Cable Ampacity Problem

[unclebob]

Hi all,

I have a situation where low voltage cables are becoming hot, about 60 C at the hottest point in the gear B phase (middle). Customer wants to know if this is normal.

Here's the scenario:

We have to feed a 3000 A gear. We have 600V 750 MCM Nual ACWU90 single conductor armored cable. Manufacturer, see link, set the maximum ampacity of that cable at 785 A.
Cables go from an outside transformer, through a wall to the existing gear. Two cable trays are use and the cables disposition is as follow:

N A B C C B A N
N A B C C B A N

so, there's only 4 cables per phase (4 x 785 = 3140 A). Cable spacing is 1 x the cable diameter and spacers are made of wood. Cables go thru an aluminum plate at the transformer, and a fiber plate in the switchgear. Grounding is at one point only, in the transformer throat.

Contractor did the calculations, but seems to me that we're missing a couple of cables. Using CEC Table 3, I get 670 A max current per cable.
Now, table 5B doesn't apply but 5D does and show a derating factor of 0.74. They disregard the N cables... is it ok to do so?

Using this gives 496 A max per cable. 3000 A / 496A/C = 6 C per phase. I guess someone forgot Table 5D.

 

[dpc]

Where are they getting hot and how is this being measured?

Do you know what the actual current is?

 

[unclebob]

Going onsite today with a power recorder and a thermo-cam. Will be able to take a look at harmonics, power factor and current imbalance if any.

 

[unclebob]

Got some info:

Actual current is 1800 A. Power quality is good, 6% 5th harmonic that's all.
Hottest temperature is in the gear, where all the cables but the neutral are at 55C, distributed evenly along the cable length, so no hot spot there. As soon as the cables exit thru the top of the gear, temperature falls to 40 C.

Customer is worry that in summertime, the cables will be hotter. An A/C unit could be used to cool the substation.
I wonder what is the temperature rise when a cable is at it's nominal current load?

 

[7anoter4]

In my opinion the ampacity indicated in manufacturer table it is for single cable in free air at 30oC and 1/3 concentric conductor current.
The rated temperature is the conductor maximum temperature allowed for [touching] insulation .In this case 90oC.
Let's say the actual current it is 1800/4=450 A and no neutral current. In this case a single cable in free air of 30 oC will be approx. 45 oC. [according IEC 60287].
In a single open top tray [solid bottom] according to Stolpe at 450 A conductor temperature will be 65 oC. In my opinion 60 oC on outside surface of a cable is o.k.
According to Stolpe at 785 A [no neutral current] the cable temperature will be 170 oC.

 

[Skogsgurra]

I visited a paper mill yesterday and was involved in a discussion about cable temperatures (which was not the problem I was supposed to look at). There was a cable tray (an open ladder, instead of a tray) going vertically up a wall.
Someone had put a steel cabinet for spare parts and tools up against the wall and after a few days someone (another guy) noticed that the temperature in the cabinet was unusually high.

The cabinet was heated by the cables on the ladder and when we checked cable surface, it was 56 C. Removed cabinet and temperature dropped to 45 C after a few hours (don't know how long it took - watching cable temperatures is as boring as watching paint).

Some guys thought that 45 C was too hot. And others thought it was OK. Of course, it is OK. Cables are designed to run more than luke-warm. It should be printed on them - then we could avoid lots of discussions.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[crshears]

Some guys thought that 45 C was too hot. And others thought it was OK. Of course, it is OK. Cables are designed to run more than luke-warm. It should be printed on them - then we could avoid lots of discussions.

Very subtle, Skogs, I like it.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]