Instantaneous ampacity of a cable

[Voltrael]

I have 2AWG 90deg C tray cable. We have federal inspectors on site (MSHA). In order to satisfy them that an installation is safe I need to demonstrate that this cable can withstand a short circuit on the cable. The cable runs a short distance between a generator (800A breaker, 1600A instantaneous setting) and a motor starter (125A breaker).

 

[tom baker]

What is the motor HP and voltage?

 

[Voltrael]

75 HP 460V

 

[don_resqcapt19]

I have 2AWG 90deg C tray cable. We have federal inspectors on site (MSHA). In order to satisfy them that an installation is safe I need to demonstrate that this cable can withstand a short circuit on the cable. The cable runs a short distance between a generator (800A breaker, 1600A instantaneous setting) and a motor starter (125A breaker).

You are going to need the short circuit study showing the available fault current and the clearing time of the upstream OCPD, and then look at a cable withstand table.
Electrical engineering software will do these calculations.

 

[Voltrael]

Thank you. One follow up question for people. How many cycles is considered "instantaneous"? Just one?

 

[don_resqcapt19]

Thank you. One follow up question for people. How many cycles is considered "instantaneous"? Just one?

The cable withstand has to be based on the clearing time of the upstream device. It could be 3 or more cycles, or could be 1/4 cycle.

 

[jim dungar]

All UL489 Listed breaker are tested with conductors sized per the NEC, this is partly to prove that the conductors are not damaged during short circuits.

I seem to recall very few LV cable damage curves that actually show coordination with standard breaker curves.

 

[wwhitney]

All UL489 Listed breaker are tested with conductors sized per the NEC, this is partly to prove that the conductors are not damaged during short circuits.

Is that testing using the minimum sized conductor based on the 75C ampacity, or the 90C ampacity?

The NEC would allow us to use 4' of conductor sized based on the 75C ampacity, and then splice in a separate enclosure to a smaller conductor based on the 90C ampacity. So I'm wondering if the the UL testing covers protection of that smaller 90C ampacity conductor.

Cheers, Wayne

 

[tom baker]

Your ocpd for the #2 seems too large, but MSHAV rules may be different than the NEC

 

[herding_cats]

Why is a 75 hp motor not on a drive?

 

[zbang]

Your ocpd for the #2 seems too large, but MSHAV rules may be different than the NEC

I was thinking that, especially downstream of an 800 amp breaker, again MSHA rules may be different.

Why is a 75 hp motor not on a drive?

Why should it be? That really depends on the load and environment - for something like a fan that's running full-tilt 24/7, what's the point of a drive?

 

[jim dungar]

Is that testing using the minimum sized conductor based on the 75C ampacity, or the 90C ampacity?

The NEC would allow us to use 4' of conductor sized based on the 75C ampacity, and then splice in a separate enclosure to a smaller conductor based on the 90C ampacity. So I'm wondering if the the UL testing covers protection of that smaller 90C ampacity conductor.

Cheers, Wayne

I don't know the insulation type, but most 100% rated equipment terminations require 90°C conductors applied at the NEC 75°C values.

 

[wwhitney]

I don't know the insulation type, but most 100% rated equipment terminations require 90°C conductors applied at the NEC 75°C values.

OK, my question wasn't about 100% rated equipment. Rather, it was about a 110.14(C)(2) installation, such as:

225A OCPD -- 4' of 4/0 Cu -- Junction Box -- Splice to 90C insulated 3/0 Cu

110.14(C)(1) limits the ampacity of the 4/0 Cu to the 75C termination rating of the 225A OCPD, or 225A. But 110.14(C)(2) says that with 90C rated pressure connectors we can use the 90C ampacity of the conductors, so 225A for the 3/0 Cu.

So if UL 489 only requires testing 225A OCPD with 4/0 Cu, and if the conductor damage curve and the OCPD trip curves don't coordinate, how do we have confidence that the 225A OCPD will protect the 3/0 Cu?

Thanks,
Wayne

 

[jim dungar]

OK, my question wasn't about 100% rated equipment. Rather, it was about a 110.14(C)(2) installation, such as:

225A OCPD -- 4' of 4/0 Cu -- Junction Box -- Splice to 90C insulated 3/0 Cu

110.14(C)(1) limits the ampacity of the 4/0 Cu to the 75C termination rating of the 225A OCPD, or 225A. But 110.14(C)(2) says that with 90C rated pressure connectors we can use the 90C ampacity of the conductors, so 225A for the 3/0 Cu.

So if UL 489 only requires testing 225A OCPD with 4/0 Cu, and if the conductor damage curve and the OCPD trip curves don't coordinate, how do we have confidence that the 225A OCPD will protect the 3/0 Cu?

Thanks,
Wayne

I believe breakers are short circuit tested with all of the conductor sizes listed on their terminations. For the most part breaker short circuit elements are common over the full range of the available trip ratings for its frame.

 

[Julius Right]

Let's say the switchgear is close to generator so cable impedance is 0 and the highest fault current they can produce is I"k.
According to Advanced Power Technologies , in this case, as a rule of thumb, if the 80% of nameplate interrupting capacity of the feeder circuit breakers connected to this generator bus is above I"k, no further calculations are generally required. If the name plate interrupting capacity of the feeder is 1600 A then I"k=1600*0.8=1280 A
According to NEC Art 240.92 Table 92(B) :
(I/A)^2*t=0.0297*log10((T2+234)/(T1+234) for copper conductor
(I/A)^2*t=0.0125*log10((T2+228)/(T1+228) for aluminum conductor
From here we get:
A=I/sqrt(0.0297/t*log10((T2+234)/(T1+234))) for copper or:
A=I/sqrt(0.0125/t*log10((T2+228)/(T1+228))) for aluminum.
If we take I=1600 A, t= 0.100 sec we get:
If it is XLPE or EPR insulation [usually all XHHW ,XHWN or else with X first letter]
T1=90 T2=250
A=1600/SQRT(0.0297/0.1*LOG10(250+234)/(90+234)))=7032.2 cmils required for copper conductor
If it is PVC insulation [usually THHW or else with T first letter] for copper conductor
T1=90 T2=150
Then the maximum required circular mils is for aluminum conductor PVC insulation=16517.6
2 awg is 66369 cmils any way.