is why the 75C is not used. If the allowable ampacity should be based on the weakest link ( the 75C terminations) then why are we going off the 90C of the cable?

 

[roger]

See 110.14

 

[StevodaSparky]

Let's see, Table 310.16 says that the ampacity of 2/0 Cu is 175A @ 75C and 195A @ 90C. So there are 3 different limitations:

(1) Source end termination limit. The problem specifies that this is a 75C termination. So this has a 175A ampacity.
(2) Run of the conductors. The conductor insulation temperature is 90C, so we start with 195A. There are 12 CCCs in the raceway, so the adjustment factor is 0.5. There is no mention of ambient temperature, so we will assume no correction is required. That makes an ampacity of 97.5A here.
(3) Load end termination. The problem specifies that this is a 90C termination. So this has an ampacity of 195A.

Calculation (2) is the lowest result, so 97.5A is the circuit ampacity. Note that the 0.5 adjustment factor in calculation (2) does not apply to the termination calculation, as at the terminations the conductors are no longer in a common raceway.

It is also worth noting that install is not NEC compliant, as the kilns require a minimum circuit ampacity of 136A, and 97.5A < 136A.

Cheers, Wayne

So here's another one if you don't mind

Based on the following information, what standard size overcurrent protection device (OCPD) and what size copper conductors, with type XHHW-2 insulation, are required for the following circuit configuration?:

A individual 3-phase branch circuit that supplies a 42 ampere load, that will operate as a continuous load.
Some electrical terminations (connections) in the circuit are rated @ 75°C and some terminations are not temperature rated.
Question 5Select one:
a.
60 Amp OCPD with size 6 AWG conductors
b.
50 Amp OCPD with size 6 AWG conductors
c.
50 Amp OCPD with size 8 AWG conductors
d.
45 Amp OCPD with size 8 AWG conductors

I based it off 60C because if terminations are not labeled then that's what your supposed to use apparently. So why use the 60C here and not the 90C that the cable is rated for?

 

[StevodaSparky]

See 110.14

what about 110.14 C?

temperature limitations when connecting conductors to terminals and equipment. It requires that the temperature rating associated with the ampacity of a conductor be selected and coordinated to not exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with higher temperature ratings than the terminations may be used, but the ampacity must be based on the lowest temperature rating of the termination.

 

[roger]

Exactly, the answer did not exceed the 75 deg rating.

 

[StevodaSparky]

Exactly, the answer did not exceed the 75 deg rating.

I'm sorry we are talking about the first question right? about the 2/O. The answer is A) 97.5 which is based on the 90 deg rating. Sorry if this seems repetitive I just really want to understand the equation.

 

[wwhitney]

so I guess my confusion is why the 75C is not used. If the allowable ampacity should be based on the weakest link ( the 75C terminations) then why are we going off the 90C of the cable?

The short answer: the conductor temperature at the termination is basically not affected by the conductor temperature within the raceway. So if the termination is 75C and we size the conductor accordingly, the termination won't overheat. Even if the actual conductor temperature within the raceway is above 75C because we are taking advantage of 90C rated conductor insulation.

Cheers, Wayne

 

[wwhitney]

I'm sorry we are talking about the first question right? about the 2/O. The answer is A) 97.5 which is based on the 90 deg rating. Sorry if this seems repetitive I just really want to understand the equation.

You can think of sizing the conductor at a termination as a straight lookup. The equipment calls for a certain MCA, the termination has a particular temperature rating, you use that column of Table 310.16.

It might be clearer if the NEC didn't use the word ampacity to refer to this process and instead just called it termination sizing. So then you'd find the minimum conductor size for the terminations, and find the minimum conductor size for the ampacity (within the run of the raceway/cable), and the overall minimum would be the larger of the two.

Cheers, Wayne

 

[wwhitney]

I based it off 60C because if terminations are not labeled then that's what your supposed to use apparently.

Correct, 110.14(C)(1)(a) tells you when you are to assume that unmarked terminations are rated only 60C.

So why use the 60C here and not the 90C that the cable is rated for?

Well, the termination is only rated for 60C, so that's the column we use for termination sizing, and termination sizing will control. Now if we had a 0.5 ampacity adjustment factor, like in the previous example, we could still apply that to the 90C table ampacity to determine our conductor ampacity. Then I expect you'd find that ampacity controls the choice of size, rather than termination sizing.

BTW, as a test taking strategy, the continuous load requires a 125% sized breaker, so at least 52A, so you immediately know the answer has to be (a) without considering any temperature questions.

Cheers, Wayne

 

[StevodaSparky]

Correct, 110.14(C)(1)(a) tells you when you are to assume that unmarked terminations are rated only 60C.


Well, the termination is only rated for 60C, so that's the column we use for termination sizing, and termination sizing will control. Now if we had a 0.5 ampacity adjustment factor, like in the previous example, we could still apply that to the 90C table ampacity to determine our conductor ampacity. Then I expect you'd find that ampacity controls the choice of size, rather than termination sizing.

BTW, as a test taking strategy, the continuous load requires a 125% sized breaker, so at least 52A, so you immediately know the answer has to be (a) without considering any temperature questions.

Cheers, Wayne

Correct! I knew that one fairly quickly from learning what I have in the field. I journeyed out in a less conventional way I guess and I am really trying to educate myself and understand the math behind all the decisions that are made for sizing. Thank you for helping break things down for both questions. I will for sure have more to come