Special permission?

[tortuga]

You cannot put 1200A of continuous load on the 1200A MCC.

Interesting discussion but I wonder if 215.2(A)(1)(a) even applies to a Motor Control Center feeder?
I would use part V of 430 , 430.63 and part VIII of 430, in particular 430.94
to size the feeder
Just wanted to point that out.

EDIT I am not saying you would get a different answer, just that 430.62 -> 430.24 would be the code reference.

 

[Dale001289]

You do realize that this is the NEC section of the Forum?

Of course I’m just demonstrating what may seem like semantics to one person may change the entire meaning of a statement to another when talking NEC. It’s important to be accurate here


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[Dale001289]

Interesting discussion but I wonder if 215.2(A)(1)(a) even applies to a Motor Control Center feeder?
I would use part V of 430 , 430.63 and part VIII of 430, in particular 430.94
to size the feeder
Just wanted to point that out.

I think you’re right, but you have took at both. 430.94 allows the OCPD to be LESS than the bus rating. So, if the calculated load of the mcc is 1000A (or even 100A) it doesn’t matter what size the bus is (bus could be 3000A)as long as the MCB doesn’t exceed the calculated load value combination of continuous and non continuous loads as shown in 215.2


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[wwhitney]

You are missing the point of scenario (B), the load is 1000A continuous in all scenarios. There is no safety issue (overloading/overheating anything), other than the breaker tripping when we don't want it to, and whatever knock-on effects that could have.

Absolutely incorrect.

Please consider scenario (B) further, as you are still missing the point. Let me try putting it this way:

Say you had a normal breaker whose trip characteristics, within the allowable range permitted under the listing standard, along with the conditions of its installation (ambient temperature, spatial density of breakers in its enclosure) meant that it would, in fact, carry 100% of its rated current continuously. This certainly can and does happen. But the breaker is not listed as a 100% breaker, it has not been tested to perform this way, and under different conditions of installation it might not perform this way.

We agree that scenario (A) is allowed. Now substitute our hypothetical breaker for the 100% rated breaker (and enclosure), putting us in a version of scenario (B). What has changed, performance-wise? Absolutely nothing. What new safety issue or problem has been introduced? Absolutely none. If scenario (A) is OK in practice, then this version of scenario (B) is OK in practice.

As a code issue, of course, we can't distinguish between this version of scenario (B), and the version where the regular breaker will undesirably trip. That is the sole reason that scenario (B) is prohibited by the NEC.

According to this statement section 215.2(A)(1) has no reason to exist.

Actually, it is correct that 215.2(A)(1) has basically no reason to exist. Setting aside for the moment the issue of 240.4(B), 215.2(A)(1) is a logical consequence of 215.3 (which tells you how to size the OCPD) and 240.4 (which tells you that the conductors need to be protected by the OCPD). So 215.2(A)(1) is just there to spell out that logical consequence for the reader.

The one exception to the above is the use of 240.4(B), the "next size up rule". If we deleted 215.2(A)(1), then the following would be permitted (to use a common example with small conductors): a 48A continuous load, 55A ampacity conductors, and a 60A breaker. The conductors are sufficient for the load, and the breaker is sufficient for load, and the breaker protects the conductors per 240.4(B). [So to me this should be allowed.] However, because of 215.2(A)(1), this is not permitted, and 60A ampacity conductors are required.

Cheers, Wayne

 

[david luchini]

Of course I’m just demonstrating what may seem like semantics to one person may change the entire meaning of a statement to another when talking NEC. It’s important to be accurate here


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It's not semantics. The Code rules are either Mandatory or Permissive. Where the Code says SHALL NOT, it means it is specifically prohibited by the Code. And following the Code rules, you shall not put a 1200A continuous load on a 1200A MCC.

Nor, getting back to the OP, is there any rule in the NEC that specifically requires (SHALL) a 1500A feeder ampacity for a 1200A feeder circuit breaker, as the original poster is suggesting.

 

[Dale001289]

It's not semantics. The Code rules are either Mandatory or Permissive. Where the Code says SHALL NOT, it means it is specifically prohibited by the Code. And following the Code rules, you shall not put a 1200A continuous load on a 1200A MCC.

Nor, getting back to the OP, is there any rule in the NEC that specifically requires (SHALL) a 1500A feeder ampacity for a 1200A feeder circuit breaker, as the original poster is suggesting.

Putting 1500A worth of feeder on a 1200A Breaker isn’t a code violation. It just means you have a fat feeder.


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[wwhitney]

And following the Code rules, you shall not put a 1200A continuous load on a 1200A MCC.

Can you point out what rule that is? I would think a 1200A 100% rated breaker supplying 1200A conductors to a 1200A continuous load on a 1200A MCC would be allowed. [Not so familiar with MCCs, so a more familiar example to me would be, say, 400A and an MLO panelboard.]

Cheers, Wayne

 

[david luchini]

I think you’re right, but you have took at both. 430.94 allows the OCPD to be LESS than the bus rating. So, if the calculated load of the mcc is 1000A (or even 100A) it doesn’t matter what size the bus is (bus could be 3000A)as long as the MCB doesn’t exceed the calculated load value combination of continuous and non continuous loads as shown in 215.2

Nothing would prohibit the MCB from being larger than the calculated load value from 215.2. If the load is 1000A, the MCB can be 3000A on the 3000A bus mcc.

Putting 1500A worth of feeder on a 1200A Breaker isn’t a code violation. It just means you have a fat feeder.

Never suggested that it was a Code violation. The whole point is that it is NOT REQUIRED to have a conductor ampacity of 1500A on a 1200A feeder, as the OP suggested was required.

 

[david luchini]

Can you point out what rule that is? I would think a 1200A 100% rated breaker supplying 1200A conductors to a 1200A continuous load on a 1200A MCC would be allowed. [Not so familiar with MCCs, so a more familiar example to me would be, say, 400A and an MLO panelboard.]

Cheers, Wayne

This is why I hate when people go off on tangents. The OP's confusion is directly related to the fact that the c/b is NOT 100% rated.

The MCB is not rate 100% - its rated 80%

 

[Dale001289]

Nothing would prohibit the MCB from being larger than the calculated load value from 215.2. If the load is 1000A, the MCB can be 3000A on the 3000A bus mcc.


Never suggested that it was a Code violation. The whole point is that it is NOT REQUIRED to have a conductor ampacity of 1500A on a 1200A feeder, as the OP suggested was required.

I have no issues with the MCB being larger as you’ve stated BUT it can also be smaller according to 430.94


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[tortuga]

I have no issues with the MCB being larger as you’ve stated BUT it can also be smaller according to 430.94


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Right I dont think article 215 applies at all to a MCC, article 430 modifies other articles.
If you look at 430.62 wich sends us to 430.24, you could have some continuous motors not included in the continuous part of the calc.

 

[Dale001289]

Right I dont think article 215 applies at all to a MCC, article 430 modifies other articles.
If you look at 430.62 wich sends us to 430.24, you could have some continuous motors not included in the continuous part of the calc.

Interesting point and you’d be correct about 430.24 except MCCs usually supply more than just motor loads. What about lighting transformers, HVAC, heaters etc that are normally connected to a mcc?


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[tortuga]

Here is what is says about that
430.24 Several Motors or a Motor(s) and Other Load(s). Conductors supplying several motors, or a motor(s) and other load(s),
shall have an ampacity not less than the sum of each of the following:

1. 125 percent of the full-load current rating of the highest rated motor, as determined by 430.6(A)
2. Sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A)
3. 100 percent of the noncontinuous non-motor load
4. 125 percent of the continuous non-motor load.

 

[Dale001289]

Here is what is says about that
430.24 Several Motors or a Motor(s) and Other Load(s). Conductors supplying several motors, or a motor(s) and other load(s),
shall have an ampacity not less than the sum of each of the following:

1. 125 percent of the full-load current rating of the highest rated motor, as determined by 430.6(A)
2. Sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A)
3. 100 percent of the noncontinuous non-motor load
4. 125 percent of the continuous non-motor load.

I think you’re right on this one


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[tortuga]

They have this nice little diagram in the beginning of 430:

 

[Dale001289]

They have this nice little diagram in the beginning of 430:

View attachment 2558397

Right - But since we don’t know the exact loads involved we have to stick with assumptions. I believe this is why you still need 215.2


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[jap]

Right - But since we don’t know the exact loads involved we have to stick with assumptions. I believe this is why you still need 215.2


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With electricity and elements being in an ever changing state, no loads could ever be considered exact.

Therefore, everything we do is calculated on assumptions to a degree.

JAP>

 

[Dale001289]

With electricity and elements being in an ever changing state, no loads could ever be considered exact.

Therefore, everything we do is calculated on assumptions to a degree.

JAP>

True statement


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[tortuga]

Right we would need some real numbers to size this thing, or at least the motors
so for discussion I'll provide a fictional scenario , (with a historic twist of course), Say you are working on tunnels deep under the liberty bell, and we have a existing 5 wire 480V two phase MCC rated 1200A, with a 1200A feeder and fused at 1200A.
It powers 460V lift pumps to pump water out of the subway and the sub tunnels for an old portion of the city.
There are

• 13 existing 50 HP 4 wire two phase pumps,
• Two 75 HP 4 wire two phase pumps,
• when there is a major storm the pumps can easily run 3 hours or more.
• There is also 240V LPS tunnel lighting fed; say 2 amps per ballast, 30 lights, the lights run 24/7.

Due to bigger storm surges the city needs to add two new 50 HP pumps, and does not want to spend an extra nickel.
So now using these numbers show if the existing MCC can handle the existing load plus two new 50 HP two phase pumps?
What numbers do you all come up with?

 

[jap]

Right we would need some real numbers to size this thing, or at least the motors
so for discussion I'll provide a fictional scenario , (with a historic twist of course), Say you are working on tunnels deep under the liberty bell, and we have a existing 5 wire 480V two phase MCC rated 1200A, with a 1200A feeder and fused at 1200A.
It powers 460V lift pumps to pump water out of the subway and the sub tunnels for an old portion of the city.
There are

• 13 existing 50 HP 4 wire two phase pumps,
• Two 75 HP 4 wire two phase pumps,
• when there is a major storm the pumps can easily run 3 hours or more.
• There is also 240V LPS tunnel lighting fed; say 2 amps per ballast, 30 lights, the lights run 24/7.

Due to bigger storm surges the city needs to add two new 50 HP pumps, and does not want to spend an extra nickel.
So now using these numbers show if the existing MCC can handle the existing load plus two new 50 HP two phase pumps?
What numbers do you all come up with?

911

JAP>