200A vs 400A Non-Fusible Disconnect Switch

[frwilliams]

I would like to find in the NEC Code concerning which Disconnect Switch shall be required (200A or 400A Non-Fusible Disconnect Switch) upstream of a 208/120V, 225A Bus Rated Panelboard, with a calculated of 180A.

FRWilliams

 

[infinity]

No local disconnect is required for a panelboard.

Welcome to the Forum.

 

[frwilliams]

No local disconnect is required for a panelboard.

Welcome to the Forum.

I understand it is not required, but in this situation a Disconnect Switch is upstream toa 208/120V, 225A Main Bus Rated Power Panel with a calculated load of 180A. My question is, which disconnect switch to use per NEC Code. Where in the NECCode explains the requirements of using a 200A vs a 400A Rated Non-FusibleDisconnect Switch in this situation.

 

[infinity]

What size OCPD is ahead of the panel?

 

[Carultch]

I would like to find in the NEC Code concerning which Disconnect Switch shall be required (200A or 400A Non-Fusible Disconnect Switch) upstream of a 208/120V, 225A Bus Rated Panelboard, with a calculated of 180A.

FRWilliams

Check whether or not the disconnect is continuous duty rated. Generally, unfused units meet this condition. This means that 100% of continuous load + 100% of non-continuous load sizes the unit, rather than 125% of the continuous load.

There are a couple of pitfalls to keep in mind.
1. The terminals may not be large enough for your application, if a serious amount of voltage drop governs wire sizing.
2. Typical unfused disconnects are may only allowed with 10kA fault currents and less (SCCR rating), unless otherwise wired in series with fuses. This is more of a rule due to lack of testing to prove it is safe, than any lilkihood of a serious failure, because the disconnects have not been tested in combination with breakers to allow use when higher fault currents are present. The internal switching units are the same part as those used in the fused disconnects, so I don't anticipate any serious hazard if you overlook this. But it doesn't strictly meet the product's listing.

 

[frwilliams]

Check whether or not the disconnect is continuous duty rated. Generally, unfused units meet this condition. This means that 100% of continuous load + 100% of non-continuous load sizes the unit, rather than 125% of the continuous load.

There are a couple of pitfalls to keep in mind.
1. The terminals may not be large enough for your application, if a serious amount of voltage drop governs wire sizing.
2. Typical unfused disconnects are may only allowed with 10kA fault currents and less (SCCR rating), unless otherwise wired in series with fuses. This is more of a rule due to lack of testing to prove it is safe, than any lilkihood of a serious failure, because the disconnects have not been tested in combination with breakers to allow use when higher fault currents are present. The internal switching units are the same part as those used in the fused disconnects, so I don't anticipate any serious hazard if you overlook this. But it doesn't strictly meet the product's listing.

The OCPD is a 225A Circuit Breaker in a Power Panel upstream of the Disconnect Switch.

 

[infinity]

The OCPD is a 225A Circuit Breaker in a Power Panel upstream of the Disconnect Switch.

OK so you have a 225 amp OCPD then the disconnect switch then the panel. So your question is can the disconnect be smaller (200 amps) than the 225 OCPD ahead of the feeder or does it need to be a 400 amp disconnect?

 

[frwilliams]

OK so you have a 225 amp OCPD then the disconnect switch then the panel. So your question is can the disconnect be smaller (200 amps) than the 225 OCPD ahead of the feeder or does it need to be a 400 amp disconnect?

Yes that is my question.

 

[Carultch]

Yes that is my question.

From Eaton:
"The continuous load current of fusible switches is not to exceed 80% of the rating of fuses employed in other than motor circuits. Nonfusible switches are 100% fully rated"
http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/tb00801002e.pdf

 

[infinity]

From Eaton:
"The continuous load current of fusible switches is not to exceed 80% of the rating of fuses employed in other than motor circuits. Nonfusible switches are 100% fully rated"
http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/tb00801002e.pdf

He did state the the calculated load is 180 amps so isn't the continuous/non-continuous already factored in?

 

[jaggedben]

Even if the switch is rated for 200A continuous, if you have a 225A OCPD upstream, then the switch has to have an equal or higher rating. Ask Eaton if you can protect a 200A switch with a 225A breaker. I think they'll say no.

The 180A calculated load doesn't figure into it, except in as much as it prompts the question: why isn't the OCPD 200A?

 

[wwhitney]

Even if the switch is rated for 200A continuous, if you have a 225A OCPD upstream, then the switch has to have an equal or higher rating. Ask Eaton if you can protect a 200A switch with a 225A breaker. I think they'll say no.

The 180A calculated load doesn't figure into it, except in as much as it prompts the question: why isn't the OCPD 200A?

So other than an order of magnitude, why is the above different from a 20A breaker protecting a 15A toggle switch for a 10A load?

Cheers, Wayne

 

[infinity]

Even if the switch is rated for 200A continuous, if you have a 225A OCPD upstream, then the switch has to have an equal or higher rating. Ask Eaton if you can protect a 200A switch with a 225A breaker. I think they'll say no.

The 180A calculated load doesn't figure into it, except in as much as it prompts the question: why isn't the OCPD 200A?

That's the question, if the load is limited to 180 amps and the switch is 200 amps is it code complian? I say no but am trying to come up with a code section.

 

[jaggedben]

So other than an order of magnitude, why is the above different from a 20A breaker protecting a 15A toggle switch for a 10A load?

Cheers, Wayne

a) there's no code section that specifically allows it, unlike parts of 210.
b) The 15A receptacle isn't marked as requiring a 15A OCPD.

Don't look for a logical reason though.

 

[jaggedben]

That's the question, if the load is limited to 180 amps and the switch is 200 amps is it code complian? I say no but am trying to come up with a code section.

Probably 110.3(B) when you look at the switch label. Unfortunately.

 

[Jraef]

I believe (but can’t check right now) that somewhere in the NEC the disconnect switch must be rated for a minimum of 115% of the continuous load. 115% of 180A is 207A, so that would dictate a 400A Switch.

A side issue that can be a nasty “gotcha” is going to be the SCCR of the non-fused Disconnect Switch. Most are rated for only 10kA max unless fed from a SPECIFIC fuse or listed in series with a specific breaker. So if the breaker and the switch are both Eaton, they MIGHT be listed in series for a higher SCCR, but if the breaker is Sq D and the switch is Eaton, that’s not going to fly and you are left with 10kA (if you are lucky, a few are only 5kA).

To put that in perspective, let’s assume the best case scenario, your 225A breaker was put down stream of a 112.5kVA 280Y120V transformer with around a 1.2% impedance. Your available fault current is going to be around 35kA (thumbnail guess), too much for a typical NF disconnect without being series rated with that specific breaker. Most likely since that breaker is part of a larger panel board, the transformer is a lot larger and the AFC is much higher.

NF disconnects have become more difficult to use now with all the emphasis on SCCR.

 

[Cow]

At some point you have to inject a little common sense.

If someone took the time to design a 225 amp feeder to a 225 amp panelboard with a calculated load of 180 amps, it stands to reason your disconnect should be capable of 225 amps as well.

The only way I would install a 200 amp disconnect is if the feeder OCPD was reduced to 200 amps. But that point is moot anyhow. I know from experience that 200 amp disconnects loaded 160-180 amps continuously have a much shorter lifespan. Even more reason I would push for the 400 amp disconnect.

 

[kwired]

I believe (but can’t check right now) that somewhere in the NEC the disconnect switch must be rated for a minimum of 115% of the continuous load. 115% of 180A is 207A, so that would dictate a 400A Switch.

A side issue that can be a nasty “gotcha” is going to be the SCCR of the non-fused Disconnect Switch. Most are rated for only 10kA max unless fed from a SPECIFIC fuse or listed in series with a specific breaker. So if the breaker and the switch are both Eaton, they MIGHT be listed in series for a higher SCCR, but if the breaker is Sq D and the switch is Eaton, that’s not going to fly and you are left with 10kA (if you are lucky, a few are only 5kA).

To put that in perspective, let’s assume the best case scenario, your 225A breaker was put down stream of a 112.5kVA 280Y120V transformer with around a 1.2% impedance. Your available fault current is going to be around 35kA (thumbnail guess), too much for a typical NF disconnect without being series rated with that specific breaker. Most likely since that breaker is part of a larger panel board, the transformer is a lot larger and the AFC is much higher.

NF disconnects have become more difficult to use now with all the emphasis on SCCR.

Also keep in mind that the available fault current decreases over the length of the circuit, one might have 35kA at the transformer secondary, but it will be less at the far end of a 100 foot run of conductor because of resistance of those conductors.

 

[Jraef]

Also keep in mind that the available fault current decreases over the length of the circuit, one might have 35kA at the transformer secondary, but it will be less at the far end of a 100 foot run of conductor because of resistance of those conductors.

Yes, but that much less?

 

[kwired]

Yes, but that much less?

For a 400 amp conductor, might not drop down to 10kA max but will still be a significant drop even over 25-50 feet of conductor. For a 50 amp conductor I wouldn't be surprised to see available fault current being less than 10 kA at the end of a 50 foot run of conductor.