New Service Gear Ampacity versus 100% continuous duty

[Bill K]

Services article is confusing. Once a service size is calculated, I understand it is not needed for the "service size" to be larger than that calculated.
But can one then infer, unlike for other calculated loads, that the main service gear need not be sized 125% larger? A fellow contractor is convinced he doesn't need 100% duty rated gear, he just needs to size the main gear just at or above the calculated service ampacity. If that's the case, why is 100% continuous duty main service gear even manufactured in the first place?
The wording of the services article is confusing at best in this regard.

 

[electrofelon]

Services article is confusing. Once a service size is calculated, I understand it is not needed for the "service size" to be larger than that calculated.
But can one then infer, unlike for other calculated loads, that the main service gear need not be sized 125% larger? A fellow contractor is convinced he doesn't need 100% duty rated gear, he just needs to size the main gear just at or above the calculated service ampacity. If that's the case, why is 100% continuous duty main service gear even manufactured in the first place?
The wording of the services article is confusing at best in this regard.

Part if the confusion may be due to the fact that "gear" isnt rated 80%/ 100%/ etc and doesn't need to be sized at 125% of the continuous load. Ocpd's do, but not panelboards and switchboards

 

[iwire]

If you calculate the loads correctly you will already have applied 125% to any loads that require it.

That being the case there is no reason to add 125% to all of the loads.

 

[Smart $]

230.23 and 230.31 for overhead and underground service conductors state... "conductors shall have sufficient ampacity to carry the current for the load as calculated in accordance with Article 220 and shall have adequate mechanical strength."

230.42 for service-entrance conductors in short state the conductors must be rated for "The sum of the noncontinuous loads plus 125 percent of continuous loads".

Note there are instances where there are no service-entrance conductors when there are underground service conductors.

230.79 requires the service disconnecting means to be rated not less than the calculated load.

230.90 requires the OCPD to be not greater than the allowable ampacity of the service conductors (but permits next higher rating under Exception No. 2)...

...but in no case are we required to have a disconnecting means or OCPD rating which considers continuous loads at 125%.

 

[electrofelon]

230.23 and 230.31 for overhead and underground service conductors state... "conductors shall have sufficient ampacity to carry the current for the load as calculated in accordance with Article 220 and shall have adequate mechanical strength."

230.42 for service-entrance conductors in short state the conductors must be rated for "The sum of the noncontinuous loads plus 125 percent of continuous loads".

Note there are instances where there are no service-entrance conductors when there are underground service conductors.

230.79 requires the service disconnecting means to be rated not less than the calculated load.

230.90 requires the OCPD to be not greater than the allowable ampacity of the service conductors (but permits next higher rating under Exception No. 2)...

...but in no case are we required to have a disconnecting means or OCPD rating which considers continuous loads at 125%.

So in summary, is the following correct:

Say I have only luminaires on a service and their nameplate is their actual load which all adds up to 400 amps. Say they are all continuous loads. My branch circuits and feeders and the OCPD's protecting them of course must be sized at 125% per 210.19(A)(1), 210.20(A), 215.2(A)(1), 215.3. I can have a 400 amp panelboard with 400 amp main breaker - no requirement for 125% for the panelboard or the main breaker. The service conductors however must be sized at 125% per 230.42.

Correct?

 

[Smart $]

So in summary, is the following correct:

Say I have only luminaires on a service and their nameplate is their actual load which all adds up to 400 amps. Say they are all continuous loads. My branch circuits and feeders and the OCPD's protecting them of course must be sized at 125% per 210.19(A)(1), 210.20(A), 215.2(A)(1), 215.3. I can have a 400 amp panelboard with 400 amp main breaker - no requirement for 125% for the panelboard or the main breaker. The service conductors however must be sized at 125% per 230.42.

Correct?

Correct except the service conductor part. Those depend on whether they are service-entrance or not. If not, they can be 100%.

Also, do not take that completely out of context. I'm just saying that's all Article 230 requires in and of itself. For example, you may run into a problem with using a main breaker within its listing if you size it without factoring continuous loads by 125%.

Ultimately, the safest approach is to size for noncontinuous and 125% continuous.

 

[david luchini]

I can have a 400 amp panelboard with 400 amp main breaker - no requirement for 125% for the panelboard or the main breaker.

Correct?

That is not correct. The load side of the main breaker is a feeder. The breaker must be sized for 215.3 for feeder protection.

 

[electrofelon]

That is not correct. The load side of the main breaker is a feeder. The breaker must be sized for 215.3 for feeder protection.

David, it sounds like you are saying that the panelboard bus rating must be 125% then? I was in a similar debate about this a while back but that time the question was if the buss rating of a MLO service panelboard (2-6 disconnects) needed to be sized at 125%. IMO the parts of a panelboard are parts of a panelboard and not feeders, or service conductors and there is not article 408 requirement for the 25% adder.

 

[Smart $]

David, it sounds like you are saying that the panelboard bus rating must be 125% then? I was in a similar debate about this a while back but that time the question was if the buss rating of a MLO service panelboard (2-6 disconnects) needed to be sized at 125%. IMO the parts of a panelboard are parts of a panelboard and not feeders, or service conductors and there is not article 408 requirement for the 25% adder.

Factoring continuous load at 125% is not required by Article 230 for service equipment, other than service-entrance conductors, up to and including a service OCPD. However, because a service OCPD also serves as load-side OCP, other articles may require the OCPD rating to be determined by factoring continuous loads at 125%. If so, this determination could also affect service equipment ratings (e.g., you can't have a service panel bus rated less than its overcurrent protection rating).

 

[david luchini]

David, it sounds like you are saying that the panelboard bus rating must be 125% then? I was in a similar debate about this a while back but that time the question was if the buss rating of a MLO service panelboard (2-6 disconnects) needed to be sized at 125%. IMO the parts of a panelboard are parts of a panelboard and not feeders, or service conductors and there is not article 408 requirement for the 25% adder.

"Feeder" is defined as ALL circuit conductors between the service equipment...and the final branch-circuit overcurrent device. I don't see how you could exclude panelboard bus from that.

Similarly, for you MLO service panelboard with 2-6 disconnects, the panelboard bus would fall under the definition of "Service Conductors."

As a thought experiment, what if you took your 400A continuous load example in post #5 and used a standalone 400A MCB on the outside of the building with 2' of cabling to a panelboard in the inside of the building. By your reasoning, would you then need a 500A MCB and a panelboard with bussing rated at not less than 500A to supply the exact same load as your example?

Why would the requirements be different if the load is the same?

 

[Smart $]

"Feeder" is defined as ALL circuit conductors between the service equipment...and the final branch-circuit overcurrent device. I don't see how you could exclude panelboard bus from that.

...

By that reasoning an installation with an MCB panel used as service equipment and with only branch circuits would not have any feeders, including the busses. See Service Equipment definition.

 

[david luchini]

By that reasoning an installation with an MCB panel used as service equipment and with only branch circuits would not have any feeders, including the busses. See Service Equipment definition.

The definition of Service Equipment doesn't support this idea. The MCB of the panel is the "service equipment," it constitutes the main control and shutoff of the supply. The panelboard bus is a feeder.

 

[Smart $]

The definition of Service Equipment doesn't support this idea. The MCB of the panel is the "service equipment," it constitutes the main control and shutoff of the supply. The panelboard bus is a feeder.

The definition...

Service Equipment. The necessary equipment, usually consisting
of a circuit breaker(s) or switch(es) and fuse(s) and
their accessories, connected to the load end of service conductors
to a building or other structure, or an otherwise designated
area, and intended to constitute the main control and cutoff of
the supply.

Is an enclosure necessary equipment for that breaker?
Are the bus and load breakers accessories?

Seems to me you are adapting the definition to your interpretation.

 

[david luchini]

Is an enclosure necessary equipment for that breaker?
Are the bus and load breakers accessories?

No, the bus and load breakers are not accessory to the Main Circuit Breaker. They provide no function in main cutoff and control of the supply.

Seems to me you are adapting the definition to your interpretation.

Seems to me that you are doing the same.

Service Equipment. The necessary equipment, usually consisting
of a circuit breaker(s) or switch(es) and fuse(s) and
their accessories, connected to the load end of service conductors
to a building or other structure, or an otherwise designated
area, and intended to constitute the main control and cutoff of
the supply.

 

[Smart $]

No, the bus and load breakers are not accessory to the Main Circuit Breaker. They provide no function in main cutoff and control of the supply.

Seems to me that you are doing the same.

The panel is purchased as a unit. It has to be rated Suitable for Use as Service Equipment (SUSE).

We can probably go back and forth on this all day and not get anywhere... :lol:

 

[electrofelon]

"Feeder" is defined as ALL circuit conductors between the service equipment...and the final branch-circuit overcurrent device. I don't see how you could exclude panelboard bus from that.

Similarly, for you MLO service panelboard with 2-6 disconnects, the panelboard bus would fall under the definition of "Service Conductors."

I do not believe it is the intent of the code to classify the buss in the former as a feeder, or the buss in the latter as a service conductor. One possible explanation could be that the bussing of a factory assembled and UL listed panelboard is not considered a conductor in the article 310 sense. Or maybe the definition just needs a little tweaking. Or, as is a common occurance in the code, when there is a specific article on something, it overrides the general rules so in the case of a panelboard we go to 408.

As a thought experiment, what if you took your 400A continuous load example in post #5 and used a standalone 400A MCB on the outside of the building with 2' of cabling to a panelboard in the inside of the building. By your reasoning, would you then need a 500A MCB and a panelboard with bussing rated at not less than 500A to supply the exact same load as your example?

I dont follow you. Wouldn't "my reasoning" be that I DONT need the 125% for the inside panelboard? I guess I dont quite follow the thought experiment. Perhaps my belief that the panelboard is not a feeder explains it.

[/QUOTE]

 

[david luchini]

I dont follow you. Wouldn't "my reasoning" be that I DONT need the 125% for the inside panelboard? I guess I dont quite follow the thought experiment. Perhaps my belief that the panelboard is not a feeder explains it.

If you had a 500A feeder (215.2(A)(1)) from the main c/b outside to the mlo panelboard inside, and a 500A main c/b to protect that feeder (215.3), but rated the panelboard for only 400A (because you reason that you don't need 125% for inside the panelboard) then you would be violating section 408.36. The panelboard must also be rated at least 500A.

I do not believe it is the intent of the code to classify the buss in the former as a feeder, or the buss in the latter as a service conductor. One possible explanation could be that the bussing of a factory assembled and UL listed panelboard is not considered a conductor in the article 310 sense.

I don't see anything in the definition of "feeder" that references Article 310...it says ALL circuit conductors. If conductor only refers to conductor in the Article 310 sense, then section 368.17 doesn't make any sense.

And, if the definition of service conductor is the "conductors from the service point to the service disconnecting means," how would the buss not be service conductors in the MLO panel example? The lugs of the panel are certainly not the service disconnecting means.

 

[kwired]

The panel is purchased as a unit. It has to be rated Suitable for Use as Service Equipment (SUSE).

We can probably go back and forth on this all day and not get anywhere... :lol:

Typical "loadcenters" are purchased as a unit. Most other panelboards you get to select components that are added and basically either build your own or at least specify what is assembled before it is sent to you. It is possible to put together an assembly that has code issues, but the complete loadcenter is listed as it is for the most part, and sometimes you still can make field modifications that are in accordance with listing by adding or removing certain components. Some will be SUSE - but only if certain conditions are followed, take away (or add) the wrong component and you are in violation.

I do not believe it is the intent of the code to classify the buss in the former as a feeder, or the buss in the latter as a service conductor. One possible explanation could be that the bussing of a factory assembled and UL listed panelboard is not considered a conductor in the article 310 sense. Or maybe the definition just needs a little tweaking. Or, as is a common occurance in the code, when there is a specific article on something, it overrides the general rules so in the case of a panelboard we go to 408.



I dont follow you. Wouldn't "my reasoning" be that I DONT need the 125% for the inside panelboard? I guess I dont quite follow the thought experiment. Perhaps my belief that the panelboard is not a feeder explains it.

[/QUOTE]

Art 100 definitions:
Feeder.


All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.

Conductor, Bare.
A conductor having no covering or electrical insulation whatsoever.
Conductor, Covered.
A conductor encased within material of composition or thickness that is not recognized by this Code as electrical insulation.
Conductor, Insulated.
A conductor encased within material of composition and thickness that is recognized by this Code as electrical insulation.

There is no definition of "conductor", so we go to common definitions of words from a dictionary. Wouldn't the panel bus be a conductor since it is not otherwise defined as anything else in the NEC? If the bus is a conductor - it is included in "all circuit conductors" in the definition of a feeder.

You can have in this order - circuit breaker - wire type conductor - panel bus - wire type conductor (supplied by subfeed lugs attached to opposite end of panel bus as the supply conductors) - second panel bus, and assume all mentioned components are rated for same current level.

Isn't the first panel bus part of the feeder circuit between the circuit breaker and the second panel bus? If not what do you call it? sure it is a panel bus but it is also a part of a feeder circuit.

Aren't branch breakers plugged on/bolted to the bus essentially feeder taps of very short length?

 

[Smart $]

Typical "loadcenters" are purchased as a unit. Most other panelboards you get to select components that are added and basically either build your own or at least specify what is assembled before it is sent to you. It is possible to put together an assembly that has code issues, but the complete loadcenter is listed as it is for the most part, and sometimes you still can make field modifications that are in accordance with listing by adding or removing certain components. Some will be SUSE - but only if certain conditions are followed, take away (or add) the wrong component and you are in violation.

Absolutely correct.

Point is, where compliant, the unit is listed as SUSE... not just the breaker. You can't connect the main bonding jumper in a breaker.

This discussion is a bit overboard. The short of it is, no Article 230 section requires the service disconnecting means and overcurrent protection to be rated for noncontinuous plus 125% continuous load. Requirements of other Articles may require such. However, those Articles also conditionally permit said equipment to be 100% rated.

 

[electrofelon]

If you had a 500A feeder (215.2(A)(1)) from the main c/b outside to the mlo panelboard inside, and a 500A main c/b to protect that feeder (215.3), but rated the panelboard for only 400A (because you reason that you don't need 125% for inside the panelboard) then you would be violating section 408.36. The panelboard must also be rated at least 500A.

I am aware of 408.36 and agree that it certainly makes it "hard" to load a panel bus to its rating with continuous loads. The only way I can think to do it would be to have a 100% rated breaker supplying the feeder.

I don't see anything in the definition of "feeder" that references Article 310...it says ALL circuit conductors. If conductor only refers to conductor in the Article 310 sense, then section 368.17 doesn't make any sense.

I agree that 368.17 adds some weight to your point of view. OTOH, wow the first instance of the NEC not making sense :roll:

And, if the definition of service conductor is the "conductors from the service point to the service disconnecting means," how would the buss not be service conductors in the MLO panel example? The lugs of the panel are certainly not the service disconnecting means.

There is no definition of "conductor", so we go to common definitions of words from a dictionary. Wouldn't the panel bus be a conductor since it is not otherwise defined as anything else in the NEC?

IMO the bussing is an integral part of the equipment defined in 408. See 310.1. Also the code was not written by god. It is not perfect and logical and without faults - this isnt mathematics. I dont think the intent in the statement "....all conductors...." is meant to apply to everything. If every conductive thing was a conductor than OCPD's are conductors, a wire nut is a conductor, a set screw butt spice is a conductor - how do I verify the circular mills and ampacity of that?? Where to the service conductors end EXACTLY? At the breaker terminals? The breaker terminals are conductors. Lets follow the breaker bussing into the mechanism.......