Cable Operating Temperature and Method of Installation

[123MB]

Hi All

Here in Australia we commonly use 'V-90' PVC (thermoplastic) insulated and PVC sheathed power cable for cabling to final equipment, i.e. 3 phase motors, and for cabling to small distribution boards. This cable has a 'maximum operating temperature of 90 degrees"

The standards here tell us the maximum current carrying capacity of V-90 cable is based on the cable operating at a maximum temperature of 75 degrees - not 90 degrees. The standard further tell us that it is allowed to operate the cable at the full 90 degrees as long as the following conditions are met by the installation:

1) The cable is installed in a manner that is not subject to, or is protected against, severe mechanical pressure at temperatures higher than 75 degrees C.

I am trying to determine, practically, what this means, with respect to actual cable installation and what is permitted?

For example, if an insulated and sheathed cable is installed on cable ladder, without any further mechanical protection, and it is supported through its entire length, surely it would comply with these requirements.

To be honest I am struggling to come up with a practical example of cable installation that would not meet the installation requirements to operate the cable at the higher temperature. Can anyone give me some practical examples of installations which would and would not meet the requirements?

Thanks all.

 

[jghrist]

In the USA, our code restricts the ampacity to the temperature rating of terminations, which generally is 60°C or 75°C for low voltage equipment. Cable temperature correction and adjustments do not necessarily affect the termination temperature, however.

 

[123MB]

I have heard this recommendation before, however I have never seen termination ratings for equipment published by manufacturers - for example, the equipment in question here is a variable speed drive. I have never seen a manufacturer state that the terminals on a VSD are rated for a specific temperature.

Terminals, for example, are specified with maximum operating temperatures (i.e. a Weidmuller SAK35 terminal is rated to 100 deg C max, implying that you should not expect to operate 110 deg C rated cable above 100 degrees C).

However I have never seem an item of electrical equipment with this specification.

It seems to be a poorly treated area of specificity when it comes to cable sizing. I am lost.

It seems the safe thing to do is just consider a maximum operating temperature of 75 degrees C, but what about cables designed to operate at 110 degrees C for example - I see switchboard manufacturers use these often and I seriously doubt they have checked if the terminations on the equipment they are connecting to is rated for 110 degrees C.

Thanks.

 

[123MB]

It has just occurred to me...

The operating temperature limits of the cable are different depending on the environment in which it is installed.

For example, a cable which is sized for a high ambient temperature, and perhaps with derating due to being in proximity to other cables, will operate at its maximum operating temperature outside the panel, hence the current carrying capacity at the maximum operating temperature is used, combined with the temperature derating and ambient temperature derating, to give the current carrying capacity of the outdoor section of the cable.

In the panel, where derating is not applicable and, say, for argument's sake, the ambient temperature is 40 deg C - the cable is designed so that it operates only at 75 deg C - and hence the current carrying capacity for the cable at 75 deg C is checked against the load.

Can anyone concur with this?

Thankyou.

 

[7anoter4]

See:

https://www.voltimum.com.au/articles/difference-between-xlpe-higher-rated

In a conduit-for instance- the cable bunch is pressed when installed and also by thermal expansion and the same in a cable tray around a corner or curvature.

 

[123MB]

Hi 7anoter4

Are you saying that a cable that goes around a corner on tray is subject to severe mechanical pressure that would deform the insulation if the cable was operating at its maximum temperature?

This sounds a little off. So what conditions would be allowed then? a straight run of cable without bends etc?

I was brainstorming and I thought of some conditions that could include:

1) Cable bend radius half of manufacturer's recommendation or less
2) Cable cleats used or cable ties applied with a gun to a specified cable tension to avoid over pressuring the insulation.

Thoughts?

 

[HamburgerHelper]

ETAP has a module for doing thermal analysis of underground cables. Any installation that makes it hard to dissipate heat might require derating.

https://etap.com/cable-thermal-analysis-software/underground-thermal-analysis-raceway-rulebook

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If you can't explain it to a six year old, you don't understand it yourself.

 

[waross]

It is common for cables in tray to be derated due to mutual heating.
Our code has specific rules and tables for derating multiple cables in contact or close proximity..
With long runs, allowances for voltage drop often result in the actual current being less than the 90 degree rating.
Generally motor terminations must be limited to a 75 degree rating.
In some instances, we consider installing a junction box close to a motor to transition from a cable used at the 90 degree rating to a larger cable at a 75 degree rating.
These instances are not common as derating due to more than three conductors in contact and derating due to voltage drop generally reduce the current of the 90 degree rated cable to the 75 degree rating or less.
On short runs the cost of material and labour to install transition junction boxes is generally more than the cost of using the next larger size cable.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

In the panel, where derating is not applicable and, say, for argument's sake, the ambient temperature is 40 deg C - the cable is designed so that it operates only at 75 deg C - and hence the current carrying capacity for the cable at 75 deg C is checked against the load.

Yes, that is what the USA National Electrical Code (NEC) allows. You can use the un-derated 75°C rating of the cable if the terminals are rated for 75°C.

 

[7anoter4]

By the way,in the interpretation of Articles 110.14 (C) and 310.15 (B) NEC there is a little confusion. Article 110.14 (C) allows the use of ampacity corresponding to the rated temperature of conductor, but Article 310.15 (B) introduces a condition:
„The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the TEMPERATURE RATING of the conductor, IF the corrected and adjusted ampacity DOES NOT EXCEED the AMPACITY for the temperature rating of the termination in accordance with the provisions of 110.14(C)”.What ampacity?
110.14(C):
(C) Temperature Limitations. The temperature rating associated
with the ampacity of a conductor shall be selected
and coordinated so as not to exceed the lowest temperature
rating of any connected termination, conductor, or device.
Conductors with temperature ratings higher than specified
for terminations shall be permitted to be used for ampacity
adjustment, correction, or both.