15kV Ventilated Tray

[dy11]

I'm trying to confirm the ampacity/design intent of an existing installation. It consists of 4x 15kV copper MV-105 500 mcm /phase in tray with more than 1 cable diameter spacing between cables, single layer. It's an outdoor ladder tray. It has a ventilated cover which is where I'm getting stuck. 392.80.B.2 only discusses uncovered tray or solid covered tray, neither of which seem to apply here. I'm thinking, based on the cable spacing, whoever designed it was going for the 392.80.B.2 (2) requirements which do not require a derate, but that language specifically says "uncovered". To me, a ventilated cover is not uncovered.

Other elements in the circuit (bushings, breaker) are rated for 3000A but I'm not sure this cable installation meets that rating.

Looking for some feedback here. Thanks!

 

[bwat]

I've been hoping someone else jumps in on this. I've heard it said that the ventilated is considered uncovered because the covered specifically calls out "unventilated", and those are the only two options listed in 392.80(B) but I never had that warm fuzzy feeling about it.

 

[zbang]

I'd be asking this question- with a ventilated cover, what's the heat rise underneath? If sufficient air can move upwards through the tray & cover to keep the temp. rise reasonable, I'd tilt towards calling it open.

Maybe one of the tray manufacturers has some into on this.

 

[petersonra]

4X 500 MCM does not seem to get one to 3000 Amps.

But I have no experience with MV cables, maybe it does get you there with MV cables.

 

[dy11]

I'd be asking this question- with a ventilated cover, what's the heat rise underneath? If sufficient air can move upwards through the tray & cover to keep the temp. rise reasonable, I'd tilt towards calling it open.

Maybe one of the tray manufacturers has some into on this.

I’m not sure I’m able to say what’s sufficient air flow or not. With an open bottom, it seems like it shouldn’t be an issue but I’ve been burned by making assumptions like that before. That said, I’m leaning toward uncovered too. The 70% derate doesn’t seem applicable because this isn’t a solid unventilated cover.

4X 500 MCM does not seem to get one to 3000 Amps.

But I have no experience with MV cables, maybe it does get you there with MV cables.

Looking at table 310.60(C)(69) I get 765A for 15kv MV-105.

 

[paulengr]

I’ve started using the “peaked” covers as “open”. The ones with about a 30 degree angle. They tend to keep dirt from accumulating which is what frequently converts vented covers into unvented.