Underground installations over 1000V

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adunbar

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lowell, ma
The Solar industry is moving toward designing ground mounted PV systems with 1500V dc solar modules. The requirements in NEC 2017 article 300.50(A)(3) require underground nonshielded cables rated over 1000V be installed in conduit encased in not less than 3" of concrete. This would require all our underground PV source circuits to be encased in concrete.


If I look at article 300.50(A)(2), it permits nonshielded single conductor cables up to 2000V to be direct buried in industrial establishments if listed for direct burial.


Our PV systems are installed behind a fence, serviced only be qualified persons. Our interpretation is that our 1500V dc source circuits do not need to be concrete encased when installed underground, in conduit, behind a fence.

Please let me know if you agree with our interpretation as we made a similar argument when we moved to 1000Vdc systems during previous code cycles and the code spoke to over 600V. I appreciate any input you can provide as we expect to start designing 1500V systems in the next few months and the cost of encasement would be huge.


 
The Solar industry is moving toward designing ground mounted PV systems with 1500V dc solar modules. The requirements in NEC 2017 article 300.50(A)(3) require underground nonshielded cables rated over 1000V be installed in conduit encased in not less than 3" of concrete. This would require all our underground PV source circuits to be encased in concrete.


If I look at article 300.50(A)(2), it permits nonshielded single conductor cables up to 2000V to be direct buried in industrial establishments if listed for direct burial.


Our PV systems are installed behind a fence, serviced only be qualified persons. Our interpretation is that our 1500V dc source circuits do not need to be concrete encased when installed underground, in conduit, behind a fence.

Please let me know if you agree with our interpretation as we made a similar argument when we moved to 1000Vdc systems during previous code cycles and the code spoke to over 600V. I appreciate any input you can provide as we expect to start designing 1500V systems in the next few months and the cost of encasement would be huge.



I agree with you assessment of 300.50(A)(2). But I also dont see why PV wire wouldnt fall under (A) general. What conductors/cables would fall under 300.50(A)(3)? What is an example of something that is "not covered in 300.5(A)(2) or (3)"? This section is not written well.
 

adunbar

Member
Location
lowell, ma
Not sure what you mean. 300.5(A) General says underground cable shall be installed per 300.50(A)(1), (A)(2) or (A)(3). (A)(1) is for shielded or MC cables, so doesn't apply to PV wire. (A)(2) is for direct buried nonshielded cable up to 2000V. We are installing nonshielded PV wire in conduit , so this doesn't technically apply. (A)(3) is for other nonshielded cables not covered in (A)(1) or (A)(2). I believe that our PV wire would fall here and need to be encased in concrete.

I'd like to use the 2000V limit of (A)(2) and apply it to (A)(3) so that our 1500V PV wire in conduit would not need to be encased in concrete.
 
Not sure what you mean. 300.5(A) General says underground cable shall be installed per 300.50(A)(1), (A)(2) or (A)(3). (A)(1) is for shielded or MC cables, so doesn't apply to PV wire. (A)(2) is for direct buried nonshielded cable up to 2000V. We are installing nonshielded PV wire in conduit , so this doesn't technically apply. (A)(3) is for other nonshielded cables not covered in (A)(1) or (A)(2). I believe that our PV wire would fall here and need to be encased in concrete.

I'd like to use the 2000V limit of (A)(2) and apply it to (A)(3) so that our 1500V PV wire in conduit would not need to be encased in concrete.

Ok I reread it a few times and I think it makes sense. I dont see any reason why you cant apply (A)(2). However I am not sure what you mean by "use the 2000V limit of (A)(2) and apply it to (A)(3)" - why apply it to (A)(3) and not just use it directly?
 

adunbar

Member
Location
lowell, ma
I interpret (A)(2) to apply to direct buried cables. I don't include underground conductors installed in conduit as direct burial. If I can, then (A)(2) will apply to our 1500V PV wire in PVC conduit.
 
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