MLO service panel rating with continuous loads

[electrofelon]

Say I have 1000 amps of continuous load and the service equipment is a MLO service rated panel with 4 breakers serving as the service disconnects, and the buss has a 1000 amp rating. Compliant?

 

[Smart $]

Say I have 1000 amps of continuous load and the service equipment is a MLO service rated panel with 4 breakers serving as the service disconnects, and the buss has a 1000 amp rating. Compliant?

No, typically. Yes, atypically, if the panel and all associated equipment therein is rated for 100% operation. The panel bus is considered a service entrance conductor (or portion thereof). Refer to 230.42(A) for the specifics.

 

[electrofelon]

No, typically. Yes, atypically, if the panel and all associated equipment therein is rated for 100% operation. The panel bus is considered a service entrance conductor (or portion thereof). Refer to 230.42(A) for the specifics.

Hold on I meant to say 6 breakers serving as the disconnects. Assume the load is evenly distributed. Now what say you?

 

[electrofelon]

The panel bus is considered a service entrance conductor (or portion thereof).

Can you give some justification for that statement?

 

[Smart $]

Hold on I meant to say 6 breakers serving as the disconnects. Assume the load is evenly distributed. Now what say you?

The number of or ratings of service disconnects have no bearing on being non-compliant.

Can you give some justification for that statement?

The panel bus is on the line side of the 6 service disconnects.

1000A of continuous calculated load requires 1250A minimum rated service entrance conductors including the bus of an MLO panelboard [230.42(A)(1) & (2)]. The only "exception" would be a 100%-rated panelboard, enclosure, and all contained breakers [230.42(A)(3)].

 

[electrofelon]

The number of or ratings of service disconnects have no bearing on being non-compliant.

well in my OP each of those breakers would be carrying 100% of their capacity and I wanted to take that variable out of play.


The panel bus is on the line side of the 6 service disconnects.

1000A of continuous calculated load requires 1250A minimum rated service entrance conductors including the bus of an MLO panelboard [230.42(A)(1) & (2)]. The only "exception" would be a 100%-rated panelboard, enclosure, and all contained breakers [230.42(A)(3)].

Can you elaborate, I am not seeing panel boards or panel board busses mentioned in 230.42. Am looking at the 2008 code, has this changed recently?

How about this. Say I have a non service MLO panelboard rated 250 amps. Say it is fed from a 100% rated 250 breaker and 250 amp feeder. Can I serve 250 amps of continuous load off that panel?

 

[jim dungar]

Can I serve 250 amps of continuous load off that panel?

Yes.

 

[electrofelon]

Yes.

Ok. I concur. I am not seeing a prohibition from the service equipment (OP) case however. There is no requirement that a panelboard be rated for 125% of the continuous load. ?

 

[Smart $]

Ok. I concur. I am not seeing a prohibition from the service equipment (OP) case however. There is no requirement that a panelboard be rated for 125% of the continuous load. ?

I believe you are looking at it from the perspective of service OCPD, which is actually required to be not greater than the ampacity of the service conductors... though there are several exceptions under 230.90(A).

However, that only covers the breaker rating of the service equipment... not the panelboard bus!

As for you being under 2008 Code, I was using 2014, which has a change, but not relevant other than reference number. Looking at the 2011 Code, in your case, 230,42(A)(1) or (A)(2) applies to the bus. The former being the requirement for 125% continuous, and the latter being the "exception" for 100%-rated service equipment. I don't have a copy of 2008 Code readily handy... but will get access if necessary.

 

[electrofelon]

I believe you are looking at it from the perspective of service OCPD, which is actually required to be not greater than the ampacity of the service conductors... though there are several exceptions under 230.90(A).

However, that only covers the breaker rating of the service equipment... not the panelboard bus!

As for you being under 2008 Code, I was using 2014, which has a change, but not relevant other than reference number. Looking at the 2011 Code, in your case, 230,42(A)(1) or (A)(2) applies to the bus. The former being the requirement for 125% continuous, and the latter being the "exception" for 100%-rated service equipment. I don't have a copy of 2008 Code readily handy... but will get access if necessary.

This is an interesting situation. I am trying to follow the logic and reasoning for all the 125% stuff. It is actually a real world issue too as this could safe thousands of dollars on a prospective job where the difference would be a 1000 panel board vs a 1600 amp swithboard. It seems to me that it all starts because of the breaker. Although the 125% applies to conductors too, I think they ultimately dont have an issue with the conductor taking full continuous load, its just because of the terminations and perhaps temperature of the conductor near the breaker. Now the panelboard/breaker assembly has to be tested/approved for 100% continuous use because the breaker will generate more heat and the panelboard needs appropriate ventiliation and clearances. Panel boards themselves dont have a derating requirement for continuous loads. So putting that all together, what about my case of a MLO serving continuous loads? If all the branch breakers/service disconnects are only used at 80% or less I see no requirement to derate the buss of the panel board. Oddly the service conductors would have to be sized to 125% but not the bus - or so I argue. I dont see that 230.42 (2008 nec) applies. It does say, "the maximum allowable current of busways shall be that value for which the busway has been listed or labeled." I'm not sure if that is what you are referencing, but a busway is not a panel board. Can you explain why 230.42 applies to the MLO bus?

 

[Smart $]

... Can you explain why 230.42 applies to the MLO bus?

Just my impression over the years of interpreting Code, along with other opinions voiced here on the forum. I agree the issue merits debate, and my opinion is not so steadfast that it cannot be changed.

230.42(A) applies to the general rating of service entrance conductors. Under Article 100 definitions for the two types of service entrance conductors, both say they end at the terminals of the service equipment.

Now look at 230.62 and note that service equipment is required to be enclosed or guarded. To me, that means the panelboard enclosure is not actually service equipment, but rather contains service equipment.

From there, the only service equipment mentioned by name under Article 230 are disconnecting means and overcurrent protection. In the case we are discussing, breakers serve as both. As such, and by definition, the service entrance conductors end at the line-side terminals of the breakers. Bus and tangs connected thereto are service entrance conductors by my reasoning.

 

[jim dungar]

... As such, and by definition, the service entrance conductors end at the line-side terminals of the breakers. Bus and tangs connected thereto are service entrance conductors by my reasoning.

This would be my reasoning also.:thumbsup:

 

[kwired]

This is an interesting situation. I am trying to follow the logic and reasoning for all the 125% stuff. It is actually a real world issue too as this could safe thousands of dollars on a prospective job where the difference would be a 1000 panel board vs a 1600 amp swithboard. It seems to me that it all starts because of the breaker. Although the 125% applies to conductors too, I think they ultimately dont have an issue with the conductor taking full continuous load, its just because of the terminations and perhaps temperature of the conductor near the breaker. Now the panelboard/breaker assembly has to be tested/approved for 100% continuous use because the breaker will generate more heat and the panelboard needs appropriate ventiliation and clearances. Panel boards themselves dont have a derating requirement for continuous loads. So putting that all together, what about my case of a MLO serving continuous loads? If all the branch breakers/service disconnects are only used at 80% or less I see no requirement to derate the buss of the panel board. Oddly the service conductors would have to be sized to 125% but not the bus - or so I argue. I dont see that 230.42 (2008 nec) applies. It does say, "the maximum allowable current of busways shall be that value for which the busway has been listed or labeled." I'm not sure if that is what you are referencing, but a busway is not a panel board. Can you explain why 230.42 applies to the MLO bus?

I learned that a standard breaker depends on the attached conductors to function as a "heat sink" and that is why we need to size conductors for continuous loads connected to these standard devices at 125%.

That means a breaker designed for 100% continuous operation doesn't depend on the conductor being a heat sink and that is why we can size the conductor to 100% of the load in those instances.

Chances are with a breaker that attaches to a bus that means it is sinking some heat into the bus as well so the bus maybe would have to be rated for 100% continuous operation as well if using it with a 100% breaker.

I don't really know much first hand about 100% rated breakers, I would guess most of them today are electronic trip devices and that would help explain a lot about why they wouldn't necessarily need a heat sink from a conductor compared to one with a thermal type trip operation.

 

[electrofelon]

Just my impression over the years of interpreting Code, along with other opinions voiced here on the forum. I agree the issue merits debate, and my opinion is not so steadfast that it cannot be changed.

230.42(A) applies to the general rating of service entrance conductors. Under Article 100 definitions for the two types of service entrance conductors, both say they end at the terminals of the service equipment.

Now look at 230.62 and note that service equipment is required to be enclosed or guarded. To me, that means the panelboard enclosure is not actually service equipment, but rather contains service equipment.

From there, the only service equipment mentioned by name under Article 230 are disconnecting means and overcurrent protection. In the case we are discussing, breakers serve as both. As such, and by definition, the service entrance conductors end at the line-side terminals of the breakers. Bus and tangs connected thereto are service entrance conductors by my reasoning.

Hmmmm... With all due respect, part of me cant believe what I am hearing from you and Jim. So panel board busing is a conductor? I dont even want to look at 310 and see how many paradoxes would arise Ill buy the argument that the enclosure of a panel board, may not be service equipment, but to start dissecting the interior....? Besides, no one is suggesting the service entrance conductors terminate on the enclosure, in the MLO case the terminals of the service equipment would be the lugs on the busing. I admit that using a MLO panel board with multiple breakers as disconnecting means has always seemed a little odd to me; when planning to use one, I always do a double take thinking at first its a big code violation. If you follow and agree with my reasoning for the 125% rule and what its purpose is, then what would be the need to upsize the MLO busing?

 

[kwired]

Hmmmm... With all due respect, part of me cant believe what I am hearing from you and Jim. So panel board busing is a conductor? I dont even want to look at 310 and see how many paradoxes would arise Ill buy the argument that the enclosure of a panel board, may not be service equipment, but to start dissecting the interior....? Besides, no one is suggesting the service entrance conductors terminate on the enclosure, in the MLO case the terminals of the service equipment would be the lugs on the busing. I admit that using a MLO panel board with multiple breakers as disconnecting means has always seemed a little odd to me; when planning to use one, I always do a double take thinking at first its a big code violation. If you follow and agree with my reasoning for the 125% rule and what its purpose is, then what would be the need to upsize the MLO busing?

How about the load side of a main breaker connected to a panel bus and with no additional overcurrent protection we install subfeed lugs? Wouldn't any thing connected to those lugs be a feeder or a feeder tap? And isn't the bus essentially a separate segment of conductor protected by the same supply side device? It is not an Art 310 conductor but still is a conductor.

Art 100 doesn't have a definition for conductor, but does mention three types of conductors - bare, covered and insulated.

 

[jim dungar]

I learned that a standard breaker depends on the attached conductors to function as a "heat sink" and that is why we need to size conductors for continuous loads connected to these standard devices at 125%.

Almost but not quite.

All breakers and fuses are tested, per UL standards, at 100% loading in a controlled ambient in free air. Yes, the conductors play a part as a heat sink.

Because the ambient is not controlled, and especially because the devices are not in free air in the real world, a 125% , or 80%, compensation/adjustment factor has been adopted.

For 100% rated devices, the conductor is still a heat sink, but the enclosure has much more ventilation, bringing its inside closer to free air.

 

[Smart $]

Hmmmm... With all due respect, part of me cant believe what I am hearing from you and Jim. So panel board busing is a conductor? I dont even want to look at 310 and see how many paradoxes would arise

See below...

310.1 Scope. This article covers general requirements for
conductors and their type designations, insulations, markings,
mechanical strengths, ampacity ratings, and uses.
These requirements do not apply to conductors that form an
integral part of equipment, such as motors, motor controllers,
and similar equipment, or to conductors specifically
provided for elsewhere in this Code.

Ill buy the argument that the enclosure of a panel board, may not be service equipment, but to start dissecting the interior....? Besides, no one is suggesting the service entrance conductors terminate on the enclosure, in the MLO case the terminals of the service equipment would be the lugs on the busing.

Please reference the section of Code that says the service entrance conductors end at the MLO terminals... or the section that states the busing is service equipment.

I admit that using a MLO panel board with multiple breakers as disconnecting means has always seemed a little odd to me; when planning to use one, I always do a double take thinking at first its a big code violation. If you follow and agree with my reasoning for the 125% rule and what its purpose is, then what would be the need to upsize the MLO busing?

Busing is still a conductor, just not a wire conductor. It will heat up as a result of handling current just like wire. It will also act as a heat sink the same as a wire conductor. The sinking of breaker heat is not limited to just the load side of the breaker. Busing may not have any insulation covering to be concerned with regarding temperature, but what heat is generated and sinked will transfer to the supply conductors all the same.

Look at 408.30. Note panelboards must be rated not less than the minimum feeder (supply conductors) ampacity. The minimum feeder ampacity under 215.2(A) must be noncontinuous plus 125% continuous unless the assembly is 100% rated. In this case, regardless of where they end, the service entrance conductors are the supply conductors.

 

[iwire]

... As such, and by definition, the service entrance conductors end at the line-side terminals of the breakers. Bus and tangs connected thereto are service entrance conductors by my reasoning.

This would be my reasoning also.:thumbsup:

I agree with Smart and Jim on this.

 

[electrofelon]

See below...

​

Please reference the section of Code that says the service entrance conductors end at the MLO terminals...

here:

service entrance conductors: the service conductors between the terminals of the service equipment and.......

Service equipment: the necessary equipment, usually consisting of a circuit breaker(s) or switches(s) and fuses(s) and their accessories....

I personally consider the busing to fit under the accessory category. And this implies overcurrent devices and disconnects are not the only service equipment:

230.94 relative location of overcurrent device and other service equipment

or the section that states the busing is service equipment.

here:

408.3(A)(2) Service switchboards: Barriers shall be placed in all service switchboards such that no uninsulated, ungrounded service busbar

I looks like busing is differentiated from a conductor:

408.3 support and arrrangement of busbars and conductors

Look at 408.30. Note panelboards must be rated not less than the minimum feeder (supply conductors) ampacity. The minimum feeder ampacity under 215.2(A) must be noncontinuous plus 125% continuous unless the assembly is 100% rated. In this case, regardless of where they end, the service entrance conductors are the supply conductors.

I dont disagree. This would not apply to the OP situation

310.1 Scope. This article covers general requirements for
conductors and their type designations, insulations, markings,
mechanical strengths, ampacity ratings, and uses.
These requirements do not apply to conductors that form an
integral part of equipment, such as motors, motor controllers,
and similar equipment, or to conductors specifically
provided for elsewhere in this Code.

I dont see panelboards listed and just from their prevalence and importance and the items that are specifically listed in that statement, I wouldnt consider them "similar equipment"
Besides you cant have it both ways: If the bus of the panelboard is one of these exempted conductors, we dont really have to check it, size, it, coordinate with it other than looking at the nameplate - just like you would with a motor or other conductor integral to manufactured equipment.

I agree with Smart and Jim on this.

Fine, Im starting my own clubhouse

 

[Smart $]

...

Fine, Im starting my own clubhouse

You make some fair points. Here's some counterpoints...

I would consider a breaker bolt or lug, or fuse holder and lugs as accessories... but not busbar and tangs, barriers, etc.... JMO, as it is not clearly defined.

While 408.3 differentiates busbar and conductor, and (A)(2) therein uses the term service busbar, Article 230 mentions nothing about busbar or busing (other than busway and cablebus) in its stipulating the requirements of service conductors and equipment.

It could take a formal request for interpretation to clear this up, but FWIW another forum participant got an informal interpretation just by emailing the NFPA. I suggest you do that, and post their reply here... or you could just ask the AHJ for the project.