Overcurrent Protection Requirements for Services

Xptpcrewx

Senior Member
Location
Las Vegas, Nevada, USA
Occupation
Power System Engineer
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.
 

tom baker

First Chief Moderator
Staff member
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.
 

Xptpcrewx

Senior Member
Location
Las Vegas, Nevada, USA
Occupation
Power System Engineer
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...
 

tom baker

First Chief Moderator
Staff member
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
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
Service equipment must be rated for the available short circuit current.
Downstream, as mentioned, are feeders and they require short circuit and overload protection.
 

Xptpcrewx

Senior Member
Location
Las Vegas, Nevada, USA
Occupation
Power System Engineer
Service equipment must be rated for the available short circuit current.
Downstream, as mentioned, are feeders and they require short circuit and overload protection.
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.
 
Last edited:

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
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.
 

Xptpcrewx

Senior Member
Location
Las Vegas, Nevada, USA
Occupation
Power System Engineer
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.
Perfect. Thanks. I am not used to thinking of the Service OCPD as both the Service OCPD and Feeder OCPD.
 

Xptpcrewx

Senior Member
Location
Las Vegas, Nevada, USA
Occupation
Power System Engineer
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
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.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
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
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
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.
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)
 

Xptpcrewx

Senior Member
Location
Las Vegas, Nevada, USA
Occupation
Power System Engineer
Overcurrent is not a "vague" term. It is a term that is defined in Article 100 of the NEC.
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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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
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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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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