Electroendosmosis

[jdlovins]

I am the installation manager for a renewable energy company that specializes in photovoltaic systems. I have a technical question about a condition that may arise in some of our projects. I have not been able to get a clear answer on this topic from wire manufacturers, internet searches, electrical engineers, or a member of an NEC code making panel. Article 310.13 of the 2008 NEC, or Article 310.104 from 2011, has a fine print note that references a condition called electroendosmosis. I understand that FPNs are not enforcable parts of the Code but we do not want to do an installation that may result in wire failure. From my understanding, this condition may occur when DC conductors with thermoplastic insulation are installed in wet locations. From the research that we have done by looking at other systems, it seems that THWN-2 rated wire is used in outdoor installations in conduit for photovoltaic systems. This rating gives a 90 degree rating in a wet location. Again, this seems to be an industry standard but it opens up the possibility for electroendosmosis to occur and damage the insulation of the wire. The solution would be to install XHHW-2 but this increases costs as this wire is more expensive. So, my question is if this condition ever occurs in the real world or if this is just a situation that can be created in a laboratory setting. Two leading wire manufacturers have told us that it is not an issue and most of the other methods that we have explored have never even heard of this. Online searches do not reveal too much useful information either. Thank you for taking the time to read this and I look forward to any feedback.

David

 

[rbalex]

Technically, electro-endosmosis (EED) can occur in an ac circuit as well, but the effect is much more pronounced with dc. The ultimate effect is that the “plasticizers” may be leached out of thermoplastic type insulation which leaves tiny “voids” in the insulation and there may be a modest ingress of water. Loss of plasticizers can also occur when there are extreme ambient temperature swings.

Multiconductor cables are rarely affected by dc alone. Oddly enough, if the installation is fundamentally stable, the water ingress will have virtually no effect on the overall installation integrity.

The plasticizers have very little effect on the electrical properties and after installation they play little role in mechanical properties of the insulation as well.

One caution: there is very little “safety margin” in the electrical properties for conductors with only 15 mil insulation. The second sentence in the FPN (pressure deformation) can be a problem. My usual recommendation is to avoid No. 12 and 14, THHN/THWN-2 in the application you described; otherwise, you should be fine.

 

[K8MHZ]

Technically, electro-endosmosis (EED) can occur in an ac circuit as well, but the effect is much more pronounced with dc. The ultimate effect is that the ?plasticizers? may be leached out of thermoplastic type insulation which leaves tiny ?voids? in the insulation and there may be a modest ingress of water. Loss of plasticizers can also occur when there are extreme ambient temperature swings.

Multiconductor cables are rarely affected by dc alone. Oddly enough, if the installation is fundamentally stable, the water ingress will have virtually no effect on the overall installation integrity.

The plasticizers have very little effect on the electrical properties and after installation they play little role in mechanical properties of the insulation as well.

One caution: there is very little ?safety margin? in the electrical properties for conductors with only 15 mil insulation. The second sentence in the FPN (pressure deformation) can be a problem. My usual recommendation is to avoid No. 12 and 14, THHN/THWN-2 in the application you described; otherwise, you should be fine.

This would probably explain the difference between the insulation used on vehicles and what we use.

I have seen THHN used on vehicles that, after a period of years, the insulation became brittle, discolored and started to crack. I never attributed the condition to DC vs. AC. I always thought it was environmental.