1000V DC Systems

[Andrew445]

Per NEC 300.50(A)(2), do 1000V DC cables need to be trenched in concrete or RMC? This has always been a point of contention for us. We have typically compromised with PVC conduit under a 3" min. concrete slab. What are the thoughts on this?

 

[jaggedben]

What exactly is the point of contention?

The section you cite specifically mentions "rigid nonmetallic conduit" a.k.a. PVC so there should be no issue there. This seems to go back at least as far as the 2005 code cycle.

My only question is whether "under a 3" minimum slab" is the same as "encased in not less than 3" of concrete". I'd say the proper method would be to dig your trench 3" deeper than the requirement, 6" wider than OD of conduit, put 3" spacers below the conduit, and pour directly over the installed conduit.

 

[Andrew445]

Sorry, the point of contention is that these 1000V DC cables do not fall under that code section. Client would like to go with direct-buried cables as it is obviously cheaper, but typically these are not shielded. In fact, I cannot even find PV cable that is manufactured with a metal covering for this application. I contacted a cable manufacturer and was told that since article 690.31 states in part that ....all other wiring systems and fittings specifically intended and identified for use on photovoltaic arrays shall be permitted, that this allows the use of direct buried unshielded/sheathed PV cable at 1000V that they manufacture, since they are listed for and specifically intended for use on pv arrays.

I believe that technically the code section applies, and the cables must be in either RMC/IMC or RNMC in concrete-encased ductbank. This is a fairly major issue, as the cost difference is large.

 

[Andrew445]

To clarify a bit more, here is a portion of the response from a cable engineer:

"Article 690 is the ruling code article for solar photovoltaic systems. In 690.3, the code states that ?Wherever the requirements of other articles of this Code and Article 690 differ, the requirements of Article 690 shall apply?? Wiring Methods are specifically addressed in Section IV of Article 690, and 690.31 states that ?All raceways and cable wiring methods included in this Code and other wiring systems and fittings specifically intended and identified for use on photovoltaic arrays shall be permitted? (emphasis mine). Nowhere in Article 690 are underground installations specifically mentioned.

Neither the language in 690 or 300.50 is perfectly clear. However, based on the language in 690.31, my position is that since our PV conductors are Listed for and ??specifically intended for use on photovoltaic arrays?? our customers should be able to direct bury the single conductors, even when the nominal system voltage is over 600 volts."

 

[GoldDigger]

To clarify a bit more, here is a portion of the response from a cable engineer:
... 690.31 states that ?All raceways and cable wiring methods included in this Code and other wiring systems and fittings specifically intended and identified for use on photovoltaic arrays shall be permitted? .... Nowhere in Article 690 are underground installations specifically mentioned.

Neither the language in 690 or 300.50 is perfectly clear. However, based on the language in 690.31, my position is that since our PV conductors are Listed for and ??specifically intended for use on photovoltaic arrays?? our customers should be able to direct bury the single conductors, even when the nominal system voltage is over 600 volts."

But does directly burying individual wires constitute a raceway or cable method?

 

[jaggedben]

Now that I get what the issue is, some thoughts...

1) I myself have never directly buried PV conductors or seen it done. Always THWN-2 inside PVC. Not that I've done any 1000V systems, but still.
2) It seems to me that single conductors do not constitute a 'cable assembly' subject to 300.50 (A)(1), no matter if they are listed for direct burial or not.

However, based on the language in 690.31, my position is that since our PV conductors are Listed for and ??specifically intended for use on photovoltaic arrays?? our customers should be able to direct bury the single conductors..."

Call me skeptical. Photovoltaic arrays are not buried, so 'intended for use on PV arrays' doesn't imply suitability for burial. Is he saying their conductors are listed both as PV wire and as some type subject to 300.50(A)(1)? I've certainly never heard of a product that resembles this.

 

[Andrew445]

I am in agreement with you. I contend that the intent of 300.50 is personnel protection, should someone come into contact with the cable. The grounded metal sheath would provide a return path. Direct buried cables that are not protected with a ground return path are not in compliance. This is why PVC in earth would not be acceptable as well.

 

[don_resqcapt19]

I don't agree with the cable rep. The code rules say that for Chapter 6 the rules in Chapters 1-4 apply unless they have been specifically modified by a rule in the Chapter 6 article. 90.3

 

[Kuru]

Having designed a number of systems with direct burial 1kV conductors I have referred to 310.10(F) which allows the use of identified unshielded direct-burial cable up to 2,000 V. It's an internal conflict in the NEC but my experience is that the interpretation in 310.10(F) is accepted over that in 300.50. This is also the interpretation used by many others building 1kV systems and so far so good.

 

[SolarPro]

+ 1. My understanding is that the more permissive statement in 310.10(F) is the more widely accepted interpretation.

Some of this internal confusion in the NEC relates to the fact that a fair amount of the content that supposedly applies generically to "systems over 600 volts" really applies specifically to medium-voltage systems, which are systems that operate over 2,001 volts and have very unique requirements for conductors, terminations and wiring methods. The conductors and wiring methods used for 1,000 volt PV systems are more closely related to those used in 600 volt PV systems than they are to those used in MV systems. There is an awkward gap in the Code between 601 volts and 2,000 volts. Note that Table 310.15(B)(16) and 310.104(A) & (B) and similar apply to conductors rated up to 2,000 volts, whereas there are different tables for systems over 2,001 volts, like 310.60(C)(70), 310.104(C) and so forth.

 

[Andrew445]

Thanks for the input guys! It looks like the best course of action is to get with the AHJ and iron it out before design gets too deep. I am all for using 1000V direct-buried cable if it is allowed.

 

[jaggedben]

I am in agreement with you. I contend that the intent of 300.50 is personnel protection, should someone come into contact with the cable. The grounded metal sheath would provide a return path. Direct buried cables that are not protected with a ground return path are not in compliance. This is why PVC in earth would not be acceptable as well.

In the case of a PV system, it's unlikely that the metal sheath would act as a return path, since the GFDI is usually in the inverter. Fire dangers could potentially be worse if it did act as such. Not that that's anything but trivia as far as what the code allows.

Whether or not 1000V direct burial is allowed, I still don't think you can use PV wire. You'll need to transition to something else that is identified for the purpose.

 

[TheElectrician]

By following the NEC 2014, 310.10(F) and 300.5, can we install a 2kV PV (PV and RHW-2) wire (Circuit voltage is 1000V) in a PVC/IMC/RMC raceway 24" below grade? Can it also be directly buried 24" below grade? Or is there any other code that I have t follow? Thanks

 

[electrofelon]

By following the NEC 2014, 310.10(F) and 300.5, can we install a 2kV PV (PV and RHW-2) wire (Circuit voltage is 1000V) in a PVC/IMC/RMC raceway 24" below grade? Can it also be directly buried 24" below grade? Or is there any other code that I have t follow? Thanks

There is nothing special you need to do for 1000 volts and under. The MV requirements are for OVER 1000 volts.