Equipment Termnation Temperature Limitations

[Fergie]

After sizing cables for installation conditions including ambient temperature, it is understood that the conductor ampacity must also be checked to verify that the equipment termination temperature limit is not exceeded. 60 Deg C or 75 Deg C temperature limits are used per Table 310.15(B)(16).

Our question is: For a 40 Deg C ambient (outside of the equipment enclosure) is an additional ambient temperature derating required on the 60 Deg C or 75 Deg C termination ampacity?

 

[augie47]

Perhaps I don't understand the question as it seems the 1st paragraph answered the 2nd but, for clarity, an ambient correction is required on the conductors. That correction factor may be applied to the ampacity based on the insulation (ie: a THWN-2 conductor can be adjusted based on it's 90° ampacity)
After all adjustments you can then check to make sure you ampacity does not exceed the termination temperatures shown in 110.14.

 

[Carultch]

After sizing cables for installation conditions including ambient temperature, it is understood that the conductor ampacity must also be checked to verify that the equipment termination temperature limit is not exceeded. 60 Deg C or 75 Deg C temperature limits are used per Table 310.15(B)(16).

Our question is: For a 40 Deg C ambient (outside of the equipment enclosure) is an additional ambient temperature derating required on the 60 Deg C or 75 Deg C termination ampacity?

As counterintuitive as it may seem, you don't need to correct your equipment/termination ampacities for ambient temperature. Only the ampacity of the wire itself, needs adjustment for ambient temperature, rooftop added temperature (if applicable), and conductor bundling.

110.14(C) states that conductors with temperature ratings greater than the rating of the equipment shall be permitted to be used for both temperature corrections and bundling ampacity adjustments.

As an example, consider a 60A circuit fully utilized, with a 40C ambient temperature and Cu THWN-2 wire. Terminations on both sides are listed for 75C in this example. No other derate factors apply.

Start with #6 Cu THWN-2. The 75C termination ampacity is 65A. #6 Cu is required for terminations.

Now look at the 90C column, where we have 75A. Correct this for ambient temperature, and elevated conduit temperature.
Derate of 40C = 0.912
75*0.912 = 68.4A
#6 is sufficient.

 

[Fergie]

Equipment Termination Temperature Limitations

Equipment Termination Temperature Limitations

Thank you for your response. I agree that it is 'counterintuitive' and that is why I asked the question.

 

[Smart $]

Thank you for your response. I agree that it is 'counterintuitive' and that is why I asked the question.

I don't see it as counterintuitive because the termination temperature limitation determination pads any continuous load current by 25% for minimum conductor sizing. For minimum conductor ampacity, the continuous load current is not padded.

 

[nukem2k5]

the termination temperature limitation determination pads any continuous load current by 25% for minimum conductor sizing. For minimum conductor ampacity, the continuous load current is not padded.

Necro'ing an older thread. What do you mean by this? I thought that conductor ampacity should be sized for 125% of the continuous load as per 210.19(A)(1)(a) and 215.2(A)(1)(a).

I'm still unclear why temperature adjustment factors do not apply to terminations. It would make sense to me in event that the terminations are made inside of a building where the climate is maintained to 26-30C. Otherwise, what makes the terminations special such that ambient temperature won't affect their performance/integrity? Is it because the ambient temperature adjustment is mainly to protect the insulation, of which a termination has none?


For example, below is a practice test problem that I'm pondering. Why would we not apply temperature correction to the #10 AWG termination rated for 60°C, making the effective ampacity of the termination equal to 30A x 0.82 = 24.6 amps? Surely the equipment to which the termination is mated could be outdoors in this 50°C environment.
.
.

A branch circuit is designed using 10 AWG THHN 90°C copper conductors. The ambient temperature is 50°C for a desert installation. The circuit uses a switch whose terminals are rated for 75°C and a circuit breaker whose terminals are rated for 60°C. What is the maximum permitted ampacity of the circuit?​

These are several issues in this problem. The first is the basic ampacity of the conductors. This is given by Table 310.15(B)(16) on Page 161 as 40 Amps. The temperature deration for 50°C is 0.82. (40A) x (0.82) = 32.8 Amps. This is the ampacity of the actual conductors. Now, we must look to see how much current the terminations can handle. One end is listed for 75°C but the other end is only listed for 60°C. Therefore, we must go with the smaller per Article 110.14(C) on Page 38. Per Article 110.14(C)(1), for 60°C, the maximum permitted ampacity is 30 Amps. So, although the conductor itself is good for 32.8 Amps, the ends are only good for 30 Amps. (Actually, one end is good for 30 Amps and the other is good for 35 Amps). In any event, the weakest link in the chain is 30A.​

(C) 30A​

 

[Smart $]

Necro'ing an older thread. What do you mean by this? I thought that conductor ampacity should be sized for 125% of the continuous load as per 210.19(A)(1)(a) and 215.2(A)(1)(a).

These two subsections are what gets coordinated with 110.14(C). It is actually easier to understand if you call this part the minimum conductor SIZE determination.

210.19(A)(1)(b) and 215.2(A)(1)(b) is the conductor AMPACITY determination. Note there is no 125% factoring of the continuous portion of the load in this part.

I've tried to get the CMP to rewrite this section to be more concise... but alas, they keep making it worse.

I'm still unclear why temperature adjustment factors do not apply to terminations. ...

I agree, correction for ambient should apply to terminations. The problem is convincing the CMP. On this one, they will not accept theory as substantiation. They want a comprehensive study with analysis of empirical evidence. What it amounts to is someone has to prove the current method has a fault. That "someone" will likely have to be an organization recognized by the panel as authoritative in the matter at hand.

 

[nukem2k5]

These two subsections are what gets coordinated with 110.14(C). It is actually easier to understand if you call this part the minimum conductor SIZE determination.

210.19(A)(1)(b) and 215.2(A)(1)(b) is the conductor AMPACITY determination. Note there is no 125% factoring of the continuous portion of the load in this part.

I'm confused. (a) says that the conductor ampacity should be at least equal to 125% of the continuous load. (b) says that the conductor ampacity should be not less than the maximum load, after applying any correction or adjustment factors. I assume (b) is referring to 100% of the expected load, regardless of whether it's continuous.

So the cable is sized for either:

125% of the continuous load WITHOUT adjustment factors,

OR

100% of the continuous load WITH adjustment factors (like ambient temperature, >3 conductors in a conduit, etc.) -- whichever is greater.

Is that right?

 

[Smart $]

I'm confused. (a) says that the conductor ampacity should be at least equal to 125% of the continuous load. (b) says that the conductor ampacity should be not less than the maximum load, after applying any correction or adjustment factors. I assume (b) is referring to 100% of the expected load, regardless of whether it's continuous.

So the cable is sized for either:

125% of the continuous load WITHOUT adjustment factors,

OR

100% of the continuous load WITH adjustment factors (like ambient temperature, >3 conductors in a conduit, etc.) -- whichever is greater.

Is that right?

either should be both

OR should be AND

whichever is greater should be [circuit ampacity is] whichever is less

...for the size and type of conductor used.

 

[Smart $]

To add to that, (a) is determined using the termination temperature limit column of 310.15(B)(16), while (b) is determined using the appropriate table and temperature column for the conditions of use and the type of conductor.

 

[nukem2k5]

the termination temperature limit column of 310.15(B)(16)

The temperatures in the columns of Table 310.15(B)(16) are referring to insulated conductor temperature ratings. Terminations aren't mentioned in the table.

So in the end, we apply the correction factor(s) (e.g. ambient temperature >30C; more than 3 conductors, etc.) -- and the 125% padding for continuous load -- only when selecting the cable ampacity, and not when assessing the terminatoin ampacity limit. Then, if the resulting corrected cable ampacity is greater than the equivalent size termination's allowable ampacity (using the column which corresponds to the termination's temperature rating), we select the cable size using the ampacities in that column which corresponds to the termination temperature rating (60C for <100 A circuits), without any further adjustments -- is this correct? If we're using 60C rated cable, the default method is to use the adjusted/corrected cable ampacity, since the cable and the termination are already at equivalent ambient temperature ratings and we only apply the correction factors to the cable ampacity.

Or did I confuse myself further?

 

[Smart $]

The temperatures in the columns of Table 310.15(B)(16) are referring to insulated conductor temperature ratings. Terminations aren't mentioned in the table.

Refer to 110.14(C)(1) general statement. You are using the table for a purpose other than stated in its headers.

So in the end, we apply the correction factor(s) (e.g. ambient temperature >30C; more than 3 conductors, etc.) -- and the 125% padding for continuous load -- only when selecting the cable ampacity, and not when assessing the terminatoin ampacity limit. Then, if the resulting corrected cable ampacity is greater than the equivalent size termination's allowable ampacity (using the column which corresponds to the termination's temperature rating), we select the cable size using the ampacities in that column which corresponds to the termination temperature rating (60C for <100 A circuits), without any further adjustments -- is this correct? If we're using 60C rated cable, the default method is to use the adjusted/corrected cable ampacity, since the cable and the termination are already at equivalent ambient temperature ratings and we only apply the correction factors to the cable ampacity.

Or did I confuse myself further?

The first sentence is not exactly correct but the latter part of the paragraph says you are getting there.

Some additional reading to help you over the hump...
http://www2.schneider-electric.com/...7/en_US/Wire Terminations 0110DB9901R2-02.pdf
The Code references are dated because of the NEC edition in effect when authored.