2 hr. Conduit

I

ISpud

Member
Location
Prudhoe Bay, AK
Occupation
Electrical QC Inspector
We are required to have the fire pump service entrance conductors protected, that may be prone to damage outside a facility, (according to NEC 2017 which the client is governing the construction). The code, Article 692.6 requires "...conductors shall be protected from fire for 2 hrs...". As we won't be using concrete, what conduit is rated for 2 hrs. of protection that can be partially buried (18"-24"), and enter the bottom (3'-4' AGL) of a Water/Waste Water (W/WW) facility?
We'll run 3#6+#6GC, MC-HL, 600V Insul Rating, 480V/ 60A from one facility motor control center to the W/WW facility partially in cable tray, buried for a few feet, then from direct burial up to the bottom of the W/WW facility (3'-4' AGL). Can we use RMC that will certainly hold up better than the 20 sec EMT mentioned in a previous thread?
Lastly, even though the RMC or 2 hr rated conduit might not melt as fast as EMT, would the conductor insulation not withstand the high temperature and eventually short out?
 
I’m pretty sure you can’t run it from the motor control center. It must be fed either directly from the poco transformer, or tapped ahead of the main in an enclosure that does not contain an ocp, such as a pull section, then from there, it either has to be outside the building, or protected by a two hour firewall.
 
Thank you for the reply. The transformer for the feed happens to be in an MCC module. The cable route is not my concern. The concern is the route from underground, about 3'-4' into the building. That's the section that needs the 2 hr. rated conduit protection.
 
ISpud said:
The concern is the route from underground, about 3'-4' into the building. That's the section that needs the 2 hr. rated conduit protection.
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There is no 2 hour rated conduit. RMC can be encased in concrete to give it a 2 hour rating or it can be protected with a 2 hour rated enclosure built from core board and sheetrock. If you want something with a 2 hour rating then Vitalink MC cable can be used because it’s listed as 2 hour rated.
 
ISpud said:
Lastly, even though the RMC or 2 hr rated conduit might not melt as fast as EMT, would the conductor insulation not withstand the high temperature and eventually short out?
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This is essentially the crux of the issue. Conduit is noncombustible so it does not have a rating. Installing conductors in conduit using typical article 300 methods does not provide a fire rating. Unless otherwise listed, the conductor insulation will not withstand the temperatures of a fire.

To get a 2-hour fire rating for conductors, you need to either encase in 2" of concrete or provide a listed fire rated cable or circuit protective system (as noted in the NEC). To my knowledge, there are a couple options for a 2-hour rated cable/system: Vitalink MC, DuraLife RHH in EMT, or MI cable.

I am assuming you have a feeder and not a service as 2-hour ratings don't apply to service conductors. The requirements for a 2-hour rating only apply where routed inside of a building. If you are going underground, you do not have to worry about the 2-hour rating because you are not inside of the building.
 
Based on changes that appear in the first draft report, the 2" of concrete is being replaced with 5" of concrete and 2" of concrete does not prevent heat damage to the conductors for 2 hours.
 
infinity said:
Any idea who tested this? My guess the 2 hour cable manufacturers.
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It was news to me that the draft is increasing the thickness of concrete encasement for fire rating so I looked it up. There is a report published by NFPA that provides some background information on this change. Per a quick skim, it seems like 5" of encasement is the minimum requirement per multiple studies to reach a 2-hour rating without specifying the concrete mix design.

https://www.nfpa.org/education-and-...istance-of-concrete-for-electrical-conductors
 
Knightryder12 said:
I have never installed it, but I have seen in after the fact, and it did not appear to be to bad to install. Looked sorta like the wrap you put on headers in a car. EC said it was not that bad.
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A gc I do work for called me a couple months ago on a remodel job on a Costco, inspector caught an existing feed to a fire pump that was running open through the store, I told them about the wrap, and that’s what they did, it was that, or re-route the conduit outside for a couple hundred feet!
 
Pinnie said:
Do you know the most cost effective option of wrap. And how about boxes? Do they need to be 2hr rated as well?
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I do not know what the most cost effective option of wrap is, but I have heard it is expensive. But when you get in to an existing condition, it can be a huge savings. To be honest, I have never come across a run that has had a box it, so I do not know the answer to that question.
 
Pinnie said:
Do you know the most cost effective option of wrap. And how about boxes? Do they need to be 2hr rated as well?
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We used the wrap on a job. It is required to be installed by a professional insulation company who are trained and certified to install it. For a 2-hour rating it was about 6" around the cable. Also the wrap is very heavy so we had to go back and provide additional supports. If I remember correctly for a few hundred feet of installed wrap it was about $50k. I would avoid this like the plague.
 
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