Article 230.79: Rating of Service Disconnecting Means

[Jim Bujtor]

Hi, I have a two-part question about the interpretation of this small paragraph: "The service disconnecting means shall have a rating not less than the calculated load to be carried....".

We have an existing panel where the future projected load is going to be much lower than the building was originally designed for. The first question is: if you have a main breaker in the main service entrance panel that has the ability of accommodating multiple trip settings, say it is a 3000 amp frame breaker and you set the "trip setting" to 50% or 1500 amps, did you change the "rating" of the service disconnecting means? If the answer to the first question is "no, the rating did not change", then we know we can change the trip setting without affecting the rating and the second question is, "what should the trip setting be set to: the engineering calculated maximum load or the connected load?" (the maximum load is typically much lower than the connected load calculation).

I believe there is a difference between a 'rating' and a 'setting', but my local building official does not agree yet. I believe that the intent of the NEC and that section also is so that you don't have a panel, busswork, and breaker frame size that is smaller than your calculated maximum load. That makes sense. Yet, if you find that your highest load is only going to be half of the 'rating' of the existing panel, it would be prudent to set the trip to be lower, for life safety and equipment damage precaution in sync with the overall intent of the NEC to begin with!?! To me, lowering a trip setting in this situation is prudent and still maintains the overall "rating" of the system.

I believe lowering the trip setting to match near what your calculated maximum load demand is safer and better protection for personnel and equipment, than leaving the trip setting higher. Basically, in the event of an overload or fault condition, the breaker would trip sooner with the lower trip setting, which would be a safer installation. For those of us who have seen melted main breakers, bussway, etc., it is obvious that limiting the overcurrent trip setting is more prudent and more safe.?

 

[pfalcon]

Hi, I have a two-part question about the interpretation of this small paragraph: "The service disconnecting means shall have a rating not less than the calculated load to be carried....". ... , did you change the "rating" of the service disconnecting means? If the answer to the first question is "no, the rating did not change", then we know we can change the trip setting without affecting the rating ... .?

"no, the rating did not change"

The rating of the disconnecting means is not affected by the setting of the disconnecting means. A comparable example would be placing a 10A fuse in a 30A fuse block. The block is still rated for 30A. The rating of a component is established by the manufacturer of the component not by its usage.

 

[jfandre]

I don't think the answer really addresses the question. The question was specific to an adjustable trip circuit breaker. If you look at 240.6(B), it clearly states that the rating of the breaker is the maximum setting possible, in the case described, 3000 amperes. However, if conditions in 240.6(C) are met, specifically refering to restricted access, it says that the rating may be considered equal to the setting. Refering back to 230.79, the language is specifically refering to the rating of the disconnecting means, not the setting.

So, in the case in question, if the adjustable trip breaker does not comply with (C), it is a 3000 ampere device, regardless of the trip setting and the service equipment and conductors being protected by this device would need to be rated a minimum of 3000 amperes. (no allowance for the next higher standard device for the conductors if over 800 amperes)

If there is restricted access as described in (C), then the rating is whatever the setting is. In this case, the setting must still be sufficient to carry the calculated load as required by 230.79. If the building in question has a calculated load, per the NEC, of say 1800 amperes, the setting of the adjustable trip breaker could be no less than 1800 amperes, and the conductors would need to be suitble for 1800 amperes of more. The same rules would apply to any other claculated load for the building.

So, I think the answer to the question is: If the breaker does not provide limited access, yes, the rating does not change regardless of the trip setting - it is a 3000 ampere device for application of 230.79. If there is restricted access to the breaker, the rating becomes the setting, and service equipment and conductors may be sized accordingly.

Is a lower setting and rating safer than a higher one? Sure, as long as it is set high enough to carry the calculated load.

 

[Jim Bujtor]

Ok. But then today the Official pointed me to 240.6 (B) & (C) that appears to say that the the circuit breaker (disconnecting means) is permitted to be the ampere rating of the adjustable trip setting when using the trip setting for long-time pickup and there is restricted access, which is our case.

Am I correct that there are two different 'ratings' being discussed here? One (from 240.6) is an 'ampere' rating of the disconnecting means, and the other is distinct and separate (from 230.79) an 'equipment' rating for the disconnecting means?

Is so, how confusing is that!?

If I'm correct, then in my example above inserting a 50% trip in a 2000 amp breaker, we still have an equipment rating that has not changed (3000 amp), but a disconnecting means ampere rating that has changed to 1500 amps. Or am I all wrong -- meaning their is only one rating and changing the trip setting changes the rating of the disconnecting means?

Thanks in advance for any more help!

 

[lilpky]

NEC 240.6 (B)(C)/Service Disconnectin Means

NEC 240.6 (B)(C)/Service Disconnectin Means

If you read NEC section 240.6 (C), and you have restricted access, the rating of the service disconnect does change if you adjust the trip setting. The rating of the circuit breaker then has the rating of the adjusted trip setting. If there is no access, then the rating of the breaker is at the maxium trip setting. Before setting the trip setting you must have NEC load calculaitons to verify that you do not violate NEC 230.79 by setting it below the NEC calculated load.

 

[Jim Bujtor]

This conflicts with PFalcon and with common sense, doesn't it?

 

[Jim Bujtor]

It still seems there are two definitions: a 'current' rating, and an 'equipment' rating. Does anyone else have a take on this?

 

[augie47]

To me, 230.79 states the "rating not less than the calculated load" and 240.6(C) says a breaker with restricted access "shall have a rating"; therefore, IMO, the 3000 amp frame with a 1500 amp trip would need a minimum of 1500 amp conductors.
Often times the frame sizes are increased to accommodate a higher AIC rating but the trip unit still dictates the rating.

 

[Jim Bujtor]

Under the conditions of restricted access, I agree with the last poster (augie) that a breaker with a 1500amp trip would require a minimum of conductors that can handle 1500amps, but what is still confusing is the use of the word 'rating'. The new 'current rating' is 1500amps in the example given here, but isn't the 'equipment rating' still 3000 amps?

As pfalcon said, the rating of a component is established by the manufacturer of the component not by its usage, which makes sense to me when discussing "equipment ratings". With that logic, the 'equipment rating' is 3000 amps. My interpretation of augie's post is that the 'current rating' is 1500 amps.

So, I'm still searching for confirmation that both of these NEC sections are talking about two different ratings, correct?

 

[jim dungar]

If you take a 3000A breaker and set the Long Time to 1.0 you have a current rating of 3000A. If you set the Long Time to .5, your current rating is now 1500A.

If you mount that 3000A breaker into a bus structure with 1500A bussing, you have an equipment rating of 1500A, regardless what Long Time setting you choose.

I guess I don't see the conflict you are saying exists.
230.79 wants the conductors and equipment to be rated for the load they need to carry.
240.6(C) is about determing the rating when the equipment has an adjustable trip.

Section 240.6(C) allows adjustable trip breakers to be treated the same as interchangeable fuse holders. Prior to it, it was not uncommon for inspectors to require that adjustable trip breakers be wired for with conductors rated for the 1.0 long time setting, yet these same inspectors were allowing #14 conductors to be connected to 30A fusible switches based on 15A fuses being installed.

 

[don_resqcapt19]

...
Section 240.6(C) allows adjustable trip breakers to be treated the same as interchangeable fuse holders. Prior to it, it was not uncommon for inspectors to require that adjustable trip breakers be wired for with conductors rated for the 1.0 long time setting, yet these same inspectors were allowing #14 conductors to be connected to 30A fusible switches based on 15A fuses being installed.

That was just part of the ongoing war between the fuse manufacturers and the breaker manufactures in their education and code writing programs I am sure that there was information out there from the fuse people that led the inspectors to do that. Then the breaker people responded with a code change to make sure the fuse and breaker equipment were treated equally.

 

[jim dungar]

That was just part of the ongoing war between the fuse manufacturers and the breaker manufactures in their education and code writing programs I am sure that there was information out there from the fuse people that led the inspectors to do that. Then the breaker people responded with a code change to make sure the fuse and breaker equipment were treated equally.

Its not like turning the dial on the breaker was a whole lot more difficult than installing a larger fuse.

 

[Jim Bujtor]

I'm having forum problems and wasn't able to see all the posts till now. Jim Dungar and others have confirmed the clarification. Thank You.

"230.79 wants the conductors and equipment to be rated for the load they need to carry. " = **"equipment rating** (exceeding will cause equipment damage and/or personnel danger). This section is clearly not related to changing the adjustable trip at all.
"240.6(C) is about determing the rating when the equipment has an adjustable trip. " = **current rating** (exceeding will trip the breaker or blow the fuse, as designed). This is the only section in the NEC that is related to adjustable trips.

They are indeed two different ratings.

1: "Equipment rating" is made up of conductors, frame size of devices (breakers or fuse holders), bussing size, and hardware to ensure the backbone electric system is rugged enough to handle the NEC calculated loading plus some -- typically at least 125% of the NEC calculated loading, but sometimes up to and beyond the connected load calculation for overzealous design engineers. This is where you see a 3000 amp equipment rated system for a 1500 amp current rated load and represents the building's backbone wiring and panels, sized as a permanent installation.

2: The "current rating" is the circuit breaker trip setting or actual fuse, not the 'breaker' itself. Typically, when the term 'breaker' is used generically, it refers to the frame size. It is usually something less than the 'equipment rating' -- typically 60-80% of the NEC calculated loading for commercial loads in my experience. This "current rating" is more easily changed than the "equipment rating".

It should be clear more than before from this exchange that the "equipment" rating does not change when you change the trip setting or fuse -- proving that 230.79 does NOT change the rating of the equipment when you change the trip setting. Instead, it is only the "current" rating of the device that changes (240.6C). The "equipment" rating doesn't change unless you change the actual conductors, frame size, bussing, etc.

So, to summarize, changing the trip setting of a breaker:

230.79 (equipment rating) does NOT change the rating of the service disconnecting means as that is the rating of the more permanent, fixed electric system, if you will, and
240.6(C) (current rating) does NOT change the "equipment rating" of the service disconnecting means either, but DOES change the "current rating".

In essence, by making a lower trip setting, you are only limiting the ability of the breaker to pass more current. This is not less safe. Since this setting has nothing to do with the 'equipment rating', you can lower the trip setting as low as you want -- even below the NEC calculated load, as long as you are prepared to deal with the consequences of nuisance tripping. 240.6(C) only says that it is permitted to make this trip adjustment and doesn't address the consequences. This is the confusing part that I am glad we have wrapped our thoughts around and resolved. There is no way that lowering a trip setting creates a less safe installation-- instead it only has the potential of creating nuisance tripping, which is not an NEC concern (equipment failure and life safety), which is why the consequence isn't addressed in the NEC. Lower trip setting protects equipment.

With a lower trip setting, no equipment will melt or burn up and no personnel will be exposed to any more harm than with a higher trip setting. Instead, the breaker will simply trip before equipment failure or personnel danger presents itself. This is a more stringent and safer installation, as many have agreed. On the other hand, changing the "equipment rating" (i.e. conductors or breaker frame or bussing, etc.) below the NEC calculated loading is a violation of the NEC (230.79) and could cause equipment failure and/or personnel danger, but has nothing to do with the original question of this posting to adjust the trip setting, which clearly does not change the equipment rating.

I'm sure this summary contains grammatical errors and definites that are not always the case, but for purposes of this discussion for this installation, it suffices to provide a good enough resolution to move forward.

I now understand that the NEC limited itself to saying only what it has said in these two sections so that engineers are allowed to make trip setting changes in controlled environments as they see fit. These settings are more stringent than NEC requirements and build in even more equipment protection and personnel safety. Makes great sense. Good job NEC!

 

[Jim Bujtor]

Thanks in advance for hanging in there on this one. I'd like confirmation on what I said above if anyone would.

The building officials still don't believe there are two different 'ratings' discussed in 230.79 and 240.6. In line with that thought, they also believe that the NEC overcurrent requirements are also meant to limit under-current tripping, which seems opposite of the intent of overcurrent tripping. I still say that lowering a trip setting below the amperage value of the loading calculations is allowed in the NEC, with careful nuisance tripping considerations by the engineer. This limits the amount of current allowed before a breaker interruption, which is safer for personnel and equipment. Is there a problem with this?

I'm having a hard time understanding why the building officials would prefer to allow the maximum current to flow before a breaker interruption; basing their entire stance on an interpretation of 230.79 that doesn't appear to make sense here. It seems that interpretation of rules in a solely text-book way of looking at this has become more important than common sense.

Please respond if you agree or disagree.

 

[Smart $]

...

The building officials still don't believe there are two different 'ratings' discussed in 230.79 and 240.6. ... I still say that lowering a trip setting below the amperage value of the loading calculations is allowed in the NEC, with careful nuisance tripping considerations by the engineer. This limits the amount of current allowed before a breaker interruption, which is safer for personnel and equipment. Is there a problem with this?

I'm having a hard time understanding why the building officials would prefer to allow the maximum current to flow before a breaker interruption; basing their entire stance on an interpretation of 230.79 that doesn't appear to make sense here. It seems that interpretation of rules in a solely text-book way of looking at this has become more important than common sense.

Please respond if you agree or disagree.

Well I agree there are two ratings. That's easily discernable from Article 230 having Part VI. Service Equipment — Disconnecting Means, and Part VII. Service Equipment — Overcurrent Protection, even though in many instances they are integrated in the form of a circuit breaker, or an isolated piece of equipment such as a fused disconnect switch. The respective ratings are established by 230.79 (& 230.80) and 230.90.

As for OCPD rating (or setting), I don't see anywhere that it is specified the rating (or trip setting) must equal or exceed the calculated load current value. Only requirement I can find is that the rating (or trip setting) cannot be greater than the allowable ampacity of the connected conductor. I have a feeling I'm overlooking something, but that is what I'm coming up with at present. I'm open to correction...

Ohh! ...and safety is a subjective term. While maintaining power can cause safety issues, so can cutting power.

 

[Jim Bujtor]

Thanks Smart$. You bring up an important point that Article 230 is located in the equipment section as a piece of hardware disconnect, which solidifies my conjecture a bit more. It follows, as you have said, that the rating (or trip setting) cannot be greater than the allowable ampacity of the connected conductor (or the disconnect, or breaker frame size, etc). I believe what you have is complete. It clearly does not set an overcurrent requirement regarding trip settings with respect to calculated load.

240.6(C) does permit you to consider the overcurrent rating of a circuit breaker to be equal to the trip setting, but it also doesn't specify that the trip setting should be lower, equal, or higher than the calculated load. Common engineering sense would say to have the trip setting high enough to avoid nuisance tripping, but that's about the extent of this.

I agree that cutting power to a hospital or other life-safety institution or load can be a safety issue. But the load in question is office space and computer rooms.

 

[pfalcon]

So, to summarize, changing the trip setting of a breaker:

230.79 (equipment rating) does NOT change the rating of the service disconnecting means as that is the rating of the more permanent, fixed electric system, if you will, and
240.6(C) (current rating) does NOT change the "equipment rating" of the service disconnecting means either, but DOES change the "current rating".

Sounds like you've got it to me. And yes, the NEC would have done us all a favor if 240.6 had made the distinction. You can plan the wiring at the setting value provided you protect it from elves.

 

[Smart $]

...

I agree that cutting power to a hospital or other life-safety institution or load can be a safety issue. But the load in question is office space and computer rooms.

Well you have safety issues there, too. For example, emergency lighting kicks in on power loss, but it cannot compete with the safety provided by normal lighting.

 

[jfandre]

So let me try to understand this: The NEC requires all of the service equipment/conductors/disconnecting means to be rated to no less than the calculated load (load to be served). In order to comply, that means the installation contains only gear that will continuously carry that calculated demand indefinitely without failure, overheating, deterioration, or any other bad consequences. That gear, wire, busing, etc. was tested and listed by a nationally recognized testing lab, and there is certainly a safety margin built into the test standard. By my way of thinking, installing a main overcurrent device set to trip at the calculated demand is a safe installation. So where is the added safety that is being touted by going to a lower trip setting? I don't see it. just saying it's safer doesn't make it so. Please, someone explain how it's safer.

On the other hand, reducing the trip setting of the device does certainly lessen safety in that any unplanned, unexpected loss of power, while the building is operating within design parameters mind you, can result in a host of unsafe conditions: elevators stuck between floors, people in stairways that suddenly go dark, kitchens with hot surfaces, deep fat fryers, sharp implements being used when the lights suddenly go out, a machine shop with equipment that continues to spin from centrifugal force in the dark, security systems that shut down, critical ventilation in a laboratory that stops, and the list could go on and on. Yes, some buildings require emergency lighting, but not all, and not in all areas. how safe is it for the lights in your home to go out when you're carrying an armload of something down your stairs? Remember, this is in Chapter 2 that applies generally to all installations unless modified in Chapters 5, 6, or 7. I can't begin to agree with a statement that says a sudden, unexpected loss of power is just an inconvenience and doesn't pose a safety risk.

And consider the building owner, who pays for an electrical system to handle the full calculated load, and is then told that he/she can only use 2/3 of that capacity before the facility shuts down without warning and for no reason whatsoever. I would not want to be that design professional having to explain that one.

Lastly I have to wonder why you need to install all fully rated equipment if you are going to set the trip significantly below that. If I have a 200 amp service on a connected load of 160 amps, and I put a 125 amp fuse in the service, why couldn't I then reduce my wire size to 125 amps? and the service rating? and the busing? after all, it's fully protected. And if I do that, what is my service size? Is it still a 200 amp service, or is it now a 125 amp service because that's the maximum it can ever carry?

I think this discussion about the difference between "ratings" is not the issue; it is clouding the real issue. The issue is, would an AHJ approve an installation that will not let a building operate up to the capacity that the NEC says it needs? I for one would never approve such an installation, and in the course of my career as an AHJ have turned down such designs. If the utility thinks there is additional safety in reducing the building capacity, why not just fuse the transformer down to whatever level it determines is appropriate? Then there is no discussion about the NEC's intent and the utility can explain to the building owner why the lights went out when there was no overload/short circuit/ground fault condition. :slaphead:

 

[Smart $]

So let me try to understand this: The NEC requires all of the service equipment/conductors/disconnecting means to be rated to no less than the calculated load (load to be served). In order to comply, that means the installation contains only gear that will continuously carry that calculated demand indefinitely without failure, overheating, deterioration, or any other bad consequences. That gear, wire, busing, etc. was tested and listed by a nationally recognized testing lab, and there is certainly a safety margin built into the test standard. By my way of thinking, installing a main overcurrent device set to trip at the calculated demand is a safe installation. So where is the added safety that is being touted by going to a lower trip setting? I don't see it. just saying it's safer doesn't make it so. Please, someone explain how it's safer.

While lowering the trip setting is part of the OP contention, you missed part of it. The scenario includes a change in calculated load... a substantially lower load... and I believe no change in service equipment other than an adjusted main breaker trip setting. As I recall without re-reading, 3000A to 1500A.