Why Underground Conductors

[Rachit]

So I feel the NEC is clear in sections 230.3 by saying do not pass service conductors through the interior of another building, and from my interpretation a work around to routing through a building is stated in 230.6 where 2 in. concrete in-cased conductors are considered outside the building. BUT WHY? What is the intent of this? The way I am interpreting this makes me think it is better to route outside the building instead of concrete in-cased conduit. My original assumption of why, was to prevent the potential issues that may come up if the building was ever to be demolition far into the future, but I am getting told it is do to the potential of a fire hazard. Does anyone have any information as to why this exists in the NEC. Is there any benefits to routing your service conductors outside the building rather than concrete in-casing them?

 

[tom baker]

I will answer your question with a question. You may not realize that you already know the answer.
For secondary service conductors (EG 120/240) to a building, from the utility transformer to the line side of the service disconnect:
Are these conductors protected against (yes or no)
Overload
Short circuit
Ground fault

Lets let the OP answer before we answer again.

 

[Dennis Alwon]

Cost is the biggest issue. If you have a slab then I would run the conduit under there otherwise I would try and stay outside the building. You have to way which would be the best. In big commercial areas where expansion can happen it may be best to keep them inside in concrete. Another workaround would be to add exterior disconnects and not worry about concrete inside.

 

[GoldDigger]

So I feel the NEC is clear in sections 230.3 by saying do not pass service conductors through the interior of another building, and from my interpretation a work around to routing through a building is stated in 230.6 where 2 in. concrete in-cased conductors are considered outside the building. BUT WHY? What is the intent of this? The way I am interpreting this makes me think it is better to route outside the building instead of concrete in-cased conduit. My original assumption of why, was to prevent the potential issues that may come up if the building was ever to be demolition far into the future, but I am getting told it is do to the potential of a fire hazard. Does anyone have any information as to why this exists in the NEC. Is there any benefits to routing your service conductors outside the building rather than concrete in-casing them?

Among other things, the NEC allows service conductors to be used in a smaller wire gauge than would be chosen for the same expected load with interior wiring. Also the OCPD for service conductors is usually very high compared to the wire size. And that OCPD is governed by NESC rather than NEC standards.
The result is that the conductors are probably (although not necessarily) protected against a bolted fault, but may get overloaded to a level that causes overheating and fire.

This is considered an acceptable risk outside the building but not inside it. Putting the conductors inside 2" or more of concrete is assumed to mitigate the fire risk.
This is also why it is acceptable to run service conductors above a building but not under it (unless concrete protected.)

 

[Rachit]

Among other things, the NEC allows service conductors to be used in a smaller wire gauge than would be chosen for the same expected load with interior wiring. Also the OCPD for service conductors is usually very high compared to the wire size. And that OCPD is governed by NESC rather than NEC standards.
The result is that the conductors are probably (although not necessarily) protected against a bolted fault, but may get overloaded to a level that causes overheating and fire.

This is considered an acceptable risk outside the building but not inside it. Putting the conductors inside 2" or more of concrete is assumed to mitigate the fire risk.
This is also why it is acceptable to run service conductors above a building but not under it (unless concrete protected.)

I will answer your question with a question. You may not realize that you already know the answer.
For secondary service conductors (EG 120/240) to a building, from the utility transformer to the line side of the service disconnect:
Are these conductors protected against (yes or no)
Overload
Short circuit
Ground fault

Lets let the OP answer before we answer again.

AH HA! NO Overload protection, No Short Circuit protection, and No Ground Fault protection. So the service conductors are unprotected unless a disconnect means is installed, and in the case of unprotected conductors you have the risk of fire due to short circuiting. From your statement above I am to assume the risk of a bolted fault is especially present within a building and requires concrete in-casement compared to outside a building where the risk is low enough that no concrete encasement is needed. Am I on the right track here? Which brings me to another question is a parking garage considered outside the building? And in the case of non-concrete in-cased conduits running above a building is the risk of fire just so low no protection is needed? Thank you all for the information, y'all have been very informative.

 

[GoldDigger]

AH HA! NO Overload protection, No Short Circuit protection, and No Ground Fault protection. So the service conductors are unprotected unless a disconnect means is installed, and in the case of unprotected conductors you have the risk of fire due to short circuiting. From your statement above I am to assume the risk of a bolted fault is especially present within a building and requires concrete in-casement compared to outside a building where the risk is low enough that no concrete encasement is needed. Am I on the right track here? Which brings me to another question is a parking garage considered outside the building? And in the case of non-concrete in-cased conduits running above a building is the risk of fire just so low no protection is needed? Thank you all for the information, y'all have been very informative.

"By George, I think he's got it!"

 

[RB1]

Short-circuit protection: No

Ground-fault protection: No

Overload protection: Yes.

 

[tom baker]

Great!
Service entrance conductors are often called "unfused". Due to the fire danger we protect them by a specific wiring method, distance into the building, and service grounding. By encasing the conductors in 2" of concrete they are considered outside of the building.
Washington is very restrictive on the wiring methods for service conductors and only allows 8 methods. EMT is not allowed.

 

[GoldDigger]

Overload protection: Yes.

Only because there is overload protection at the building end of them. If that fails or is bypassed in some way, then there is no overload protection remaining on the service conductors.

 

[electrofelon]

Great!
Service entrance conductors are often called "unfused". Due to the fire danger we protect them by a specific wiring method, distance into the building, and service grounding. By encasing the conductors in 2" of concrete they are considered outside of the building.
Washington is very restrictive on the wiring methods for service conductors and only allows 8 methods. EMT is not allowed.

Tom I believe emt is allowed for service conductors outside of a building only. Wac 296 46b 230 043. City of seattle has replacement pages to the nec and has emt crossed out as service wiring method but has "reserved" next to it. Im not exactly sure what reserved means, perhaps by special permission. The washington emt restriction has always drove me crazy. I assume they are worried about the fault carrying ability of joints.