Cable Sizing for 100% UL MCCB's for Diesel Generator Application

And this is why this site is one of the most important sites that exists today.

Not only to follow NEC, NFPA, but also UL. And when to follow which!

Sir,

JRaef has it right. I will add that the enclosure size rules also are to allow adequate space for the bending radius of the conductors.

The reason for the 75°C column limit for 90°C rated cable is because the breakers are tested and rated for proper operation with their terminals not getting any hotter than the 75°C limit in an ambient temperature not exceeding 40°C. If your breaker is in a higher ambient location, then additional derating of the cable size will be needed. Cables with 90°C insulation are often desirable for use because they allow a higher current or allow for temperature or conduit fill derating. However, in your case, you have to use the 75°C table because of the limit on temperature at the termination.

--JMM

Thank you for you reply. The information you provided is generally what I have been finding there is actually an additional requirement that may change between breaker manufacturers and that is they list them with specific cable lug terminals also and therefore we must use them to avoid re-cert.

With these 100% rated CB's we are required to follow ABB's listing instructions (cable sizing to 75ºC, specfic box sizes, bend radii of cabling and terminal lugs) according to ABB's UL listing to UL489. If we deviate we must have a review with UL and that is pretty clear. What I am trying to gather is a level of confidence before testing (as this is probably going to have to happen anyway). I am trying to gauge what table would be most applicable to my application and i've found some information that I would like to get someone else's opinion on.

I have the NEC2011 with me and i've noted somethin that i've found. There is an information note:

"Adjustment factors shall not apply to conductors in raceways having a length not exceeding 600mm (24in.)."

The raceway i am installing the cables through (from the alt to the breaker) is less than 600mm. This would say to me that bunching factors are no longer required the same as ambient de-rates are no longer required.

The question I am facing at the moment is are the bunching factors and the de-rates as per tables 16 & 17 still applicable considering the raceway is less than 600mm? In which case I could use current values for cabling the breaker based on single conductors in free air table 310.15(B)(17) instead of using table 310.15(B)(16).

Your thoughts?

Best regards.

Sir,

Regarding de-rating, I believe the only one you can ignore is the conduit/raceway fill. All these code considerations for wires and cables are based on the need to keep the conductor insulation temperature within safe limits. Free air has easy radiation and convection. When the ambient temperature is higher, the radiation and convection are reduced, so ampacity is correspondingly reduced (depending on the difference between the ambient temperature and the conductor's insulation temperature limits). When more conductors are in a given conduit/raceway, then you must assume that each one is carrying the current it is rated for and you have more heat sources trying to be cooled by radiation or convection from the outside surface of the conduit. However, when the conduit/raceway is fairly short then conduction of excess heat along the length of the conductors will also help to reduce their temperature. For this reason, the code permits the conductor fill (only) to be ignored when calculating ampacity.

If you are purchasing components and assembling them together without additional testing and listing of your finished product, then you must comply with all the requirements of the listing for each component. This includes the maximum temperature rating for the conductors attached to the breaker terminals (you can use a conductor with a higher-rated insulation temperature, but must limit its ampacity to either the 75 or 90 degC table (depending on the breaker's rating).

If you are having your completed product assembled and tested to the U/L standards, then you would go by their book and can frequently use a smaller conductor size. In the mid- to late-90's I worked in a U/L panel shop and met with their inspector a couple times when he was by to see what we were doing (all custom jobs) and verify that we were complying with their rules.

Hope this helps.--JMM

Jraef is exactly correct.

If you are not having your work inspected by a UL Inspector, you MUST follow NEC.