Emergency Rating of cable

[GreyGoose]

anybody know how to go about calulating the emergency rating of a power cable? In other words, if you have a copper cable with a temperature rating of 75oC (or 90oC or 105oC), what is the ampacity of that cable at the emergency rated temperature of 130oC?

 

[jbartos]

Suggestion: The proper way to proceed is to have a cable damage curve (different from cable ignition curve) available and leave some margin. Visit

http://www.uscg.mil/hq/msc/PRGuidance/e2-04.pdf

etc. for more info

 

[peebee]

What jbartos said.

Also, here's what ANSI/IEEE Std. 141-1986 (Red Book) (1986 is superseded but still relevant), has to say, from 11.4.4, "Cable Emergency Overload Criteria":

". . . the life of cable insulation is about halved and the average rate of . . . failures about double for each 5-15 deg. C increase in normal daily load temperature. . . the ICEA has established maximum emergency overload temperatures for various types of insulation [not exceeding] 100 h per year . . . [or] five [times] during the life of the cable . . . Table 78 gives uprating factors [ranging from about 5% to 80% depending on temperature, voltage, and insulation type]. . . . A more detailed discussion on emergency overload and cable protection is contained in ANSI/IEEE Std 242 (Buff Book). . . . "

 

[GreyGoose]

Thanks.

Question came up because of the apparent disparity in requirements for fire pump protection in the NEC. The NEC requires the protective device to be sized for locked rotor current (LRA), but but the feeder only needs to be sized for 125% of motor FLA. So, if the motor does happen to draw close to LRA for whatever reason (for an extended period of time), the breaker or fuse would never trip/blow but the cable may burn out.

 

[cuky2000]

…. if the motor does happen to draw close to LRA for whatever reason (for an extended period of time), the breaker or fuse would never trip/blow but the cable may burn out.

The design philosophy for fire pump should be treated as special cases designing for indefinitely run with locked-rotor current. It is common practice to select the electrical components to support the concept that the motor-pump and other components will operate to destruction if necessary.

Fortunately cables used inside buildings are tested for high temperature and flame retardant. For instance the “All-Wires Test Procedure provide a temperature of 1,500 oF for short time.

Record and experience shows that this method of considering 1.25 time of the largest motor plus other FLA motors appear to be adequate.

I hope this address some of your concerns.

 

[peebee]

Re: "So, if the motor does happen to draw close to LRA for whatever reason (for an extended period of time), the breaker or fuse would never trip/blow but the cable may burn out."

Two comments:

1. As mentioned by cuky2000, the NFPA would rather have the motor & cable run to destruction & keep pumping water right up until the feeder shorts out & trips if need be.

2. Even on non-emergency motors, the overcurrent protective device provides ONLY short-circuit protection for the feeder, NOT overload protection, which is why Table 430.52 permits overcurrent devices sized at anywhere from 175% to 1100% of motor full load. Overload protection is provided by the overload relay at the starter. This is about the same as for fire pump motors, they simply have the additional *requirement* that the overcurrent device be able to indefinitely carry the locked rotor current, whereas in 430, using an instantaneous trip breaker set at 1100%, you're *permitted* to let it indefinitely carry the locked rotor current (which is usually about 600 or 700%).

There is still an overload device in the UL-listed Fire Pump Controller, as indicated in NFPA 20 7-4.4.

The way the NEC is written, it appears at first that motor feeders are permitted to be unprotected for both life-safety and general-purpose motors. In fact, they are typically protected against overload by downstream overload relays, and against overcurrent by upstream fuses or breakers. I agree that it is somewhat confusing the way it is currently written. The NEC handbook helps understand this with general-purpose motors, and NFPA 20 helps makes this clear with fire pumps.

 

[RajT]

There is no disparity in the NEC clause, its just that you have misunderstood it.
What must be realised is that the scenario of breaker not opening or the fuse not blowing is discounted in the sizing of the cable and as such cable sizing NEVER allows for such condition i.e if that were to happen and the cable caught fire then so be it.Therefore the cable should be sized for full load current and volt drop requirements only.