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#71525 - 11/01/06 06:57 AM How long can a circcuit be?
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Another thread raised the issue of breakers not tripping. It was suggested that impedance was the cause.

So- how long can a circuit bem before impedance becomes an issue in breaker performance?

The answer seems to be available in the GEMI study. Free software, that lets you compare different solutions, is available at:
http://www.steelconduit.org/gemi.htm

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#71526 - 11/01/06 08:33 AM Re: How long can a circcuit be?
SteveFehr Offline
Member

Registered: 03/19/05
Posts: 1192
Loc: Chesapeake, VA
There is no length restriction; the breakers are designed to protect the cable, which fails thermally. The instant trip is there to kill the circuit immediately if a short circuit is detected (and may not ever trip if the cable impedance is too high) but thermal trip should always work regardless of the fault circuit and protect the cables. If the breaker doesn't trip (and isn't malfunctioning), it's because it didn't see levels of fault current that would pose a danger to the building cabling, merely elevated fault current. It's not smart enough to know in-rush from a cheap compressor from a kid with a fork in the socket; AFCIs aren't even that smart. It just looks at current and trips if there's too much for too long.

On better breakers, all the trip settings are calibrated and manually configrable, so it's just a matter of taking system impedance into account when dialing in the trip settings.

[This message has been edited by SteveFehr (edited 11-01-2006).]

[This message has been edited by SteveFehr (edited 11-01-2006).]

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#71527 - 11/01/06 10:28 AM Re: How long can a circcuit be?
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Steve brings up an important point. Branch overcurrent protective devices (fuses and breakers) are designed to protect the branch circuit wiring not the wiring after the receptacle.

This is a paraphrased quote from a major manufacturer concerning BOLTED short circuits:
If a low voltage phase conductor is properly sized per NEC 240, it is not possible to damage the phase conductor during short-circuits below the AIC rating of the breaker protecting the phase conductor. UL tests to verify the short-circuit rating of a circuit breaker are performed considering 75C cable. The corresponding phase conductor is sized according to the NEC and must pass the fault tests without compromising its integrity."

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#71528 - 11/01/06 11:33 AM Re: How long can a circcuit be?
RODALCO Offline
Member

Registered: 12/08/05
Posts: 863
Loc: Titirangi, Akld, New Zealand
Cable length is an interesting point raised here.
If a circuit is very long and the maximum load on the circuit equals the circuit braker value, the voltage at the outlet may be half the voltage and the other half of the voltage can be dissipated as heat in the cable or connection.
When motors are connected, lower voltage, less torque is available, motors may not be able to start properly and draw excessive start current adding to the problem.

Under short circuit condition the cable impedance may be high enough not to trip the braker immidiately and excessive heating in the cable may occur and possible cause of a fire.

Certainly cable length is an important fact to bear in mind.

My $0.02 worth on the topic

Regards

Raymond ( RODALCO )
_________________________
The product of rotation, excitation and flux produces electricty.

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#71529 - 11/01/06 02:01 PM Re: How long can a circcuit be?
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Breakers are designed and tested to protect the branch circuit conductors under all conditions. This is one of the reasons that the UL test for circuit breakers (and fuses mounted in holders) includes 4' of conductor. This way we know the conductor is protected up to the AIC rating of the device and to the thermal rating of the termination. If there is enough impedance to limit the fault current below that of the short circuit portion of the protective device then its thermal portion will protect the conductor.

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#71530 - 11/01/06 02:18 PM Re: How long can a circcuit be?
wa2ise Offline
Member

Registered: 11/29/02
Posts: 771
Loc: Oradell NJ USA
If a cable is long enough to have enough impedance to cause its breaker not to clear a fault, is it also too long (for the gauge of wire used) for acceptable amounts of voltage drop when it's used normally? Say #14 loaded by a 13A load at the far end and the voltage drop seen by the load excessive? If you upsize the wire to get acceptable voltage drop, would this excessive impedance issue go away? Or are there issues involved if ferrious conduit or such us correctly used?

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#71531 - 11/01/06 03:14 PM Re: How long can a circcuit be?
SteveFehr Offline
Member

Registered: 03/19/05
Posts: 1192
Loc: Chesapeake, VA
Fault current is far in excess of normal load current, and that exacerbates voltage drop. A cable that sees 5V drop at 13A will see 50V drop at 130A. This is unrelated to normal conditions, though, where the voltage is within generally acceptable limits.

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#71532 - 11/01/06 03:51 PM Re: How long can a circcuit be?
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
JBD,
 Quote:
If a low voltage phase conductor is properly sized per NEC 240, it is not possible to damage the phase conductor during short-circuits below the AIC rating of the breaker protecting the phase conductor.

The 1/4 cycle withstand for #14 is 3370 amps. That is below the AIC rating of all breakers.
Don
_________________________
Don(resqcapt19)

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#71533 - 11/01/06 05:00 PM Re: How long can a circcuit be?
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
I think it is around 1300 feet that you could wirenut the 2 conductors of a 14ga circuit together and the breaker would never trip. You could still have good performance out there if you were just running a 100w light.
I don't know why I know this but it was either a "puzzler" or a real question on one of these BBs.
_________________________
Greg Fretwell

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#71534 - 11/01/06 05:03 PM Re: How long can a circcuit be?
frank Offline
Member

Registered: 05/29/01
Posts: 361
Loc: windsor ontario canada
Can be as long as the voltage drop allows it to be is what i always thought but i guess thats wrong.


[This message has been edited by frank (edited 11-01-2006).]

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