Overcurrent Protection Requirements for Services

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xptpcrewx

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
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Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
Hi Everyone,

Testing my understanding here... In general, Article 230 only requires Overload protection for the conductors on the supply side of the disconnecting means and Ground-Fault protection of the service equipment itself (sections 230.90 and 230.95 respectfully), yet makes no mention of Short-Circuit protection requirements. I just want to confirm that I am not missing something here as Short-Circuit protection is only spelled out in Part IX of this article. Thanks in advance.
 
Yes you have got the gist! Service Entrance Conductors, commonly called unfused conductors are protected from overload by your load calculation, but not from shorts or grounds. Protection from shorts and grounds are typically done by the utility having fuse on the transformer primary.
The NEC has additional requirements for unfused conductors, the type of bonding, restrictions on the type of raceway and how far the conductor can extend into the building, no feet, 2 ft 5 ft etc. For example in Washington EMT can not be used for service conductors (only 8 types of raceway are allowed and the maximum distance is 15 ft of conduit.
 
Thanks guys. What I don't understand is why the service is not required to have short-circuit protection for the equipment/devices served directly downstream of the service overcurrent protective device; i.e. switchgear, switchboards, panelboards, MCC's, loads etc.

Section 110.10 suggests that equipment needs to be protected and the OCPD's must be selected/coordinated to clear a fault without causing "extensive damage". In practice I can see this being easily ignored if there is no specific requirement to have short-circuit protection...
 
Downstream of the service OCPD your circuit is a feeder, providing, SS protection, overload and ground fault. Look up the definition of FEEDER, and all the definitions of SERVICES
 
So does this mean the Service OCPD doubles as a Service OCPD and a Feeder OCPD?
 
Service equipment must be rated for the available short circuit current.
Downstream, as mentioned, are feeders and they require short circuit and overload protection.
 
augie47 said:
Service equipment must be rated for the available short circuit current.
Downstream, as mentioned, are feeders and they require short circuit and overload protection.
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Maybe I am not being clear. I am specifically referring to the service OCPD protecting the load side bus. I understand equipment must be rated to withstand the prospective short-circuit current, but that is not the same as "protection".

Equipment withstand ratings are really about preventing the equipment from breaking apart due to the magnetomotive forces experienced during short-circuits. I can imagine other modes of extensive damage like conductor melting associated with long clearing times.
 
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Your first statement was accurate, service conductors are protected against overload by the downstrean device (service disconnect) which can not detect line side faults. That device does offer both overcurrent and short circuit protection for load side conductors and equipment under Art 215 Feeders and Art 240.
 
augie47 said:
Your first statement was accurate, service conductors are protected against overload by the downstrean device (service disconnect) which can not detect line side faults. That device does offer both overcurrent and short circuit protection for load side conductors and equipment under Art 215 Feeders and Art 240.
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Perfect. Thanks. I am not used to thinking of the Service OCPD as both the Service OCPD and Feeder OCPD.
 
tom baker said:
Downstream of the service OCPD your circuit is a feeder, providing, SS protection, overload and ground fault. Look up the definition of FEEDER, and all the definitions of SERVICES
Click to expand...

Sorry, I am going down a rabbit hole... Where exactly does it specify Short-Circuit protection for feeders? I only see the vague term "overcurrent" being used as well as Ground-Fault protection.
 
Don't know that it does. Protection of the overcurrent device and utilization equipment in retaliation to short circuits is covered by Art 110 and 240 Part VI
 
Xptpcrewx said:
Sorry, I am going down a rabbit hole... Where exactly does it specify Short-Circuit protection for feeders? I only see the vague term "overcurrent" being used as well as Ground-Fault protection.
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Overcurrent is not a "vague" term. It is a term that is defined in Article 100 of the NEC.
Overcurrent.
Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault. (CMP-10)
Click to expand...
 
don_resqcapt19 said:
Overcurrent is not a "vague" term. It is a term that is defined in Article 100 of the NEC.
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It is in this context because it doesn’t specify if it is Overload, Short-Circuit or Ground Fault as well as the fact that there is a separate section specific to ground fault protection only.


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augie47 said:
Don't know that it does. Protection of the overcurrent device and utilization equipment in retaliation to short circuits is covered by Art 110 and 240 Part VI
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I'm all for anything that will "retaliate" against a short circuit .... :)

JAP>
 
Xptpcrewx said:
It is in this context because it doesn’t specify if it is Overload, Short-Circuit or Ground Fault as well as the fact that there is a separate section specific to ground fault protection only.


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Click to expand...
The defined term "overcurrent" includes all three unless there is something in the section that says it does not include all 3.
 
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