I need help selecting the AIC rating of my equipment - commercial

[kgregg]

I am designing a restaurant that's within a large mixed use building. Our equipment is connecting to a retail service through that's fed from a main switchboard (MCS1). I have the base building drawings that include an arc fault riser diagram, but I am having trouble understanding how to best utilize it for selecting my equipment.

The main switchboard has an fault current of 150.4 kA. Our 400A CT cabinet, meter, and main disconnect will tie into the through coming off this switchboard. From there, it will be around 200 feet of cable to a 400A panel within out tenant space.

I know i need to size the AIC rating of my equipment to be equal or exceed the fault current available on the line side of the terminals. Should i assume that the available fault current at the terminals will be approximately 150 kA? or will there be significant losses along the way? I thought typically panels have AIC rating of 22k-65k.

Thank you for any feedback.

 

[don_resqcapt19]

There will be a substantial reduction in the available fault current at the end of a 200 foot run.
If you use 500 kcmil run in steel conduit for 200 feet, the available fault current will be less than 15kA at the end of the run per the Bussmann FC² free app.

 

[jim dungar]

Yes the cables/conductors might have a tremendous effect on available fault current.
The 200' run will definitely benefit, as Don said.
You metering and service equipment will probable need the full 150kA which sould be easy to achieve using fusible equipment with Class R fuse clips installed.

 

[Tainted]

There will be a substantial reduction in the available fault current at the end of a 200 foot run.
If you use 500 kcmil run in steel conduit for 200 feet, the available fault current will be less than 15kA at the end of the run per the Bussmann FC² free app.

Get a fully rated 200kAIC for the service equipment.

For the 200ft 400 amp circuit I'd like to add that motor contribution may slightly increase the fault current. So it may potentially have to be larger than 22kAIC, maybe 42kAIC worst case scenario. But again, you may be able to get away with 22kIAC

 

[jim dungar]

Get a fully rated 200kAIC for the service equipment.

For the 200ft 400 amp circuit I'd like to add that motor contribution may slightly increase the fault current. So it may potentially have to be larger than 22kAIC, maybe 42kAIC worst case scenario. But again, you may be able to get away with 22kIAC

Motor contribution is usually 6X FLA, which is one reason it is ignored below 50HP.
You would probably need about 250HP to get 7kA of fault current at 480V.

 

[Tainted]

Motor contribution is usually 6X FLA, which is one reason it is ignored below 50HP.
You would probably need about 250HP to get 7kA of fault current at 480V.

Why 480 though isn't this building 208?

Also I usually just add up all the motor horsepowers in the building and pretend the service is connected in parallel with 1 giant motor(from the total horsepowers). It makes the calcs easier not messy. I know it's not the right way to do it but it gives you the worst case

 

[jim dungar]

I know it's not the right way to do it but it gives you the worst case

Worst case is to assume infinite bus as the source, nothing needs to be added after that. 50HP is the IEEE ignore point regardless of voltage. Motors fed by VFDs do not contribute any fault current.

There is no reason not to overestimate available fault current, for SCCR/AIC selection, until it forces you into a higher level of equipment then you may want to refine your numbers including X/R.

 

[Tainted]

Worst case is to assume infinite bus as the source, nothing needs to be added after that. 50HP is the IEEE ignore point regardless of voltage. Motors fed by VFDs do not contribute any fault current.

There is no reason not to overestimate available fault current, for SCCR/AIC selection, until it forces you into a higher level of equipment then you may want to refine your numbers including X/R.

I know where you're coming from and I agree that motors less than 50hp are not really that significant in terms of contributing fault but I remember reading somewhere that this rule applies only for medium voltage if I'm not mistaken? I still do it anyway because why not? lol

 

[MyCleveland]

Worst case is to assume infinite bus as the source, nothing needs to be added after that. 50HP is the IEEE ignore point regardless of voltage. Motors fed by VFDs do not contribute any fault current.

There is no reason not to overestimate available fault current, for SCCR/AIC selection, until it forces you into a higher level of equipment then you may want to refine your numbers including X/R.

Jim
Following is from IEEE Violet Book : 2006

Section
"6.8 Details of induction motor contribution calculations according to
ANSI-approved standard application guides"

FIRST TWO PARAGRAPHS

For application of ac medium-voltage circuit breakers, symmetrical (ac component) short circuit
current duties are calculated according to IEEE Std C37.010-1999 [B4], using the reactance multiplying factors of Table 6-1, column 2.
The calculations omit all motors of less than 50 hp each.

For application of ac low-voltage power circuit breakers and both medium and low voltage
fuses, only first cycle calculations are necessary, and IEEE Std C37.13-1990 [B5],
IEEE Std C37.41-2000 [B6], and ANSI/IEEE C97.1-1972 [B1] recommend representing
all rotating machines in the equivalent circuit based on subtransient reactances regardless
of motor rated horsepower. IEEE Std C37.13-1990 [B5] qualifies this by suggesting that
motor short-circuit current contributions, for “typical” groups of low-voltage motors
lacking detailed information, may be estimated at four times the summation of motor rated
currents. A contribution of four times rated current corresponds to a first cycle motor Z =
(V/4) = 0.25 per-unit based on motor rated apparent power (kVA) and voltage of 1 p.u.
Usually exact motor short-circuit reactances are not readily available and is approximated
by using 1/(locked rotor current).
-----------------------------------------------------------------------------------------------------------------------------------------------
I have adopted these recommendations.

 

[jim dungar]

For application of ac low-voltage power circuit breakers and both medium and low voltage
fuses, only first cycle calculations are necessary, and IEEE Std C37.13-1990 [B5],
IEEE Std C37.41-2000 [B6], and ANSI/IEEE C97.1-1972 [B1] recommend representing
all rotating machines in the equivalent circuit based on subtransient reactances

Aren't most systems using molded case circuit breakers instead of "low-voltage power circuit breakers.

Yes, 4X (my 6X is overly conservative) motor loading is used with low voltage, non-VFD controlled, motors. So 1kA of fault current contribution from motor load would require there to be 250A of motors smaller than 50HP, which is unlikely for the 400A panel in the OP.

 

[kgregg]

There will be a substantial reduction in the available fault current at the end of a 200 foot run.
If you use 500 kcmil run in steel conduit for 200 feet, the available fault current will be less than 15kA at the end of the run per the Bussmann FC² free app.

thank you for the response and the app mention!

 

[ron]

Aren't most systems using molded case circuit breakers instead of "low-voltage power circuit breakers.

Yes, 4X (my 6X is overly conservative) motor loading is used with low voltage, non-VFD controlled, motors. So 1kA of fault current contribution from motor load would require there to be 250A of motors smaller than 50HP, which is unlikely for the 400A panel in the OP.

Are there many across the line motor starters used anymore with VFDs becoming more and more cost effective? Even if we have an application that only benefits from on/off function, we often put a vfd to function as a soft starter. If there is no bypass on the VFD, then the short circuit contribution can be ignored, as the solid state nature of the vfd will not allow it to flow upstream.

 

[MyCleveland]

Aren't most systems using molded case circuit breakers instead of "low-voltage power circuit breakers.

Yes, 4X (my 6X is overly conservative) motor loading is used with low voltage, non-VFD controlled, motors. So 1kA of fault current contribution from motor load would require there to be 250A of motors smaller than 50HP, which is unlikely for the 400A panel in the OP.

I was just trying to add the IEEE comments on the topic.
I believe they meant low voltage power "SYSTEM" circuit breakers.

I was not questioning the 6x motor fla multiplier.
I am using 5.5 + adding in a small R value for the model even though IEEE states it can be ignored.
If a computer is doing the math, why overly simplify.

 

[jim dungar]

believe they meant low voltage power "SYSTEM" circuit breakers.

No.
Low Voltage Power Circuit Breakers are completely different than Molded Case Circuit Breakers. Two different UL standards; LVPCBs are not required to have instantaneous trip function while MCCB must. These two different breakers, and their associated structures, have different performance requirements for first cycle and interrupting faults.

 

[MyCleveland]

No.
Low Voltage Power Circuit Breakers are completely different than Molded Case Circuit Breakers. Two different UL standards; LVPCBs are not required to have instantaneous trip function while MCCB must. These two different breakers, and their associated structures, have different performance requirements for first cycle and interrupting faults.

Then the entire IEEE commentary is nonsensical.
Why account for ALL motors only if served via a LVPCB or fuse...and exclude motors served via MCCBs.

 

[electrofelon]

I am designing a restaurant that's within a large mixed use building. Our equipment is connecting to a retail service through that's fed from a main switchboard (MCS1). I have the base building drawings that include an arc fault riser diagram, but I am having trouble understanding how to best utilize it for selecting my equipment.

The main switchboard has an fault current of 150.4 kA. Our 400A CT cabinet, meter, and main disconnect will tie into the through coming off this switchboard. From there, it will be around 200 feet of cable to a 400A panel within out tenant space.

I know i need to size the AIC rating of my equipment to be equal or exceed the fault current available on the line side of the terminals. Should i assume that the available fault current at the terminals will be approximately 150 kA? or will there be significant losses along the way? I thought typically panels have AIC rating of 22k-65k.

Thank you for any feedback.

Remember that you could (potentially) use series ratings instead of fully rated, although it sounds like you dont "need" to. For convenience and cost, perhaps using a fully rated 400A breaker that series rates to 10k branches would be a good approach. (Although Keep in mind some manufacturers don't have a lot of series rated options from a 400A frame down to a 10k branch).