Cable derating when covered by a fire blanket.

[kenaslan]

Has anyone know of any requirements to derate wires in RGC if the conduit is covered with a fire blanket (FULLY ENCLOSED WITH THREE 3/8 IN. LAYERS OF AN ASPEN AEROGELS PYROGEL XTF BLANKE).

This is the power for the XYZ servos on the monitor alky water wall system. My guess is because they are "fully enclosed" there must be some sort of derating.

 

[infinity]

How many CCC's?

 

[kenaslan]

6.

 

[don_resqcapt19]

I am not aware of any ampacity correction requirements for raceways in contact with thermal insulation, like there are for cables, so I doubt there is anything that requires an ampacity correction for a raceway with fire protection covering. Of course the standard ampacity adjustments for more than 3 current carrying conductors in the raceway will apply.

 

[infinity]

I agree with Don, you would adjust the ampacity due to the 6 CCC's in the raceway not the fact that it's within insulation.

 

[kenaslan]

Ampacity adjusted for both VD and 6 CCC's. 8A@208V 1PH. Worst case 340' 3-3/C #8 XHHW-2 TC-ER in 2" GRC. I need to look at IEEE 835
I adjusted for motor load as well. (better to be safe than sorry) It is a 3 axis DC servo motor. The AC feeder is to the MCU (Motor Control Unit). I also wonder if these water monitors are also used for fire suppression (dual purpose) as well as to knock down an Alky acid cloud if I need to consider Art. 695 as well. Granted this is not a "pump" but the remote controlled aiming nozzle.

 

[winnie]

Ampacity adjusted for both VD and 6 CCC's. 8A@208V 1PH. Worst case 340' 3-3/C #8 XHHW-2 TC-ER in 2" GRC. I need to look at IEEE 835
I adjusted for motor load as well. (better to be safe than sorry) It is a 3 axis DC servo motor.

These are small enough conductors (in an expensive insulation system and expensive conduit install) that I wouldn't stress too much about oversizing by a size or 2. In other words, using #8 when #10 would do simply doesn't make enough difference in cost to stress about it..

However you do not need to adjust for both VD and CCC; do the math _separately_ for each and use the one that gives you the larger conductor size.

- Jon

 

[kenaslan]

I understand what you are saying, however in hazardous chemical and refinery environments, I must cross my "T"s and dot my "I"s and have backup documentation and calculations for everything. Unfortunately, in todays litigious environment, everything is double checked, documented and recorded. I try to leave nothing to chance.

 

[kenaslan]

These are small enough conductors (in an expensive insulation system and expensive conduit install) that I wouldn't stress too much about oversizing by a size or 2. In other words, using #8 when #10 would do simply doesn't make enough difference in cost to stress about it..

However you do not need to adjust for both VD and CCC; do the math _separately_ for each and use the one that gives you the larger conductor size.

- Jon

See the following Mike Holt Forum Response . Also IEEE 848 Standard Procedure for the Determination of the Ampacity Derating Factor for Fire-Protected Cable Systems. Our SME gave me an additional 20% derating to use.

Applying voltage drop and concuctor derating.

Let say you have 8 current caring conductors in a conduit on a roof 500 feet away. I would do my voltage drop calc first to come up with a conductor size. Then based on the load I would do my conductor derating to see if the upsized conductors due to voltage drop will be sufficient. Is this...

forums.mikeholt.com

 

[winnie]

I understand what you are saying, however in hazardous chemical and refinery environments, I must cross my "T"s and dot my "I"s and have backup documentation and calculations for everything. Unfortunately, in todays litigious environment, everything is double checked, documented and recorded. I try to leave nothing to chance.

I hear you. As others have noted, the situation of conduit buried in fire blanket is not covered explicitly by the NEC, but clearly it must have an effect. Ampacity is set in part by the ability of the conductors to shed heat.

IMHO derating for the fire blanket is a 'design decision' as far as the NEC is concerned, but you as the designer have a hunch that it will be an issue, and have access to a standard for designing to accommodate that issue. I'd give that two thumbs up.

I was just noting that if the NEC said '#10 is good to go' and you said 'I am going with my gut and using a #8' that the cost of making the accurate decision between the two is probably greater than the cost difference of simply using the #8.

My other point was that you don't derate for and then do voltage drop _on top of that_. You do the two calculations, and pick the conductor size based on which comes out 'worse'. The old thread that you linked steps through this pretty clearly, and I specifically like https://forums.mikeholt.com/threads...p-and-concuctor-derating.118366/#post-1843051

-Jon