Medium Voltage Switch sizing

[Snoooorm]

I see that low voltage switches are to be sized at least 115% of the full load current. Does this also apply to medium voltage switches?

 

[JoeStillman]

Welcome to the forum.

A medium voltage switch rated for under 200A will be hard to find. For example, S&C Alduti-Rupter Switches start at 600 amps. Is your full load current more than 173 amps? That's a big load for anything over 1,000V.

 

[Snoooorm]

I'm reviewing the one-line for a solar project. There are (9) inverters connected, each with the potential to provide 60.3A at 34.5kV. It's all still preliminary, so they might find that not all will provide 60.3A. But using that as worst case, (9) would provide 543A. The full 543A are being routed to the substation through some 600A, 34.5kV sectionalizer switches. If these switches are supposed to be designed 115% of the FLA, then I need to tell them to fix the design somehow (provide different routing, increase the rating of the switch, etc.).

 

[electrofelon]

I see that low voltage switches are to be sized at least 115% of the full load current. Does this also apply to medium voltage switches?

Note that 115% figure only applies to motor applications, not generally.

 

[ggunn]

Note that 115% figure only applies to motor applications, not generally.

I don't know a whole lot about PV connectivity to MV, but do the same rules not apply? In LV connections, OCP fuses must be rated at 125% of the inverter maximum output current, and a switch cannot be rated less than the fuses it contains.

 

[ggunn]

I don't know a whole lot about PV connectivity to MV, but do the same rules not apply? In LV connections, OCP fuses must be rated at 125% of the inverter maximum output current, and a switch cannot be rated less than the fuses it contains.

At least 125%, of course.

 

[electrofelon]

a switch cannot be rated less than the fuses it contains.

I was envisioning a NF switch. But still I do not see any NEC rule supporting your statement, can you provide a reference? (Yes from up practical standpoint you can't physically put larger fuses in a less rated switch).

 

[electrofelon]

I don't know a whole lot about PV connectivity to MV, but do the same rules not apply? In LV connections, OCP fuses must be rated at 125% of the inverter maximum output current, and a switch cannot be rated less than the fuses it contains.

Part IX of article 240 would cover MV. There is no specific requirement for continuous load derating. Fuses may be three times the ampacity of the conductor, circuit breakers six times, with the general requirement that protection be analyzed with a proper analysis to prevent conductor damage.

 

[kingpb]

ALL MV equipment is 100% rated. A 1200A breaker can handle 1200A continuously. A 600A \sectionalizer can handle 600A continuously.

 

[Snoooorm]

Thanks everyone! This has been a good help!

 

[anbm]

ALL MV equipment is 100% rated. A 1200A breaker can handle 1200A continuously. A 600A \sectionalizer can handle 600A continuously.

Is this code or from product white paper? Can you provide data for reference? Thanks.

 

[electrofelon]

Is this code or from product white paper? Can you provide data for reference? Thanks.

Logically I would say the burden of proof is on you to provide a counter example showing that There IS MV gear that is 80% rated

 

[jim dungar]

Is this code or from product white paper? Can you provide data for reference? Thanks.

It is from manufacturing standards, like UL and ANSI.

 

[Jraef]

Logically I would say the burden of proof is on you to provide a counter example showing that There IS MV gear that is 80% rated

There is no such thing as “80% rated” gear, never has been. All gear is rated 100%.

CONDUCTORS must be rated for 125% of continuous loads, so indirectly, a breaker protecting that conductor will never see more than 80% of its rated load. BECAUSE of that, when breakers are crowded into low voltage panelboards and load centers, the breaker mfrs use that fact when determining the heat rise in the panel, resulting in the “80% rating” of the breakers AS USED in panels. You can use a breaker marked as 100% rated, but only if listed as such, which means it will be in its own stand-alone enclosure or cubicle, or directly stabbed onto bus (like I-Line panels) where the bus can help dissipate the heat.

Once you get away from the crowded LV breaker panel concept, as you do in MV Switchgear, everything is rated at 100%. It’s part of the design criteria.

 

[electrofelon]

There is no such thing as “80% rated” gear, never has been. All gear is rated 100%.

CONDUCTORS must be rated for 125% of continuous loads, so indirectly, a breaker protecting that conductor will never see more than 80% of its rated load. BECAUSE of that, when breakers are crowded into low voltage panelboards and load centers, the breaker mfrs use that fact when determining the heat rise in the panel, resulting in the “80% rating” of the breakers AS USED in panels. You can use a breaker marked as 100% rated, but only if listed as such, which means it will be in its own stand-alone enclosure or cubicle, or directly stabbed onto bus (like I-Line panels) where the bus can help dissipate the heat.

Once you get away from the crowded LV breaker panel concept, as you do in MV Switchgear, everything is rated at 100%. It’s part of the design criteria.

Yes I fully agree. The 80% rule only applies to conductors and then ocpds as a consequence. Many people think it applies to everything and start applying it to panel boards, switches, and pretty much everything. I was using "gear" generally referring to conductors and ocpds, not actual switch gear.

 

[ggunn]

In LV applications an unfused disconnect on a feeder protected by a breaker cannot be rated lower than the breaker. Is that true for MV as well?

 

[electrofelon]

In LV applications an unfused disconnect on a feeder protected by a breaker cannot be rated lower than the breaker. Is that true for MV as well?

It can't? Can you cite a code reference for that?

 

[jim dungar]

In LV applications an unfused disconnect on a feeder protected by a breaker cannot be rated lower than the breaker. Is that true for MV as well?

Remember while a MV breakers have 1200A contacts, it's trip rating is based on the CTs and protective relay settings.

 

[ggunn]

It can't? Can you cite a code reference for that?

Once we connected a PV system with (I'm just making up the numbers for illustration because it was so long ago) where the inverter output current was 98A. We interconnected in an MDP with a 125A backfed breaker per code, and we figured that we could use a 100A unfused disconnect since it was rated higher than the inverter Imax. An inspector called us on it and said he would only allow it if we got it in writing from the switch manufacturer that it was OK. The manufacturer told us that the switch must be rated equal or greater than the breaker.

 

[ggunn]

Once we connected a PV system with (I'm just making up the numbers for illustration because it was so long ago) where the inverter output current was 98A. We interconnected in an MDP with a 125A backfed breaker per code, and we figured that we could use a 100A unfused disconnect since it was rated higher than the inverter Imax. An inspector called us on it and said he would only allow it if we got it in writing from the switch manufacturer that it was OK. The manufacturer told us that the switch must be rated equal or greater than the breaker.

I'll just add that the manufacturer's position was understandable, since in the event of a fault on the inverter side of the switch, the OCPD would allow 125A to flow through the 100A switch.