200A vs 400A Non-Fusible Disconnect Switch

[Carultch]

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?

If that is the case, I'd like to see a description of an example of where one could take credit for the continuous duty rating. That is, an example where a fused switch would have to be a 400A switch fused at either 225A or 250A, while an unfused switch in the same location could be 200A.

If an OCPD greater than 200A elsewhere in the circut means that an unfused switch would have to be 400A anyway, what is the value of a 200A switch having a continuous duty rating?

 

[jaggedben]

If that is the case, I'd like to see a description of an example of where one could take credit for the continuous duty rating. That is, an example where a fused switch would have to be a 400A switch fused at either 225A or 250A, while an unfused switch in the same location could be 200A.

If an OCPD greater than 200A elsewhere in the circut means that an unfused switch would have to be 400A anyway, what is the value of a 200A switch having a continuous duty rating?

I suppose one example would be if you have that OCPD that the exceptions are always referring to, which is rated for continuous duty at 100%. Granted, that's a mythical device in my work experience.

Another possible example: I'm not 100% sure that the code always requires the OCPD, and not just the conductors, to be 125% rated for continuous duty. It does in the solar applications I work with, but I can't claim that's true generally. I just haven't taken a thorough look at that.

I should probably qualify my statements above: you brought up that Eaton language, but I happen to know for sure that some Eaton switches say that you need an OCPD of the same rating. I don't know if that's also true of all Eaton switches or if it's true of other brands.

 

[kwired]

Not quite the same thing the OP is asking about but sort of along similar conditions: One application I have ran into enough to remember is Square D 600 volt heavy duty safety switches and 20 HP 480 volt three phase motor applications. If you use a fused switch you need more than a 30 amp fuse to allow for starting of this motor - that puts you into the 60 amp switch size. But if you are using an unfused switch, the 30 amp versions are rated for 20 HP at 480 volts - you will most likely have ~40 amp fuse or ~60 amp breaker somewhere ahead of this switch in most cases.

 

[Carultch]

Not quite the same thing the OP is asking about but sort of along similar conditions: One application I have ran into enough to remember is Square D 600 volt heavy duty safety switches and 20 HP 480 volt three phase motor applications. If you use a fused switch you need more than a 30 amp fuse to allow for starting of this motor - that puts you into the 60 amp switch size. But if you are using an unfused switch, the 30 amp versions are rated for 20 HP at 480 volts - you will most likely have ~40 amp fuse or ~60 amp breaker somewhere ahead of this switch in most cases.

How would a motor make that any different than any other kind of load, with its own local unfused disconnecting means of 30A, ultimately connected from a 40A breaker where the branch circuit starts?

 

[kwired]

How would a motor make that any different than any other kind of load, with its own local unfused disconnecting means of 30A, ultimately connected from a 40A breaker where the branch circuit starts?

My 20 hp motor installs I based the reply on have 60 amp breakers at the beginning of the circuit, that is kind of a recommended breaker setting by Square D slide chart I have, NEC actually allows 2.5 x FLA (per NEC tables = 27) =70 amp breaker, and can be more if that doesn't allow motor to start.

Bottom line is the 30 amp non fused switch is still rated for 20 HP. Sort of no different than the motor starter is rated for a certain HP (in this case 20 HP is at the bottom end of size 2 contactor rating though). Same contactor has a maximum horsepower rating as well as a maximum current rating for other than motor loads.

You run into similar thing when you put a 15 amp snap switch on a 20 amp circuit, even though it is protected by 20 amp OCPD you can't put more than 15 amps on that switch.

 

[SceneryDriver]

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

If you need a local disconnect, can you not place a 225A main breaker in the panelboard? Seems like a simple solution to me, if the panelboard has provision for adding a main.


SceneryDriver