Maximum Circuit Length Help?

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mbrooke

mbrooke

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What is the maximum length that any one circuit can be run? What are the NEC requirements?




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I'm asking because 400% minimum short circuit current seems kind of low. According to time current curves it takes up 50 seconds for an OCPD to open:



Wouldn't this create a hazard to life and property?
 
P

paulengr

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The steel tubing institute offers GEMI for free. The accompanying technical papers explain the basis behind it. And yes, UL class B breakers won’t trip on instantaneous that low except on inverse time.

You CAN go lower and utilities do in rural areas but it requires a different approach. Effective grounding ceases to exist so extensive transient overvoltage control is a necessity. Breakers have to work based on impedance (V/I) which is called distance relaying.

But in general it’s just easier to increase voltage. Most distance problems go away quickly with increased voltage. For instance a modest increase from 230 to 480 V cuts current in half but decreases losses by a factor of 4!
 
Julius Right

Julius Right

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In my opinion it is according to BS7671 reg.411.4.5.
 

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Julius Right

Julius Right

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NEC 2017 Art.230.95 Ground-Fault Protection of Equipment.

(A) Setting. The ground-fault protection system shall operate to cause the service disconnect to open all ungrounded conductors of the faulted circuit. The maximum setting of the ground-fault protection shall be 1200 amperes, and the maximum time delay shall be one second for ground-fault currents equal to or greater than 3000 amperes.
 
romex jockey

romex jockey

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Julius Right said:
NEC 2017 Art.230.95 Ground-Fault Protection of Equipment.

(A) Setting. The ground-fault protection system shall operate to cause the service disconnect to open all ungrounded conductors of the faulted circuit. The maximum setting of the ground-fault protection shall be 1200 amperes, and the maximum time delay shall be one second for ground-fault currents equal to or greater than 3000 amperes.
Click to expand...
can i ask what calc is used for this??? ~RJ~
 
romex jockey

romex jockey

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Anecdotally , i've seen a number of long runs to 'back 40 barns' that simply do not trip the serving breaker....~RJ~
 
Julius Right

Julius Right

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Electrical Engineer Power Station Physical Design Retired
I don’t see any reference to the EMT conduit in this table.
As calculated the short-circuit it is 4 times the device rating, if the distance from source up to fault point it is not more than it is noted, indeed.
However, the voltage drop it is not 40V [ for any row].
If the circuit breaker delay setting it is 1 sec or less according to IEC/TS 60479_1 standard 120 V touch voltage is permissible.
 

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