Special permission?

[Isaiah]

It's irrelevant how lightly or how heavily the MCC is loaded. You cannot put 1200A of continuous load on the 1200A MCC. The theoretical maximum continuous load you can put on the MCC is 960A. The theoretical maximum non-continuous load you can put on the MCC is 1200A.

In either case, the feeder would never be required to have an ampacity of 1500A. It would only require an ampacity that is not less than 1200A, as in the case of your 3 sets of 600mcm (the 400 was a typo.)

Let us keep in mind at a high level, the NEC is about safety. Even though as you say "You cannot put 1200A of continuous load on the 1200A MCC" you and I both know someone out there will do it - load it up to 1200A or more 'till the CB trips. That's why the associated feeder must be sized up 125% of the 1200A.

 

[david luchini]

Let us keep in mind at a high level, the NEC is about safety. Even though as you say "You cannot put 1200A of continuous load on the 1200A MCC" you and I both know someone out there will do it - load it up to 1200A or more 'till the CB trips. That's why the associated feeder must be sized up 125% of the 1200A.

If someone loads it up to 1200A or more 'till the CB trips, the 3 sets of 600mcm still have an ampacity of more than 1200A.

There is absolutely no Code requirement to size a feeder for 1500A on a 1200A c/b, nor any physical reason to do so.

 

[Isaiah]

If someone loads it up to 1200A or more 'till the CB trips, the 3 sets of 600mcm still have an ampacity of more than 1200A.

There is absolutely no Code requirement to size a feeder for 1500A on a 1200A c/b, nor any physical reason to do so

The Code requires a safety buffer of 25% in cable size to ensure a fire does not occur due to a slow arcing fault. Hopefully using today's advanced protective relaying such as Schweitzer, Multlin, Basler and others, this never occurs. It remains a NEC requirement nonetheless. Thanks for your input.

 

[wwhitney]

The Code requires a safety buffer of 25% in cable size to ensure a fire does not occur due to a slow arcing fault.

Again, what section number requires that?

Cheers, Wayne

 

[david luchini]

The Code requires a safety buffer of 25% in cable size to ensure a fire does not occur due to a slow arcing fault. Hopefully using today's advanced protective relaying such as Schweitzer, Multlin, Basler and others, this never occurs. It remains a NEC requirement nonetheless. Thanks for your input.

Code does not require that. Glad I could help.

 

[Isaiah]

Again, what section number requires that?

Cheers, Wayne

215.2(A)(1)(a) - repeated several times in the above string.

 

[david luchini]

215.2(A)(1)(a) - repeated several times in the above string.

Again...see post #17

See also 215.2(A)(1)

See also 240.4

And maybe the Examples in Annex D.

 

[jap]

It seems the concern is over what "future" loads may be added to the MCC.

If a little extra wasn't allowed for future expansion, it may in fact be a poor initial design, but, not a code violation as it is right now.

If the future loads do exceed the Main Breaker's rating, worst case, the breaker trips.

That's what it's there for in the first place.

JAP>

 

[don_resqcapt19]

Let us keep in mind at a high level, the NEC is about safety. Even though as you say "You cannot put 1200A of continuous load on the 1200A MCC" you and I both know someone out there will do it - load it up to 1200A or more 'till the CB trips. That's why the associated feeder must be sized up 125% of the 1200A.

There is no safety issue, no matter what the load may be, with 1260 amps of conductor on a 1200 amp OCPD.

 

[don_resqcapt19]

Let us keep in mind at a high level, the NEC is about safety. Even though as you say "You cannot put 1200A of continuous load on the 1200A MCC" you and I both know someone out there will do it - load it up to 1200A or more 'till the CB trips. That's why the associated feeder must be sized up 125% of the 1200A.

You have not told us how the 1200 amp bus rating of an MCC automatically becomes a continuous load for the purposes of sizing its feeder conductors and OCPD.

 

[d0nut]

This confusion is why I hate the "80% breaker" terminology. It gets people thinking about the load calculations backwards and making statements like "A 20A circuit breaker is only good for 16A". Once you do the load calculation, you select the equipment, overcurrent protection, conductors, etc. based on that calculated value. There is no further adjustment for "continuous", "non-continuous", "80%", or "100%" ratings to be made at this point. Those adjustments were already included in the calculated load.

In the example in this thread, consider 1200A of connected load. If the entire load was continuous, we would take 1200A*125% and end up with a calculated load of 1500A. Then we would select a 1600A MCC, 1600A circuit breaker, and 1600A of conductor. If the entire load was noncontinuous, we would take 1200A*100% and end up with a calculated load of 1200A. Then we could select a 1200A MCC, a 1200A circuit breaker, and 1200A of conductor. Finally, if the load was a mixture of continuous and noncontinuous loading, we would take the continuous load*125% + the noncontinuous load*100% and end up with a calculated load. We would then select a MCC that was => the calculated load, with the circuit breaker and conductor to match.

If we didn't like the results of that calculation, we could selected 100% rated equipment. We would have a 1200A MCC, 1200A circuit breaker, and 1200A of conductor in this scenario and comply with the requirements of the NEC.

 

[wwhitney]

215.2(A)(1)(a) - repeated several times in the above string.

Yeah, that's the not the reason for 215.2(A)(1)(a). See exception No 1: if you have a 100% rated breaker, you don't need the 125% factor. So the 125% factor has nothing to do with ensuring "a fire does not occur due to a slow arcing fault," which is something that could occur just as easily with a 100% rated breaker as a regular breaker.

As I understand it, the 125% factor for continuous loads is an expression of the mismatch between the shape of the trip curve of a thermal magnetic breaker (in the environment of a panel enclosure with multiple breakers) and the damage curve of conductors. Namely, when you scale the trip curve to protect the conductors properly on short time scales (seconds, minutes), you end up with a trip curve that is too sensitive at long time scales (hours). So you'd end up with a good chance of nuisance tripping if you load a regular breaker at its full rating with a continuous load.

To avoid this risk of nuisance tripping, you have to upsize the breaker to 125% of the continuous load. But now the wires need to be upsized so they are still properly protected by the breaker (on short time scales).

That's it, it's just a limitation of thermal magnetic breakers.

Cheers, Wayne

 

[Dale001289]

Yeah, that's the not the reason for 215.2(A)(1)(a). See exception No 1: if you have a 100% rated breaker, you don't need the 125% factor. So the 125% factor has nothing to do with ensuring "a fire does not occur due to a slow arcing fault," which is something that could occur just as easily with a 100% rated breaker as a regular breaker.

As I understand it, the 125% factor for continuous loads is an expression of the mismatch between the shape of the trip curve of a thermal magnetic breaker (in the environment of a panel enclosure with multiple breakers) and the damage curve of conductors. Namely, when you scale the trip curve to protect the conductors properly on short time scales (seconds, minutes), you end up with a trip curve that is too sensitive at long time scales (hours). So you'd end up with a good chance of nuisance tripping if you load a regular breaker at its full rating with a continuous load.

To avoid this risk of nuisance tripping, you have to upsize the breaker to 125% of the continuous load. But now the wires need to be upsized so they are still properly protected by the breaker (on short time scales).

That's it, it's just a limitation of thermal magnetic breakers.

Cheers, Wayne

The simplest thing to do is to use a 100% rated MCB. Then the problem goes away. The 125% rule only applies to the 80% rated CB which is the industry standard. The caveat is the 100% version is more expensive. But when you consider upsizing the feeder based on 1200A the additional cost is probably a wash. Sometimes the Code imposes overkill in the name of safety. This is one of those.


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[infinity]

This confusion is why I hate the "80% breaker" terminology. It gets people thinking about the load calculations backwards and making statements like "A 20A circuit breaker is only good for 16A". Once you do the load calculation, you select the equipment, overcurrent protection, conductors, etc. based on that calculated value. There is no further adjustment for "continuous", "non-continuous", "80%", or "100%" ratings to be made at this point. Those adjustments were already included in the calculated load.

In the example in this thread, consider 1200A of connected load. If the entire load was continuous, we would take 1200A*125% and end up with a calculated load of 1500A. Then we would select a 1600A MCC, 1600A circuit breaker, and 1600A of conductor. If the entire load was noncontinuous, we would take 1200A*100% and end up with a calculated load of 1200A. Then we could select a 1200A MCC, a 1200A circuit breaker, and 1200A of conductor. Finally, if the load was a mixture of continuous and noncontinuous loading, we would take the continuous load*125% + the noncontinuous load*100% and end up with a calculated load. We would then select a MCC that was => the calculated load, with the circuit breaker and conductor to match.

If we didn't like the results of that calculation, we could selected 100% rated equipment. We would have a 1200A MCC, 1200A circuit breaker, and 1200A of conductor in this scenario and comply with the requirements of the NEC.

These are the answers. Bottom line is that the 1260 amps worth of conductors are protected at 1200 amps and it is code compliant. No 125% or 80% factors or 100% continuous circuit breakers are required for this load.

 

[Isaiah]

This confusion is why I hate the "80% breaker" terminology. It gets people thinking about the load calculations backwards and making statements like "A 20A circuit breaker is only good for 16A". Once you do the load calculation, you select the equipment, overcurrent protection, conductors, etc. based on that calculated value. There is no further adjustment for "continuous", "non-continuous", "80%", or "100%" ratings to be made at this point. Those adjustments were already included in the calculated load.

In the example in this thread, consider 1200A of connected load. If the entire load was continuous, we would take 1200A*125% and end up with a calculated load of 1500A. Then we would select a 1600A MCC, 1600A circuit breaker, and 1600A of conductor. If the entire load was noncontinuous, we would take 1200A*100% and end up with a calculated load of 1200A. Then we could select a 1200A MCC, a 1200A circuit breaker, and 1200A of conductor. Finally, if the load was a mixture of continuous and noncontinuous loading, we would take the continuous load*125% + the noncontinuous load*100% and end up with a calculated load. We would then select a MCC that was => the calculated load, with the circuit breaker and conductor to match.

If we didn't like the results of that calculation, we could selected 100% rated equipment. We would have a 1200A MCC, 1200A circuit breaker, and 1200A of conductor in this scenario and comply with the requirements of the NEC.

A 20A CB rated 80% doesn't mean it will trip at 16A. Likewise a 1200A MCB rated 80% will not trip at 960A. During the design phase, MCC's are sized according to their projected loading, which at the time is basically a guesstimate since 15%-20% is normally allocated as 'spare'. Obviously at this point the Bus is not considered to be a 1200A continuous load. Over time however, possibly decades, this initial loading may change significantly. Thus, the 25% is added to the feeder size to ensure a margin of safety. This is the intent of NEC 215.2(A)(1).

 

[d0nut]

Over time however, possibly decades, this initial loading may change significantly. Thus, the 25% is added to the feeder size to ensure a margin of safety. This is the intent of NEC 215.2(A)(1).

None of this is correct. The NEC has nothing to do with what might happen in the future. If in the future I added more load to the electrical equipment than it was rated to supply, I would have to change the equipment and the feeder at that point. That is why we do load calculations, meter loads on panels, and the like. The intent of 215.2(A)(1) is to manage the excess heat generated by equipment operating at its maximum rating for more than three hours at a time as described in post #32.

 

[don_resqcapt19]

A 20A CB rated 80% doesn't mean it will trip at 16A. Likewise a 1200A MCB rated 80% will not trip at 960A. During the design phase, MCC's are sized according to their projected loading, which at the time is basically a guesstimate since 15%-20% is normally allocated as 'spare'. Obviously at this point the Bus is not considered to be a 1200A continuous load. Over time however, possibly decades, this initial loading may change significantly. Thus, the 25% is added to the feeder size to ensure a margin of safety. This is the intent of NEC 215.2(A)(1).

That is not the intent of that rule...that rule is just to match up with the OCPD so that the conductors have an ampacity equal to or greater than the rating of the OCPD. Even where the loading changes, unless you change the OCPD, 1260 amps of conductor will always be protected by the 1200 amp OCPD. If there is a significant overload, the OCPD will open the circuit.

90.1(B) Adequacy.
This Code contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance result in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.

 

[Dale001289]

That is not the intent of that rule...that rule is just to match up with the OCPD so that the conductors have an ampacity equal to or greater than the rating of the OCPD. Even where the loading changes, unless you change the OCPD, 1260 amps of conductor will always be protected by the 1200 amp OCPD. If there is a significant overload, the OCPD will open the circuit.

I’ve seen OCPD’s that don’t open at their prescribed ampacities. Sometimes they fail.


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[Isaiah]

None of this is correct. The NEC has nothing to do with what might happen in the future. If in the future I added more load to the electrical equipment than it was rated to supply, I would have to change the equipment and the feeder at that point. That is why we do load calculations, meter loads on panels, and the like. The intent of 215.2(A)(1) is to manage the excess heat generated by equipment operating at its maximum rating for more than three hours at a time as described in post #32.

Actually you are wrong. Many rules in the NEC have built-in margins of safety. This is just one of them.


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[don_resqcapt19]

I’ve seen OCPD’s that don’t open at their prescribed ampacities. Sometimes they fail.


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So you are telling me when the OCPD fails, it will only fail by 25% above its rating?