Breaker Curves, Cable Damage Curves and Connection Temperature Ratings

[MG44312]

I am trying to determine if a current installation meets the NEC.

I'm looking at a breaker Time Current Curve (TCC) and comparing it with the installed cable (10 AWG 90 C). The panel connections are rated at 75 C. The breaker is a 30A 3P 480V breaker.

My understanding is that cable damage curves are determined using the following equation:
t=0.0297*log[(T2+234)/(T1+234)](A/I)^2
Where:
t = time of short circuit from .01 to 10 seconds
A = Conductor area (10380 for #10)
I = short circuit current
T1 = cable temperature rating (90 C)
T2 = max short circuit temp (250 C)

Regarding ampacity, NEC table 310.15(B)(16) (formerly 310.16) gives 35A ampacity for 75 C 10 AWG. Since the breaker is 30A and cable ampacity is 35A this should be acceptable.

The issue I have is regarding conductor temperature. If I plot the cable damage line for 90 C 10 AWG it envelopes the breaker curve. But if I am to de-rate the cable to 75 C with a max short circuit temp of 150 C, the curve no longer envelopes the breaker TCC. This suggests to me that the panel connections would potentially overheat in the given scenario. Note that the 250 C and 150 C max short circuit temps are common values I've found for 90 C and 75 C cable respectively. I don't know that the NEC specifies these values anywhere.

So my questions are:
1. Should I plot the curve for 90 C cable or 75 C cable?
2. Should I even be plotting damage curves at all or does table 310.15(B)(16) already take this into account? (I read in another post that cable damage curves shouldn't be plotted in low voltage applications)

Cable damage curves plotted for given breaker is attached:


Thanks

 

[augie47]

Way over my head but I would like to refer you to 110.14(C) to see if that provides any insight as far as how the NEC addresses your situation

 

[Julius Right]

XLPE insulation it is rated for 90oC continuous load 130oC emergency and 250oC short-circuit.
NEC art.110(14)C Temperature Limitations
The temperature rating associated
with the ampacity of a conductor shall be selected
and coordinated so as not to exceed the lowest temperature
rating of any connected termination, conductor, or device.
It is not directly limiting the insulation properties but the reduced temperature is required in order to protect the connected devices and that is for operating conditions only.
See: EPRI-EL 5036-V4 Table 4.5 Temperature Ratings of commonly Used Rubber and Plastic Insulations:
XLPE Operating conditions: 90oC; Emergency overload: 130oC ; Short-circuit Rating: 250oC
Insulation used for power and control cables is given three different temperature ratings:
operating condition,emergency overload, and short-circuit.
The emergency overload temperature is for those situations in which load current is higher than normal but not expected to last more than 100 h at any given time or more than 500 h in the life of the cable The short-circuit temperature limit is for fault current conditions normally lasting no more than 150 cycles.
IEC 60502-1/2004 Table 3 Maximum conductor temperatures for different types of insulating compound Cross-linked polyethylene[XLPE] :
Normal operation :90oC
Short-circuit (5 s maximum duration): 250 oC

 

[Ingenieur]

imo no need in the lv li application
if I were concerned I would look at conductor allowable sc i vs time
http://www.cooperindustries.com/con...ductor_Protection_Wire_Cable_Protection_2.pdf

#10
1000 amp for 16 cyc 0.27 sec
your cb trips in <0.03 sec or 2 cyc
you can check a few points, the min and max sc i the ckt will see
basically fault a cb lugs and at load

you can also use ithe icea 5 sec rating to reach an insul damage temp
#10 245 A
your cb trips in 2 to 6 sec, your chart is hard to read
bear in mind these numbers are conservative and include a sf

 

[Julius Right]

If you'll take 130oC maximum temperature and for trip time up to 5 sec using IEEE 242/2001 9.5.2 Overload capacity Formula from 9.5.2.4 Development of intermediate characteristics and from here the above adiabatic formula you used you'll get a damage curve as attached.
Translated in excel the IEEE formula is as follows:
Ie/in=sqrt(((TE-To)/(TN-To)-(Io/In)^2*exp(-t/k))/(1-exp(-t/k))*(230+TN)/(230+TE)) where
Ie=emergency current
TE=emergency temperature[130]
TN=rated temperature for normal load [90]
In=rated current[35]
To=initial temperature[75]
Io=initial current[35]