200A Disconnect Switch Fed From 225A Breaker

[turtlepokerman]

The link doesn't say that an 80% rated breaker trips at 80% of the breaker rating...Because that's simply not true.

Yep you are correct. I was mixing up thermal element and breaker thermal rating.The TCC does not physically change. It just the a 225A Breaker (80%) can not handle 225A for more than 3 hours. It will still trip at 225A.

 

[jap]

Other than the inspectors comment about the Feeder Breaker protecting the NF Disconnect. I would tend to agree with him on the disconnect rating issue.


JAP>

 

[electrofelon]

Nope I know the difference between an adjustable trip setting and a 80% continuous rating of the breaker. The breaker is a non-adjustable 225A Breaker. The disconnect Switch is UL Listed to have a short-circuit rating of 100kA. The feeder breaker upstream is UL Listed to have a SCCR of 65kA. The actual SCCR at the switch is 23kA per my calculations.

My conductors are sized for the 225A Rating of the upstream breaker. The Disconnect Switch is feeding a crane which would not operate continuously for more than 2hrs. The actual load is 120A.

When I say the 80% I mean that the breaker is 80% rated for the continuous duty. Where as a 800A Breaker for example would be rated for 100% of the continuous duty.

The 80% derating for OCPD's serving continuous loads is s NEC thing. It is there because the actual use of OCPD's is typically in an enclosure with other OCPD's with little or no air space between. This installation is prone to less heat dissipation than the standard test conditions. A 100% rated breaker is no different than a standard one, it is just in how it is used (typically it's own enclosure).

 

[kwired]

The 80% derating for OCPD's serving continuous loads is s NEC thing. It is there because the actual use of OCPD's is typically in an enclosure with other OCPD's with little or no air space between. This installation is prone to less heat dissipation than the standard test conditions. A 100% rated breaker is no different than a standard one, it is just in how it is used (typically it's own enclosure).

I learned it as being standard overcurrent devices sink heat into the conductors and therefore we must adjust conductors when we have continuous loads when using standard overcurrent devices. 100% rated devices manage heat differently and that is why they can be used at 100% even on a continuous load. That difference may be as simple as it is the only device in a certain space, but is designed to be used at 100% in some way. Electronic current sensing I would think would be another way to create less heat to have to deal with.

 

[ggunn]

So looking into it I think this is what I determined as the appropriate response to my original question. Looking through the manufacturer's TCC Curve for the 80% rated breaker. It seems that the manufacturer does not show the breaker tripping at the 80% level after 2 hours. Their TCC Curve still shows the breaker tripping at the 100% rating, even though they list the breaker as a 80% continuous duty rated breaker. <- I need to contact the manufacturer why this is the case as it does not make sense to me.

Whats more interesting is that the breaker does not even reach its 100% level until 1 hour of run-time. So per their TCC, if I replaced my 225A Breaker with a 200A Breaker of the equivalent style and somebody went up to the switch within that first hour they could technically be switching more than 200A with that switch with an incorrectly sized load. If I sized my load incorrectly, the load could operate at 300A for 10 minutes and technically the breaker would not start trip per their TCC.

All that is really beside the point. Eaton and Square D both told me that in no case should the OCPD ahead of one of their unfused switches exceed the rating of the switch. Asked and answered.

 

[electrofelon]

All that is really beside the point. Eaton and Square D both told me that in no case should the OCPD ahead of one of their unfused switches exceed the rating of the switch. Asked and answered.

But does it say that in the "instructions included in the listing and labeling?"

 

[MAC702]

What would Leviton say if we asked if we could use a 15A snap switch on a <15A load, but on a 20A branch ckt?

And who at Eaton and Square D said no about the unfused switches?

 

[ggunn]

And who at Eaton and Square D said no about the unfused switches?

Their tech support engineers. I have it in "writing" (email), pdfs on their letterheads. They weren't written at my request; the tech support departments sent me documentation they had on hand. Don't take my word for it; ask them yourself. I would be interested in knowing about it if they tell you something different.

 

[kwired]

All that is really beside the point. Eaton and Square D both told me that in no case should the OCPD ahead of one of their unfused switches exceed the rating of the switch. Asked and answered.

I have to disagree at least for motor applications. Square D catalog lists HP ratings of majority of those unfused switches higher than what the overcurrent device would need to be for same motor vs the switch ampere rating. I mentioned already my using 30 amp switch for 20 HP 480 volt motors, done it several times. I just looked in the catalog and see similar 60 and 100 amp switches being rated for more HP than what a fused switch would be rated - only reason the fused switch can't accept high enough rated fuse for holding during starting but the switch is still rated for HP that has full load current near but less than current rating of the switch.

 

[jap]

It all depends on what circumstance you're using the disconnect for. I can see a disconnect being sized lower than the OCPD on a motor circuit where the Horsepower Rating covers it.

Not so much on a Feeder.

JAP>

 

[Jraef]

It all depends on what circumstance you're using the disconnect for. I can see a disconnect being sized lower than the OCPD on a motor circuit where the Horsepower Rating covers it.

Not so much on a Feeder.

JAP>

This is what I think is pertinent. IF this is strictly a motor feeder circuit, a 200A NF disconnect switch from all of the major players is rated for a 125HP 460V motor. So 125HP is 156A in the tables, maximum T-M breaker size is 250%, so 390A, ergo it would be totally legitimate to have a 225A breaker feeding a 125HP motor circuit that then also has a NF disconnect switch in it that is rated for 125HP, but would ALSO have a 200A current rating. Checks all of the NEC boxes.

But if that 225A breaker is a feeder for other loads and that NF disconnect ALSO feeds those other loads, not just a single motor, then those motor circuit rules no longer apply and the disconnect is under sized.

Side note:
turtlepokerman, no disrespect intended, but I doubt that you have found a non-fused disconnect switch with a 100,000A SCCR stand-alone. Every one that I just looked at, Sq. D, Eaton, Siemens, GE, A-B, have a rating of just 10kA, UNLESS you have specific fuses upstream of them. In the cases where the switch mfr also makes breakers, you can often use a series rating with THEIR SPECIFIC breakers to get the rating of the BREAKER, usually limited to 65kA. So IF you have that situation, i.e. Eaton DH364FGK switch protected by an Eaton HFD, HFDE, HJD or JGH breaker, you would get a 65kA SCCR. But if it's an Eaton switch and a Sq. D breaker, or Sq. D switch and GE breaker etc. etc., they are not tested together and you are at only 10kA on the switch. Given that you misinterpreted the trip values of T-M breakers, I'm thinking you are misinterpreting this issue as well and your use of a NF disconnect in a circuit capable of 23kA is actually not valid at all, regardless of the amperage rating. If I'm wrong, post the make and model of switch, because it would be of great benefit to everyone if you found one that has a 100kA SCCR without fuses.

 

[ggunn]

I have to disagree at least for motor applications. Square D catalog lists HP ratings of majority of those unfused switches higher than what the overcurrent device would need to be for same motor vs the switch ampere rating. I mentioned already my using 30 amp switch for 20 HP 480 volt motors, done it several times. I just looked in the catalog and see similar 60 and 100 amp switches being rated for more HP than what a fused switch would be rated - only reason the fused switch can't accept high enough rated fuse for holding during starting but the switch is still rated for HP that has full load current near but less than current rating of the switch.

I know next to nothing about designing motor circuits; all I know on the subject is what the manufacturers told me. In my application (PV) I won't be going against them.