200A Disconnect Switch Fed From 225A Breaker

[turtlepokerman]

Greetings,

I recently had an inspector comment that the 200A Disconnect Switch (Non-Fused) that was installed is inadequately protected by the 80% rated 225AT Feeder Breaker. I'm confused as to why this is considered unprotected as the breaker should trip at 0.80 x 225A = 180A. Does anyone know why this is considered unprotected or if there is a section in the NEC that specifically allows this installation. Not that it matters at this point, but the actual load of the connected device is only around 120A.

All help is appreciated!

 

[petersonra]

The feeder breaker is there to protect the feeder conductors, not the disconnecting means.

A breaker rated at 225 Amps can pull 225 Amps continuously without tripping and might well be able to take 250 or 275 Amps continuously without tripping. The rating of the OCPD is essentially an operating point where the OCPD is guaranteed not to trip, at the specified ambient temperature. The colder the ambient temperature is the more current it can typically take if it is a TM type device like most MCCBs and fuses.

Having said that, he might well be right for another reason. Most nonfused disconnect switches are only rated for 5 or 10 kA of short circuit current and being fed by a 225 Amp OCPD suggests it is quite possible that the available short circuit current could well exceed that.

 

[kwired]

The feeder breaker is there to protect the feeder conductors, not the disconnecting means.

:thumbsup:

What does the disconnect supply? More important is how many amps does it carry?

This situation can happen with motors easily, higher breaker rating than the conductor rating, yet the switch is rated for the motor horsepower. One example of installation I can recall off top of my head is Square D heavy duty 600 volt 30 amp switch being rated for 20 HP @ 480 volts (supplying a 20 HP motor) and the branch circuit breaker is 60 amps. I have installed several of those like that.

Short circuit rating is also something that needs considered but has nothing to do with overcurrent protection settings. (Add and can still be in violation of SCCR even if the feeder device were 200 amps but available fault current were over 10k.

 

[ggunn]

I used to think that this was OK, but then I contacted a couple of switch manufacturers (SqD and Eaton, I believe) and asked them. Both responded that the rating of the OCPD ahead of their unfused switch should never exceed the rating of the switch. I had to eat crow for an inspector, but it wasn't the first time, and sometimes it's him.

 

[MAC702]

...80% rated 225AT Feeder Breaker. I'm confused as to why this is considered unprotected as the breaker should trip at 0.80 x 225A = 180A....

Since I don't know the answer, I wanted to make sure we were all on the same page. This is an adjustable 225A-frame breaker, set for 80%, or 180A? This isn't allowed to be considered a 180A breaker for protection of downstream stuff? The AHJs typically only look at the maximum of the breaker, assuming you might change it later? I don't do work with adjustable breakers, but I got the impression this was the gist of the question, and I'm not sure it was what was mostly addressed.

 

[electrofelon]

Since I don't know the answer, I wanted to make sure we were all on the same page. This is an adjustable 225A-frame breaker, set for 80%, or 180A? This isn't allowed to be considered a 180A breaker for protection of downstream stuff? The AHJs typically only look at the maximum of the breaker, assuming you might change it later? I don't do work with adjustable breakers, but I got the impression this was the gist of the question, and I'm not sure it was what was mostly addressed.

I think turtle is mixing up the 80% rule for OCPD's serving continuous loads with the actual trip characteristics of the OCPD.

 

[turtlepokerman]

I think turtle is mixing up the 80% rule for OCPD's serving continuous loads with the actual trip characteristics of the OCPD.

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.

 

[infinity]

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.

I see your point about the 200 amp disco since the connected load is only 120 amps however I can't think of a code section that would allow the 200 amp switch to be fed from a 225 amp circuit.

 

[david luchini]

I see your point about the 200 amp disco since the connected load is only 120 amps however I can't think of a code section that would allow the 200 amp switch to be fed from a 225 amp circuit.

I can't think of a code section that would prohibit it.

For cranes, 610.31 and 610.33 in conjunction with 610.41 and 610.42 certainly seem to permit a disconnect switch that is smaller than the OCPD.

For motors, 430.110 in conjunction with 430.52 permit a disconnect switch smaller than the OCPD.

 

[turtlepokerman]

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.

So in short, I'm coming to the conclusion that I will replace the breaker with the 200A Thermal Element. Even though the connected load is only rated for 120A and will never see the full load utilized across the switch or breaker. There is always the possibility that somebody will try to add on additional loading to the disconnect switch at a later date without checking the load or upstream breaker rating and could potentially overload the switch.

 

[kwired]

we all put 15 amp switches on 20 amp circuits for general lighting circuits quite often and that seldom gets questioned

 

[turtlepokerman]

we all put 15 amp switches on 20 amp circuits for general lighting circuits quite often and that seldom gets questioned

If I told the inspector that, I already know what the answer would be. Just because it is common practice doesn't mean that it is the right thing to do. This has happened quite a bit.

<Me banging head against wall whenever he says that>

 

[david luchini]

If I told the inspector that, I already know what the answer would be. Just because it is common practice doesn't mean that it is the right thing to do. This has happened quite a bit.

<Me banging head against wall whenever he says that>

Ask the inspector to give you what Code section is being violated. Better yet, show him that the Disconnect and OCPD are properly sized per the relevant sections in Art 610.

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.

A circuit breaker is designed to carry its rated load indefinitely in test conditions. It makes no sense that a 225A breaker would trip at 180A.

 

[turtlepokerman]

Ask the inspector to give you what Code section is being violated. Better yet, show him that the Disconnect and OCPD are properly sized per the relevant sections in Art 610.

A circuit breaker is designed to carry its rated load indefinitely in test conditions. It makes no sense that a 225A breaker would trip at 180A.

Arcticle 610.31 & 610.32 deal with the runway conductors. The disconnect switch is located on the ground and feeds the overcurrent protection for the VFDs that then feed the runway conductors. Article 610 doesn't mention anything about the protection of the disconnect switch it just mentions the sizing of the disconnect switch for the motors.

For the 80% rating question: Please see the literature from the breaker manufacturer: https://literature.rockwellautomation.com/idc/groups/literature/documents/sg/140g-sg001_-en-p.pdf
Check out Pg. 13 the 80% rating section.

 

[david luchini]

Article 610 doesn't mention anything about the protection of the disconnect switch it just mentions the sizing of the disconnect switch for the motors.

That's the point...No sections in the NEC mention anything about protection of the disconnect. The NEC tells you how to size the disconnect and how to size the OCPD. You could meet each of those requirements and have an OCPD larger than the disconnect.

For the 80% rating question: Please see the literature from the breaker manufacturer: https://literature.rockwellautomation.com/idc/groups/literature/documents/sg/140g-sg001_-en-p.pdf
Check out Pg. 13 the 80% rating section.

I'm not sure what you are asking me to look at. The 225A breaker will trip above 225A, not 180A.

 

[kwired]

If I told the inspector that, I already know what the answer would be. Just because it is common practice doesn't mean that it is the right thing to do. This has happened quite a bit.

<Me banging head against wall whenever he says that>

Which at first may seem logical, but I would be asking for code section or listing that prohibits it.

Again my earlier post mentions 30 amp unfused safety switch with HP rating for 20 HP @ 480 volts - the motor is within the current rating of the switch, but if you used a fused switch you need to jump to a 60 amp switch, not because the switch can't handle it but because the necessary fuse for this motor won't fit into a 30 amp fuse holder.


404.15(A):"Switches shall be marked with the current, voltage, and, if horsepower rated, the maximum rating for which they are designed."

There is current and voltage requirements but I see no overcurrent protection requirements for switches, it would have to be listing requirement if there is any limitation here as I don't see it in NEC.

 

[MAC702]

...A circuit breaker is designed to carry its rated load indefinitely in test conditions. It makes no sense that a 225A breaker would trip at 180A.

This is why I thought we must have been talking about an adjustable breaker. Thanks.

 

[turtlepokerman]

That's the point...No sections in the NEC mention anything about protection of the disconnect. The NEC tells you how to size the disconnect and how to size the OCPD. You could meet each of those requirements and have an OCPD larger than the disconnect.

I'm not sure what you are asking me to look at. The 225A breaker will trip above 225A, not 180A.

I agree nothing states in the NEC how to protect a switch. I have contacted the switch manufacturer to see if there is a UL requirement on the protection of the switch. The facility I am working on has a requirement that all equipment must be UL Listed.

The link above is from the manufacturer that describes how an 80% rated breaker works and what the difference is between a 100% rated breaker.

 

[david luchini]

The link above is from the manufacturer that describes how an 80% rated breaker works and what the difference is between a 100% rated breaker.

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

 

[kwired]

The 80% thing is mostly related to termination rating and has little to do with trip curve of the overcurrent device.

You can put 100% non continuous load on the device, but when you go from 2 hours 59 minutes to 3 hours suddenly the device terminations operate at higher temperature if you don't increase conductor size and raise the trip point by another 25%. Yet you can go 2 hours 59 minutes, shut the load off for just a few seconds and start the count over again and everything is supposedly fine:ashamed1: