NEC 240.21(C)(4) outside secondary conductors

I

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
We have a large pad mounted transformer located outside the building. The secondary conductors are routed underground into the building that is only about 15 feet away. Inside the building, the cable stubs up and is routed overhead approximately 150 feet to a switchgear MCB where it terminates. This appears to be in compliance with 240.21(C)(4), but it just seems odd the code would allow this considering the 25 foot rule that precedes it - or am I missing something?


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Isaiah said:
We have a large pad mounted transformer located outside the building. The secondary conductors are routed underground into the building that is only about 15 feet away. Inside the building, the cable stubs up and is routed overhead approximately 150 feet to a switchgear MCB where it terminates. This appears to be in compliance with 240.21(C)(4), but it just seems odd the code would allow this considering the 25 foot rule that precedes it - or am I missing something?


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How does that comply with 240.21(C)(4)(4)?
240.21(C)(4)
(4)The disconnecting means for the conductors is installed at a readily accessible location complying with one of the following:
a. Outside of a building or structure
b. Inside, nearest the point of entrance of the conductors
c. Where installed in accordance with 230.6, nearest the point of entrance of the conductors
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The language in (c) would require a disconnect at the point where the conduit stubs up inside the building.
 
Who owns that pad mounted transformer? If that's a utility owned pad mounted transformer, then that's a set of service entrance conductors. I don't think 240.21(C)(4) applies to service conductors - you would have to refer to article 230. Based solely on the description, the installation isn't compliant with the requirements of 230.

In the case that the transformer is not owned by the utility, your description of how the set of secondary conductors are routed still doesn't seem compliant with 240.21(C)(4). Specifically, the installation is not in compliance with 240.21(C)(4)(4). The secondary conductors cease being considered "outside" when they're inside the building.

In both instances, I think the installation as you describe is not code compliant.
 
don_resqcapt19 said:
How does that comply with 240.21(C)(4)(4)?

The language in (c) would require a disconnect at the point where the conduit stubs up inside the building.
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It’s not really a service to the building. Does this make any difference?


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If they are service conductors, SDS transformer secondary conductors, or outside feeders all require a disconnect outside or immediately upon entrance of the building.
 
augie47 said:
If they are service conductors, SDS transformer secondary conductors, or outside feeders all require a disconnect outside or immediately upon entrance of the building.
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Thanks Augie just wanted to be sure before I deliver the bad news


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Isaiah said:
It’s not really a service to the building. Does this make any difference?


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No. Nothing in 240.21 applies to service conductors. The reference to 230.6 in 240.21(C)(4)(4) is only to tell you what conductors are considered to be outside a building.
 
It's all about the risk of something damaging the conductors when there is no breaker upstream to clear the fault. It's OK to have outside conductors of "unlimited length" because if they are somehow damaged, the arcing will not set any building on fire.

Once the conductors enter a building however, you don't want them to go far before hitting an overcurrent device, which would then protect the rest of the run. That is where the a, b, and c rules cited in Don's post #2 come into play. Keeping the distance short and providing physical protection is what makes the risk level acceptable.
 
I did a job where the engineer designed a very similar situation and I discussed it with the electrical inspector and he rejected it and made the call.
 
Isaiah said:
It’s not really a service to the building. Does this make any difference?
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Physics-wise, one might think it shouldn't, but code-wise it does. Physics doesn't care about transformer ownership and circuit classification, but the NEC does, and has different standards that apply in each case.

Customer-installed conductors from the secondary of a utility-owned transformer, are service conductors, and comply with article 230. 240.21(C) doesn't apply.

Conductors from a customer-owned transformer, are transformer secondary conductors governed by 240.21(C). These rules are very similar to tap conductors, from 240.21(B). The underlying idea is that such conductors are only protected in excess of their ampacity, and "not yet protected" at their ampacity, and thus have a higher risk. Lengths are limited to reduce this risk, but still given to allow a practical length to work with, in order to connect to a grouped piece of equipment that hosts the OCPD & shutoff. Or with no limits on length, for outdoor circuits.

240.21(C) doesn't really offer guidance on how the unlimited length outdoor rule, interacts with the length limits inside a building, for a circuit in both environments. My reasoned judgement on this, would be to pick one of the length limits for the scope of the wire inside the building and comply with all context rules for both applicable subsections of 240.21(C). As an example, 20 feet inside a building, and 100 ft outside a building, would mean complying with context rules of both the 25 ft rule and the outside unlimited rule. You'd probably have to deliberately go out of your way of what most people would do anyhow, to not comply with context rules of both sections.
 
There is no interaction between the outside and inside secondary conductor rules. Trying to say there is would result in a violation of last sentence of the parent text in 240.21.
Conductors supplied under 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4.
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I would see the application of the 25' secondary conductor rule to the end of the outside conductor rule being a violation of that, as I would call the inside secondary conductor a connection to the outside secondary conductor.

The outside transformer secondary conductor rule and the service conductor rules are pretty much identical. In fact the reason that the outside transformer conductor rule was added to the code was to put the conductors from outside customer owned transformers on the same footing as service conductors.
 
don_resqcapt19 said:
There is no interaction between the outside and inside secondary conductor rules. Trying to say there is would result in a violation of last sentence of the parent text in 240.21.
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From 240.21(C)(4):
Where the conductors are located outside of a building or structure, except at the point of load termination.
And subpart 4b: Inside, nearest the point of entrance of the conductors

This part of the rule seems to imply there can be some length for entering the building, and terminating inside of indoor-mounted equipment, but it doesn't specify what that length is. Otherwise, a rigid interpretation of the conductors being outside, would require an outside disconnect/ocpd.
 
Carultch said:
From 240.21(C)(4):
Where the conductors are located outside of a building or structure, except at the point of load termination.
And subpart 4b: Inside, nearest the point of entrance of the conductors

This part of the rule seems to imply there can be some length for entering the building, and terminating inside of indoor-mounted equipment, but it doesn't specify what that length is. Otherwise, a rigid interpretation of the conductors being outside, would require an outside disconnect/ocpd.
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That does vary, but I read this just like I read the one for service conductors....you enter the building and go directly into the disconnect, or where the point of entry is at a different elevation you go directly up or down into the disconnect. That is what I read the "nearest the point of entry" language as saying.

Because the there are differences in opinions on what the language means, many areas have local amendments specifying a maximum length of conductor between the point of entrance and the disconnect.

Our local code specifies that there can be a maximum of 10' of service conductor in the building and those conductors must be in RMC or IMC. If you don't want to do that, then you put the disconnect outside.
 
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