Conductor ampacity dependent on space conditioning 1

[ViaVerde]

We have process skids and server areas that are in small, pre-fab buildings with simple HVAC.
The vendors are sizing conductors and servers based on the HVAC operating correctly. If HVAC fails then conductors will be undersized (overheat), servers and UPS may trip off, etc.

It's standard practice to have high temp alarms at these enclosures, but does community know of code that actually requires this high temp interlock?

 

[che12345]

I have the following opinion for your consideration
1. It depends on a) the ambient temperature and b) the conductor insulation (PVC-70[sup]o[/sup]C, XLPE-90[sup]o[/sup]C...).
2. In your case, the conductor amperecity should be based on the ambient temperature and the conductor insulation class. Reference BS7671. Reason: the conductor should NOT be over-heated and the system should operate normally, in the event the HVAC fails or without. The conductor operates cooler with HVAC ...no issue.
3. If you wish to have alarm interlock, I suggest to set it < the conductor insulation temperature class. Reason: the operator is alerted when the temperature inside the pre-fab building is approaching to the conductor insulation temperature class.
Che Kuan Yau (Singapore)

 

[ViaVerde]

CHE12345 -
Completely agree, and it seems obvious.
We just need to find the North American standard that states the same requirement.

Thank you!

 

[che12345]

@ Mr. ViaVerde
1. Sorry, I failed to locate any mention of this situation nor any recommendation in NEC or IEEE standards.
2. Info: Only IEC standard is adopted in Singapore.
BTW: Although Brexit from the EU, but IEC prevail in UK nevertheless.
Che Kuan Yau (Singapore)

 

[FacEngrPE]

The vendors are sizing conductors and servers based on the HVAC operating correctly. If HVAC fails then conductors will be undersized (overheat), servers and UPS may trip off, etc.

Recommend you get your vendor to explain how he justifies his approach to conductor sizing. Prefab buildings are subject to the building code in most locations (might be the manufactured building code), does the AHJ have an opinion?
If the equipment being fed trips before the wiring exceeds it's maximum design operating temperature, then the selection might be justified in the short term. What about after several rounds of equipment change outs (electronic equipment of the sort described has a relatively short lifespan compared with a structure).

 

[ViaVerde]

Thank you team.. I think we have this one closed out.
The is an enclosed water purification system. The 'building' is a lightweight steel enclosure (very little insulation, if any). Non-occupied space.
Site is a petroleum facility in Texas, so not many AHJs around.. but we do have peer-review and surveys from other engineering firms hired by investors.

We are enforcing a high-temp detection and interlock on the control system, for our own system reliability reasons. We should get remote warnings on temp trip points, and the process within the enclosure will be disabled on a simple high-temp switch. This will stop current flow and keep us from exceeding NEC wire ratings.

We never worried about this in standard occupied buildings (and maybe why NFPA70 doesn't discuss it directly). It takes allot for all HVAC in a commercial or occupied space to completely fail without it being a utility power outage. HVAC techs are on site for comfort reasons long before the conductors reach dangerous levels