Cable Ampacity At Termination Question

[cgee]

We have a run of cable tray with sets of 500KCM RHH / RHW-2 copper conductors. We used NEC Table 310.20 which gave us an ampacity of 496A per conductor at 75 degree C.

These conductors are terminated in an AL/CU 90 degree C lug (the lug will accept a range of conductors up to 750KCM). The lugs are in a Square D Switchboard.

We understand that NEC 110.14 calls for the Table 310.16 conductor ampacity to be used at terminations inside equipment (only 380A for 500KCM).

The question is: Can we use the Table 310.16 ampacity of 750KCM copper (475A) at the termination, even though we are only terminating 500KCM?

Thanks.

 

[benaround]

Table 310.20 requires the conductors to be supported on a messenger, I don't beleive

cable tray is considered a messenger.

 

[benaround]

cgee, Check out 392.11 for ampacities and tables to be used for your situation. To

answer your question, IMO., no.

 

[mweaver]

Frank has you on the correct path in his post #3

Frank has you on the correct path in his post #3

Cgee,

As Frank, has noted in his posts your conductor ampacity for cable tray installation may be in question, and Section 392.11 should direct you to a proper beginning for conductor ampacity determination for your cable tray conductors.

You also stated:

...These conductors are terminated in an AL/CU 90 degree C lug (the lug will accept a range of conductors up to 750KCM). The lugs are in a Square D Switchboard...

The conductors terminating at the switchboard will be limited by the termination temperature within the switchboard itself regardless of whether or not the switchboard lugs are stamped and marked as 90?C.

The lug marking would be appropriate where said lugs were utilized in a scenario where they were a separate termination. These lugs are no longer separate but part of the switchboard and now governed under the requirements of 110.14(C)(1)(b). (The factory lug marking is no longer the termination temperature determining factor)

With the lugs being a part of the switchboard, the termination temperature is most likely going to have a maximum rating of 75?C. A review of your switchboard literature should confirm this. If it does not, then consult the factory regarding the switchboard termination temperature.

I do hope this is helpful

mweaver

 

[Pierre C Belarge]

Equipment 600V and less, even though it may have lugs which are themselves labeled as 90C, will not have the rating as a whole for 90C. As Mike has stated, there will be labeling on the cabinet or in the instructions that limit the wiring method to 75C temperature rating for ampacity.

 

[cgee]

Clarifications

Clarifications

Thanks for the feedback so far, but I think some clarifications are needed to get back on topic:

1. We consulted Art. 392 when planning the cable trays. We are using bundled conductors, NEC 392.11(B)(4) and are using Table 310.20 as a result.

2. We are not questioning the lug temperature rating. It was only stated to show that we were landing a 75 degree C cable on a lug of equal or higher rating. I'm not sure of the temperature rating of the switchgear, but let's assume it's 75C for argument's sake.

3. Our real question is: Can we rate the lug at 475A (ampacity for a 750KCM cable at 75 degree C in Table 310.16; remember that this is the largest size cable that the lug is rated for by the manufacturer) even though we are only terminating a 500KCM cable at the lug?

In other words: Is the lug's ampacity dictated by the cable being terminated according to the ampacities listed in 310.16 per rules set forth in 110.14?

Here's an analogy: If you terminate a 500KCM cable on a 100A MLO panel, is the cable DERATED to only 100A? No. It is only LIMITED to 100A by the panel's bus rating.

Thanks in advance for any further advice.
-cgee

 

[Smart $]

... Our real question is: Can we rate the lug at 475A (ampacity for a 750KCM cable at 75 degree C in Table 310.16; remember that this is the largest size cable that the lug is rated for by the manufacturer) even though we are only terminating a 500KCM cable at the lug?

No.

In other words: Is the lug's ampacity dictated by the cable being terminated according to the ampacities listed in 310.16 per rules set forth in 110.14?

No again. Summarily, the lug's ampacity is not the heart of the issue.

A lug's ampacity is determined by several variables including the size and type of conductors, and the means by which it is connected. A lug is essentially an adapter for two means of connection. There are more details to the discussion, but none of this has any bearing on what you are trying to determine. I say this because lugs, by the very nature of their design, listing, and labeling, take their own ampacity out of the equation.

What you do have to concern yourself with regarding lugs is the compliance with installation parameters of the lug itself. This includes conductor sizes, connection hardware, torquing, compression, etc., which are all predicates of its operating temperature, and these parameters all effect the reliability of its two connections.

What it all boils down to here is, when a lug is properly installed, its operating temperature cannot exceed its maximum temperature rating, without compromising its connections reliability. If you impose a current upon a connected conductor such that its operating temperature is greater than that of the lug, that temperature is transferred to the lug, and you have compromised the reliability of the connection.

Even though the lug you are using permits a connection up to a 750 kcmil Cu conductor, ask yourself what amount of current will cause the connected 500 kcmil Cu wire to operate at a 75?C temperature.

Summary : Ampacity is determined so as not to exceed the maximum operating temperature of any circuit component.

This somewhat falls in line with your analogy(?)...

Here's an analogy: If you terminate a 500KCM cable on a 100A MLO panel, is the cable DERATED to only 100A? No. It is only LIMITED to 100A by the panel's bus rating.

 

[Pierre C Belarge]

Take a read of this SQD document, it may help.