AIC Rating

djd

Member
I am curious , mainly because it is usually done ie on plans ; what really decides the aic rating on a panel ? My situation is a medical office where there are three panels all being relocated, all being fed from main 1600 amp 120-208 , and remaining in there own individual suite. The plans call for panels with 10k aic rating they all have an separate fused disconnect from the gear. This I think is ok or is it? 2 100 amp and one 200 amp panels. My guess is the voltage would decide ?
 

topgone

Senior Member
I am curious , mainly because it is usually done ie on plans ; what really decides the aic rating on a panel ? My situation is a medical office where there are three panels all being relocated, all being fed from main 1600 amp 120-208 , and remaining in there own individual suite. The plans call for panels with 10k aic rating they all have an separate fused disconnect from the gear. This I think is ok or is it? 2 100 amp and one 200 amp panels. My guess is the voltage would decide ?
Trace where your power is sourced from. The transformer feeding your load tells a lot of how large your AIc will be. Knowing you have a 1600 amp main, I guess your design amp is around 1,200A; probably a 500kVA transformer! The typical fault amps for this size of transformer is around 25kA (at transformer terminals). If you are served with a rather long service drop, this 25kA could drop accordingly. The reason why the design engineer has chosen 10kA could be due to the length of the conductors from the source transformer.
 

fmtjfw

Senior Member
Trace where your power is sourced from. The transformer feeding your load tells a lot of how large your AIc will be. Knowing you have a 1600 amp main, I guess your design amp is around 1,200A; probably a 500kVA transformer! The typical fault amps for this size of transformer is around 25kA (at transformer terminals). If you are served with a rather long service drop, this 25kA could drop accordingly. The reason why the design engineer has chosen 10kA could be due to the length of the conductors from the source transformer.
Or because 10kA breakers are cheaper than beefier ones.
 

dionysius

Senior Member
I am curious , mainly because it is usually done ie on plans ; what really decides the aic rating on a panel ? My situation is a medical office where there are three panels all being relocated, all being fed from main 1600 amp 120-208 , and remaining in there own individual suite. The plans call for panels with 10k aic rating they all have an separate fused disconnect from the gear. This I think is ok or is it? 2 100 amp and one 200 amp panels. My guess is the voltage would decide ?
Your q is: "what really decides the aic rating on a panel ?"

You really mean what decides the SCCR on a panel.

The SCCR on a panel must be greater than the AIC available at the panel.
The AIC available at the panel is derived from the AIC at point of service and recomputed from each item it passes downstream until it arrives at the panel in question.
 

cpinetree

Senior Member
I am curious , mainly because it is usually done ie on plans ; what really decides the aic rating on a panel ? My situation is a medical office where there are three panels all being relocated, all being fed from main 1600 amp 120-208 , and remaining in there own individual suite. The plans call for panels with 10k aic rating they all have an separate fused disconnect from the gear. This I think is ok or is it? 2 100 amp and one 200 amp panels. My guess is the voltage would decide ?
The disconnects feeding the panels have fuses, so the AIC rating of the panels can be much lower, as long as the fuses spec'd can handle the AIC incoming.
 

jim dungar

Moderator
Staff member
The SCCR on a panel must be greater than the AIC available at the panel.
The AIC available at the panel is derived from the AIC at point of service and recomputed from each item it passes downstream until it arrives at the panel in question.
This is wrong.

AIC stands for Amps Interrupting Capacity, and it is applicable only to overcurrent protective devices, like fues and breakers. AIC can only be determined by the manufacturer of the device, through actual testing.
NEC 110.09 requires that a protective device AIC rating be greater than the available Short Circuit Amps (SCA).

SCCR standards for Short Circuit Current Rating, and is applied to components, like bussing and control panels, that carry fault current while waiting for the protective device to operate. SCCR can be calculated.
NEC 110.10 requires that equipment SCCR be greater than the available Short Circuit Amps (SCA).

SCA is a common abbreviation for Short Circuit Amps. At the service entrance the utility usually provides this value.
 

charlie b

Moderator
Staff member
Jim, let me ask for one clarification, and then let me offer another acronym.

You say that SCCR can be calculated. Do you mean by the manufacturer of the panel? I cannot, as a design engineer, calculate the ability of a piece of equipment to withstand a high value of current. Are we speaking different languages?

What you are calling SCA, I would instead call SCCA (Short Circuit Current Available). That is something that I would calculate. Its value depends on the utility's equipment, the sizes and lengths of any feeders between the utility and the point on the distribution system at which I am performing the calculation, and any contributions from motors installed in the facility. Are we speaking the same language, but using different words, in this case? I think we are.
 

Smart $

Esteemed Member
...
SCA is a common abbreviation for Short Circuit Amps. ...
...What you are calling SCA, I would instead call SCCA (Short Circuit Current Available). ...
Whatever you all (or, because I'm semi-close to Pittsburgh, "yinz") decide to call it, Code puts it under 110.24 Available Fault Current.

AIC falls under 110.9 Interrupting Rating.

SCCR falls under 110.10 Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics.
 

jim dungar

Moderator
Staff member
You say that SCCR can be calculated. Do you mean by the manufacturer of the panel? I cannot, as a design engineer, calculate the ability of a piece of equipment to withstand a high value of current.
SCCR can be determined by designer/engineers. You seem to be focusing on Panelboards and molded case breakers. Probably the most common SCCR field calculation is for control panels. Bus bar manufacturers can provide information on support and bracing if you wanted to evaluate older equipment. Fuse manufacturers are also sources for information on determining SCCR for different types of equipment.
 

jim dungar

Moderator
Staff member
Whatever you all (or, because I'm semi-close to Pittsburgh, "yinz") decide to call it, Code puts it under 110.24 Available Fault Current.

AIC falls under 110.9 Interrupting Rating.

SCCR falls under 110.10 Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics.
Yep.
The point is there are three different terms for three different, but related, items.
Our industry's penchant for reducing everything to 'one word', simply leads to confusion and often misdirection.
 

Carultch

Senior Member
I am curious , mainly because it is usually done ie on plans ; what really decides the aic rating on a panel ? My situation is a medical office where there are three panels all being relocated, all being fed from main 1600 amp 120-208 , and remaining in there own individual suite. The plans call for panels with 10k aic rating they all have an separate fused disconnect from the gear. This I think is ok or is it? 2 100 amp and one 200 amp panels. My guess is the voltage would decide ?
The concept is fault current, or short circuit current. The units are kA, as in kilo-amps. The equipment is either rated in terms of KAIC or SCCR. In any case, any piece of equipment needs to have a KAIC or SCCR rating that exceeds the fault current present at that point.

There is a fault current calculator available on the parent website to this forum, which is a spreadsheet where you can input the KVA and impedance spec's of the transformer, and the voltage configuration. Then you specify feeder-by-feeder, and track a point-to-point method of calculating fault current at your main panel, and up to two levels of subpaneling.

Typical trends in the calculation:
KVA of the transformer increases it
Impedance of the transformer decreases it
Greater voltage means less fault current
Greater feeder length decreases fault current from what is available at its origin
Feeders in metal raceways decrease it slightly from their plastic raceway counterparts


One way to get a lower KAIC or SCCR rating on one component, is to use it in a listed series combination with a higher rated component. Typically Class J fuses are series rated for most 10 kaic breakers.
 

djd

Member
djd

djd

Jim, let me ask for one clarification, and then let me offer another acronym.

You say that SCCR can be calculated. Do you mean by the manufacturer of the panel? I cannot, as a design engineer, calculate the ability of a piece of equipment to withstand a high value of current. Are we speaking different languages?

What you are calling SCA, I would instead call SCCA (Short Circuit Current Available). That is something that I would calculate. Its value depends on the utility's equipment, the sizes and lengths of any feeders between the utility and the point on the distribution system at which I am performing the calculation, and any contributions from motors installed in the facility. Are we speaking the same language, but using different words, in this case? I think we are.
Thank you , You answered my question and clarified my general knowledge of panel ratings , i have never read an sccr on a panelboard.
 
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