Motor Control Center Circuit Breakers with Current Limiters

[CircuitSolver]

Is it common practice to use current limiters with circuit breakers to limit short circuit current in power circuits for smaller loads fed from Motor Control Centers (MCCs)? I am finding that when sizing cables for available short-circuit currents per ICEA P-32-382-2007 that I have to upsize cables frequently for smaller loads like Motor Operated Valves (MOVs) and smaller pumps. I understand that ICEA P-32-382-2007 is generally conservative to account for faults at any point in a circuit, and this leads to the results that I am getting. Is there any additional guidance that would allow me to justify a smaller cable size for this application?

 

[CircuitSolver]

I should add that I have been investigating using current limiters with my MCC breakers. This seems like it will be costly. I don't know that it is necessary. I would like to find guidance that allows me to forego using the current limiters with a smaller wire size.

 

[jraef]

Assuming you are in the US, is this related to you wanting to use wire gauges smaller than #14 AWG? Because if not, then you don't really need to do this, the NEC sizing and protection requirements have this issue covered already, but you are just limited to using 14ga in the field as the minimum size.

The only time I use current limiters or CL breakers in MCCs is if the Available Fault Current ahead of the MCC is greater than 65kA. My experience is that this is exceedingly rare.

"Will work for (the memory of) salami"

 

[dpc]

I've never used a molded-case breaker with current limiters in a Motor Control Center. I'm not seeing how using a smaller wire size would have any impact on the required breaker interrupting ratings. The required SC rating is based on the available fault current, not the current at the end of the feeder. 65 kA ratings are readily available these days. Fault current exceeding 65 kA at the MCC level is pretty unusual.

 

[CircuitSolver]

The work is in Canada, and should follow the Ontario Electric Safety Code. For this application, I would like to use a #10 AWG wire for an MOV. My available fault current is roughly 50 kA at the MCC bus feeding the MOV through a MCCB. The available fault current appears to be too high for the wire size per ICEA P-32-382-2007. I would have to use a #2 AWG wire per ICEA P-32-382-2007 to limit the conductor temperature to less than 250 deg. C for the fault. I am trying to find some guidance that would allow me to use the #10 AWG wire, which is suitable based on ampacity and voltage drop considerations (but not short-circuit).

 

[jraef]

Hmmm... I have never seen an AHJ requiring wire sizing compliance to that standard, the generally accted wire sizing would be based on the CEC (in your case) and 50kA is nothing particularly unusual. Is this something you are being asked to do in ADDITION to normally accepted code requirements?


"Will work for (the memory of) salami"

 

[wroggent]

I can't speak to Canadian codes, but in the National Electric Code (US code) I believe such a calculation for circuits under 600V is only (optionally) required when overcurrent protection is not provided in the otherwise normally required fashion on feeder taps or secondaries of transformers (240.92) in industrial installations. I've seen several situations in a typical installation that seemed to be unprotected based on ICEA P-32-382-2007 but the code didn't require the conductors to be protected in that regard due the conditions of the installation (not being on a transformer secondary, etc.). In those cases I was using the available fault current at the MCC bus; however, by instead using the fault current at the end of the branch I was able to reduce to amount of wire size increases that needed to be done. Also, the interrupting time (usually) won't increase if you're using instantaneous only breakers. I don't know if what I did was right but I felt better about it than doing nothing.

Check if the calculation is really required by the code.
Check if you're using the correct fault current (i.e., at the correct location).

 

[rcw retired EE]

Using current limiting fuses on MCC circuit breakers is common in Canada, but not for protecting cables. With the 600V system voltage, not 480V, some breakers are derated from 65kA to 50kA and need current limiters when the MCC is fed by a larger transformer ( >2.5 MVA) with motor contribution.

But current limiting is not needed to protect the cable if the maximum short circuit at the end of the wire run is within the wire's short circuit withstand for the breaker's tripping time. If the fault is in the middle of the wire run, the wire is already damaged and needs to be replaced anyway.

Also, it doesn't take very much wire length to drop the short circuit to acceptable levels. The first 10 meters of wire may be the only portion that is not protected against melting during a short circuit. Any fault beyond that is usually within the wire's capability.