Q for the experts on AIC, SCCR, Fault Current Calculation,,,,,,

[dionysius]

Take a 480/277V 3 phase simple example as follows.......

I want to fully understand these calculations.

Stage 1: The AIC at the sec of utility trafo is 21,101 Amps.

Stage 2: Fault current at service equipment 30 ft away is 20,584 amps

Stage 3: Fault current at terminal of Panelboard 100 ft further away is 16,105 amps

Stage 4: Fault current at 3 phase branch outlet 100 ft further away is 2821 amps.

Bus bracing is rated 65 kA.

What are the minimum breaker SCCR ratings required at each stage?????

Consider Point-to-point and Series rated approach and explain when each is allowed......

 

[jim dungar]

Take a 480/277V 3 phase simple example as follows.......

I want to fully understand these calculations.

Stage 1: The AIC at the sec of utility trafo is 21,101 Amps.

Stage 2: Fault current at service equipment 30 ft away is 20,584 amps

Stage 3: Fault current at terminal of Panelboard 100 ft further away is 16,105 amps

Stage 4: Fault current at 3 phase branch outlet 100 ft further away is 2821 amps.

Bus bracing is rated 65 kA.

What are the minimum breaker SCCR ratings required at each stage?????

Consider Point-to-point and Series rated approach and explain when each is allowed......

The last sentence reads like a homework assignment.

1. The utility has available Short Circuit Amps - commonly abbreviated as SCA. AIC is an acronym for use only with overcurrent protective devices which interrupt fault current.
2. The breaker AIC rating simply needs to be greater than the fault current you have calculated at each stage. Technically you should also consider the X/R ratio of the circuit, but that step is often ignored in example questions
3. The use of series rated components is a design choice. There are several factors which must be considered. I commonly find series rated equipment employed in branch circuit applications where the feeder device is nominally 200A.

 

[ron]

The last sentence reads like a homework assignment.

1. The utility has available Short Circuit Amps - commonly abbreviated as SCA. AIC is an acronym for use only with overcurrent protective devices which interrupt fault current.
2. The breaker AIC rating simply needs to be greater than the fault current you have calculated at each stage. Technically you should also consider the X/R ratio of the circuit, but that step is often ignored in example questions
3. The use of series rated components is a design choice. There are several factors which must be considered. I commonly find series rated equipment employed in branch circuit applications where the feeder device is nominally 200A.

I agree with Jim's response. X/R multipliers are often ignored because it is only tracked when using software. The simple calculator doesn't report that info.

In addition, the individual calculated short circuit amperes you report is the point to point value, so just use equipment rated higher than that.

 

[Jraef]

And just a clarification. SCCR is in regard to equipment, what we used to call the "Withstand" rating. Breakers must also have an Interruting Capacity (IC) so although technically a breaker would be PART of an SCCR, you don't select them by that, you select them based on the IC (kAIC) rating.

 

[dionysius]

Take a 480/277V 3 phase simple example as follows.......

I want to fully understand these calculations.

Stage 1: The AIC at the sec of utility trafo is 21,101 Amps.

Stage 2: Fault current at service equipment 30 ft away is 20,584 amps

Stage 3: Fault current at terminal of Panelboard 100 ft further away is 16,105 amps

Stage 4: Fault current at 3 phase branch outlet 100 ft further away is 2821 amps.

Bus bracing is rated 65 kA.

What are the minimum breaker SCCR ratings required at each stage?????

Consider Point-to-point and Series rated approach and explain when each is allowed......

You folks are so good. Thank y'all.

So I should reword my question as follows using the correct terminology:

Stage 1: The SCA (Short Circuit Amps) at the sec of utility trafo is 21,101 Amps.

Stage 2: Fault current at service equipment 30 ft away is 20,584 amps

Stage 3: Fault current at terminal of Panelboard 100 ft further away is 16,105 amps

Stage 4: Fault current at 3 phase branch outlet 100 ft further away is 2821 amps.

Bus bracing is rated 65 kA.

What are the minimum breaker AIC (Available Interrupt Current) ratings required at each stage?????

And the answer is:

At stage 2: AIC > 20,584A (Main disconnect)
At stage 3: AIC > 16,105A (MBP Protector)
At stage 4: AIC > 2821A (Branch circuit breaker)

And for each breaker the SCCR is the maximum current it can "withstand" or survive without being shattered mechanically or electrically whilst the AIC is the maximum current that it can operate at normally and clear the fault reliably.

So then must the AIC always be < SCCR ??????? (I say yes)

And re the Series Rating......

If stage 2 were a series rated component (say a 400A breaker) then stage 3 AIC would be relaxed to 400A and stage 4 AIC would be a new point calculation so it would be <= 400A. The rationale here is because if a crowbar of the branch occurred the stage 2 series rated 400A breaker would reliably trip and clear the fault current.


Are all of the above correct????

 

[ron]

So then must the AIC always be < SCCR ??????? (I say yes)

The AIC and SCCR are similar phases and sometimes used interchangeably.

The SCCR for the equipment (AIC for equipment that breaks fault current and withstand for stuff that has to ... .withstand it like busway and cables) needs to be > than the calculated short circuit current.

Did the calculated values include any other generation such as large downstream motors that may contribute upstream during a fault (they act like generators until they spin down)

 

[Ingenieur]

The X/R s useful if you need more accurate computations than an infinite bus
say they give X/R = n and avail fault If

fault S = sqrt3 x Vll x If in VA
S = V^2 / |Z| or |Z| = V^2 / S
|Z| = sqrt( R^2 + X^2)
From above nX = R
substituting
|Z| = sqrt( nX^2 + X^2)
solve for X (Z was derived above)
then R = nX
now you have the source complex impedance (assumed lagging)
Z = R + jX
note arctan X/R is the phase angle
obviously cos of that ang is the pf

usually not required but sometimes may be useful

 

[jim dungar]

It might seem trivial, but the terminology really is important.
NEC 110.9 deals with the AIC of protective devices, while NEC 110.10 deals with the SCCR of equipment.

Series ratings typically only work for a series-combination of two devices. There are some ratings for 3 devices, but they usually involve multi-meter equipment.
The use of a series rated combination does not affect the amount of fault current that is calculated through out the system.