Kaic Rating gear and subpanels

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harmitage

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Hi everyone we are building a hotel 95 guess suites 5 floors. I?m in a debate between SQD and the engineer on the job about The KAIC Rating for the panels.
We have a 3000amp 3-phase 120/208v MDP . That is 90 feet from the power companies transformer which is IC of 61,613amps @2.25% Z for 500kva. We believe that 65,000aic is good for MDP.
There are 3- 400amp 3phase 120/208v distribution panels in the same room as the MDP. We believe 42,000aic is good for those panels they are no more than 15 feet away from the MDP.
There are 2 ? 400amp 120/208 3phase panels per floor a total of 5 floors. The closest one is about 30 feet away from the MDP and the longest one is about 320 feet away from the MDP. We believe that 22,000aic series rate is good for these panels.
There are 95 homeline 125amp MLO single phase panels for each suite that come off the 400amp panels per floor. They range from 25 feet away from the 400amp panels to about 100 feet away. We believe 10,000aic series rated are good for these panels.
Does anyone have any input on this, or understand how the calculation or code reads on this?
 
The 61 ka is the infinite bus fault current. I made some assumptions that the utility is 13.2 kv, max primary fault is 8000 amps, secondary is 6 500 kcm per phase, then the max secondary fault at the transformer is 55 ka and 46 ka at the 3000 amps mdp. If you can get the actual figures, it may change you
situation considerably.
 
Have you done any type of calculations or is this just your
harmitage said:
feel
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I just did a quick calculation for someone yesterday, 40' of 400A conductors only dropped the fault current by roughly 11kA.
 
If I'm not mistaken, you can only use lower rated systems down stream if they are "series rated" with each other. In other words, If your MDP is listed with a let-through that is lower than the AFC, then the down stream panels with lower AIC ratings can be listed by the mfr to be used behind it.

But I don't know if you can do that on your own without recalculating the AFC at each point, basically adding only the impedance of the additional cable distance to mitigate the fault current levels. So when you say "we believe..." is that based on a calculation? If it is, you had better be prepared to show your work! I have even seen AHJs insist that the calcs be signed by a PE as well.

If on the other hand, the Sq. D panels you have chosen have series ratings with each other, then it should be a no-brainer. Sq. D should be able to tell you.
 
I made a typo. Should be 8 500 kcm and not 6. Poster said the secondary was 90 ft.
 
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From an inspectors standpoint, "I believe" doesn't get you very far.
Calculations (or POCO documentation) showing no more than 65 k at the main and documentation that the entire system is SERIES RATED by the manufacturer is normally what we see.
 
to be honest, a lot of that is above me, but I would change from Homeline to QO. The cost is more but better quality. If for some reason, I don't see it on the floors, but if you need by example a shunt trip breaker, SQ D doesn't make one for Homeline. Just my 1-1/2 cents worth
 
kaic rating

kaic rating

The problem is the KAIC rating of the system is not my strong point I'm trying to understand more about it so i can talk with SQD and the PE to see who is right and who is wrong. SQD says that it can be done like i wrote the PE said he wants the MDP at 65,000 and everything else at 42,000aic.

Keep in mind this is the same PE that wanted a 5000amp 3-phase service for this building. I went around and around with him and my calculations till he finally said that my calculations were acceptable for a 3000amp. The KAIC Rating is the next hump to ordering the gear an it is not my strong point.
 
harmitage said:
The problem is the KAIC rating of the system is not my strong point I'm trying to understand more about it so i can talk with SQD and the PE to see who is right and who is wrong. SQD says that it can be done like i wrote the PE said he wants the MDP at 65,000 and everything else at 42,000aic.

Keep in mind this is the same PE that wanted a 5000amp 3-phase service for this building. I went around and around with him and my calculations till he finally said that my calculations were acceptable for a 3000amp. The KAIC Rating is the next hump to ordering the gear an it is not my strong point.
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You are concerned with the system available Short Circuit Amps (SCA), based on these amps you select the proper short circuit current rating (SCCR) of your equipment and the proper Amps Interrupting Capacity (AIC) of your breakers. this may sound like nit-picking but the words really do mean something. Technically specifing an SCCR of 65K should let me use what ever method I wanted in order to attain that level (i.e. series ratings). Specifing just 65K AIC means I need to supply devices with that level regardless of the actual SCA.

Somone needs to determine what SCCR (or typically the AIC of breakers) they want, or need, at the different points in the system. It looks like the engineer has done that by specifiying 65K and 42K devices. There is no right or wrong just like in sizing the service, however, there often are costs for oversizing equipment.

Have you offered to submit a formal Short Circuit Study to prove your equipment SCCR choices are adequate for the project requirements?
 
The engineer should really have a study in his hand as a basis of the design. Based on my hand calculation, based on assumption that there is no motor contribution, I have about 48 kAIC at MDP and 41 kAIC for the (3) 400A panels. My opinion is 42 kAIC for the three panels is too close and I would rated them for 65 KAIC. For the other subpanels, conductor size and conduit type will be required for calculation. As you know, the longer the cable, the less SCR. So, if you hire someone to perform this short circuit calculation, it might end up save you money for the cost of panels. Again, the engineer should be able to do this.

I think that I understand what series rating means, but no one can really explain how it can be applied. I way I look at it, if you have fault downstream and the branch breaker cannot safety clear the fault, then main circuit breaker would open and take all of the system down. Why risk it?
 
kroken said:
. . . My opinion is 42 kAIC for the three panels is too close and I would rated them for 65 KAIC. . . I think that I understand what series rating means, but no one can really explain how it can be applied. I way I look at it, if you have fault downstream and the branch breaker cannot safety clear the fault, then main circuit breaker would open and take all of the system down. . .
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Welcome to the forum. If the starting point is the maximum available fault current from the electric utility, the number is normally based on a bolted fault and an infinite bus. Neither of these numbers are practical so the maximum will never be reached. Therefore, sizing the panelboards to the edge is not really too close.

You are correct on your view of listed series combinations of overcurrent protection. If you have elevators, you are not permitted to use series ratings because you will lose coordination under the right faults conditions. :smile:
 
My understanding is that series rating only applies when the available fault current at a device is greater than the AIC rating of that device, and protection is being provided in combination with some other device.

If each of the subpanels has an AIC rating for the available fault current _at that panel_, then the system is 'fully rated'. Since the fault current is reduced by the impedance of the feeder conductors to the panel, the AIC rating of the subpanel may be less than that of the main, yet still be fully rated for the available fault current.

-Jon
 
charlie said:
If you have elevators, you are not permitted to use series ratings because you will lose coordination under the right faults conditions.
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I know of no general prohibition of series ratings on elevators.
Elevators on a common bus must be selectively coordinated. Series ratings do not necessarily prevent devices from being selectively coordinated, and fully rated breakers do not ensure coordination.

That said, I usually try to full rated my power panels and allow series ratings for my 100A and 200A lighting and receptacle panels. My experience is that few people look at the required AIC ratings of 1-pole breakers before adding them to these small panels, so I like them to be able to use standard 'on the truck' breakers.
 
jim dungar said:
I know of no general prohibition of series ratings on elevators. . .
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The brown text is from the 2008 Edition of the NFPA's NEC Handbook. It seems to me that "overcurrent protective devices" is plural and "selectively coordinated with any other supply side overcurrent protective devices" is also plural. That would prohibit and series coordination in front of a bank of elevators. :smile:

620.62 Selective Coordination. Where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means shall be selectively coordinated with any other supply side overcurrent protective devices.

Coordination of the overcurrent protective devices is important. For example, if a building contains three elevators and a fault occurs in the circuit conductors to one of the elevators, only the overcurrent device ahead of that faulted circuit should open. Coordination leaves the remaining two elevators in operation. This is especially important because elevators are commonly used to carry fire fighters and equipment closer to the fire during fire-fighting operations.

Where the overcurrent devices in the elevator room do not have proper coordination with the upstream feeder overcurrent device, the potential for interruption of power to all three elevators is increased.

For selective coordination of overcurrent protective devices, the manufacturer's time?current curves, let-through and withstand capacity data, and unlatching times data must be used for sizing or setting overcurrent devices.
 
charlie said:
For selective coordination of overcurrent protective devices, the manufacturer's time?current curves, let-through and withstand capacity data, and unlatching times data must be used for sizing or setting overcurrent devices.
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I do not see where the commentary is saying that series ratings cannot be used. The commentary is saying they must be selectively coordinated.
 
I don't understand

I don't understand

jim dungar said:
I do not see where the commentary is saying that series ratings cannot be used. The commentary is saying they must be selectively coordinated.
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I guess I don't understand what selective coordination is then. It seems to me that if your system is series rates, it really has the amount of fault current that has bee calculated, and a fault occurs, you would have the series rated combination of circuit breakers opening at the same time. How is that selectively coordinated? :-?

It seems to me that "overcurrent protective devices" is plural and "selectively coordinated with any other supply side overcurrent protective devices" is also plural. That would prohibit a series coordination in front of a bank of elevators. :smile:
 
charlie said:
I guess I don't understand what selective coordination is then. It seems to me that if your system is series rates, it really has the amount of fault current that has bee calculated, and a fault occurs, you would have the series rated combination of circuit breakers opening at the same time. How is that selectively coordinated?
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I have no doubt you understand selective coordination. But, I believe you do not understand what series ratings means. :-?
There is nothing in a series rating that requires the protective devices to open at the same time.
I would guess that relatively few series rated combinations are also selectively coordinated, but never-the-less some can be.

Assume the standard UL test levels for breakers are 10kAIC and 22kAIC. If I make a breaker that is capable of interrupting 18kSCA, the maximum UL rating I can give it is only 10K. But if I put a selectively coordinated 65kAIC device in front of my breaker I can series rate the combination at 18k, because series ratings do not have to match UL standard values.
 
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