Short circuit ratings for industrial machinery

[JOSEPH R. BERTRAND]

I am employeed by a machinery manufacturer.
Per NEC 409.110, the control panel on our machinery needs to include a tag with specific information, including line (3) "Short circuit rating of the industrial control panel". "Exception to line (3): Short circuit current rating markings are not required for industrial control panels containing only control circuit components".
The panel on our machinery typically consists of a fused disconnect switch, control circuit transformer or power supply, VFD and motor, safety relay, PLC or relays. The machinery operates on 208 to 600 VAC, both single and 3-phase, with controls of either 120 VAC or 24 VDC. ARTICLE 100 Definitions, does not include "control circuit components". Does the exception to line (3) apply to my type of panels?
Also, NFPA 79, 16.4.1(6) requires a short circuit rating without an exception.
Is it possible for me to calculate an acceptable rating if I do not know the customers electrical supply specifics, to meet the NEC and NFPA requirements? If so any suggestions?

 

[jcole]

I would think that the short circuit rating (aic) of the breaker or fuse that protects the branch circuit that supplies power to the your panel would meet this requirement.

Or, if you have a breaker or fuse in you disconnect the short circuit rating (aic) of those would meet the requirement.

Just an educated guess though.

 

[pfalcon]

If you have any of the items listed in 409.2 Industrial Control Panel (1) then 409.110 Marking (3) Exception to (3): does not apply. You must mark.

Disclaimer: NFPA79 is not a legal requirement unless your company has formally adopted the specification for doing business.

Thanks to crossing specs NFPA79:2007 added a blanket requirement while NFPA70:2008 granted an exception. If your company has adopted NFPA79:2007 as a required specification then you must mark.

How do you build a machine for someone without knowing their supply? Call the client.

 

[jim dungar]

I would think that the short circuit rating (aic) of the breaker or fuse that protects the branch circuit that supplies power to the your panel would meet this requirement.

Or, if you have a breaker or fuse in you disconnect the short circuit rating (aic) of those would meet the requirement.

Just an educated guess though.

You can almost never use the AIC rating of a protective device to determine the SCCR (short circuit current rating) of a panel as required by 409.2. You probably have several devices in the panel which means you are dealing with a 'series-combination' rating. the best action would be to label the panel equal to the lowest rated device in the panel or perform calculations similar to those in UL 508A.

 

[jcole]

Care to explain the difference between AIC and SCCR? I understand that AIC is the available fault current at line terminals.

What is SCCR and how is it determined?

 

[jim dungar]

Care to explain the difference between AIC and SCCR? I understand that AIC is the available fault current at line terminals.

What is SCCR and how is it determined?

The term AIC is often mis-used.

AIC = Amps Interrupting Capacity - applicable only to protective devices that operate to interrupt fault current. These items are subject to NEC 110.9.

SCCR = Short Circuit Current Rating, also called WCR (Withstand Current Rating) - applicable to equipment that does not interrupt fault (i.e. panel busing). These items are subject to NEC 110.10.

SCA = Short Circuit Amps, or Short Circuit Available - this is the amount of fault current available at the location under consideration (i.e. at the line terminals).

 

[jcole]

The term AIC is often mis-used.

AIC = Amps Interrupting Capacity - applicable only to protective devices that operate to interrupt fault current. These items are subject to NEC 110.9.

SCCR = Short Circuit Current Rating, also called WCR (Withstand Current Rating) - applicable to equipment that does not interrupt fault (i.e. panel busing). These items are subject to NEC 110.10.

SCA = Short Circuit Amps, or Short Circuit Available - this is the amount of fault current available at the location under consideration (i.e. at the line terminals).

Thanks. I learned something today.