MV services, 240.12 coordination requirements, and six disconnect rule

[malachi constant]

I have a doozy of a series of questions, by my standards of questions at least. This is under NEC 2020.

We are building a chiller plant for a campus. It will be primary power, meaning utility will supply power at 13.8 to the building, at which point the customer owns the switchgear/transformers to step it down to 480V. I have some experience designing 13.8 systems, but this is my first full blown service design and am facing some curveballs. Here's the situation:
1. Following standard NEC calculations for motors, lights, etc the building load is chiller roughly 4000kVA (roughly 5000A @ 480V, or 170A @ 13.8).
2. I am recommending a 200E fuse for the service. The MV transformers are fed from a few lower-fused switches in the same MV lineup.
3. The utility says they cannot provide lower than a 140T fuse at the pole. This does not coordinate with a 200E fuse. The contractor is adamant 240.12 Electrical System Coordination applies to this equipment. I typically only require coordination studies for healthcare projects, emergency systems, COPS, etc. I have never had it applied to a standard building type. First question: when does 240.12 apply, specifically does the presence of a MV system trigger this coordination requirement?
4. To solve the coordination issue, the utility recommends downsizing our service fuse to something like 100E or lower. (I think 80E is the highest fuse that coordinates with their 140T, but haven't confirmed that.) They say E fuses are rated to handle significantly higher ampacities than their rating so it's OK for me to downsize the main fuse. If that's even legal, and we are still following coordination requirements I would presumably have to downsize my branch MV fuses to lower than I think is correct. Second question: is there something inherently different about MV fuses such that code allows them to be sized at lower ampacities? I have reviewed NEC 490 Part II and nothing jumps out as allowing this.
5. A suggestion has been thrown out there to solve both the above conundrums: provide an unfused main switch. Utilize the utility fuses at the pole to protect the incoming underground utility cable, and utilize the six-disconnect rule to provide service disconnects. The utility's standards may allow this (they are verifying) and by my reading of 230.2 and 230.71 & 72 this appears to be legal. Third question: can you utilize the six-disconnect rule with 13.8 services?

Any and all insight is welcome here. Surely this is not the first time someone has done primary power 13.8 for a building with a 4000kVA calculated load. Thank you in advance!!!

 

[jim dungar]

MV fuses are sized by their melt curves not by their continuous current rating.
Selective coordination is not automatically required just because it is MV.
The only coordination issue between the utility source fuse and your main fuse is the expense and hassle of changing the utility fuse (of course this assumes your feeder fuse do coordinate)

 

[malachi constant]

MV fuses are sized by their melt curves not by their continuous current rating.
Selective coordination is not automatically required just because it is MV.
The only coordination issue between the utility source fuse and your main fuse is the expense and hassle of changing the utility fuse (of course this assumes your feeder fuse do coordinate)

Thanks Jim, that's helpful. Is there anything approximating a consensus as to when selective coordination is required?

 

[jim dungar]

Thanks Jim, that's helpful. Is there anything approximating a consensus as to when selective coordination is required?

No. Other than as spelled out in the NEC, it is a design decision.
Typically two devices in series do not ever need to coordinate, although there are situations like the example I gave of having a customer's device operate before a utility's.

I recall a major sports stadium where we had a lack of coordination like yours. Typically a customer's device can be reset relatively quickly, while a utility needs to get a lineman sent to the site. In our case the utility always had a lineman at the stadium, during events, so they agreed that the customer did not need to coordinate.

 

[malachi constant]

No. Other than as spelled out in the NEC, it is a design decision.
Typically two devices in series do not ever need to coordinate, although there are situations like the example I gave of having a customer's device operate before a utility's.

I recall a major sports stadium where we had a lack of coordination like yours. Typically a customer's device can be reset relatively quickly, while a utility needs to get a lineman sent to the site. In our case the utility always had a lineman at the stadium, during events, so they agreed that the customer did not need to coordinate.

In our case we have fuses, not breakers. The client has an electrician on staff, but not on-site, and may not be comfortable with opening up the MV cabinet to change the fuse. The utility says they will respond within 45 minutes so time is probably not a deal-breaker here.

Any insight on whether a non-fused switch can be used at the main, with three feeder fused switches grouped next to it? It's a weird (to me) install but the local manufacturer's vendor is saying this is how the majority of their MV service lineups are installed in our area (typically to avoid coordination issues with utility fusing). The utility says they will allow it. I have a call scheduled with the inspector.

 

[jim dungar]

Why is coordination an issue? If the utility is willing to accept a non-fused service, they are accepting the fact their fuse will open if there is a fault in the service equipment. How is this any different from having a non-coordinated main fuse?

But, yes I have seen six disconnects on MV services. One issue with a non-fused main switch could be it's short circuit rating.