Derating - NEC/NYC

[TG3]

I see a lot of installations like such - 12-#10 Copper THHN CCC in 1” Conduit for 120V 20A Branch Circuits. For this purpose let’s say the loads on these 120V circuits are for general purpose receptacles and FCUs with a 20A max nameplate OCPD rating and a RLA of 10A lets say )

From what I gather, a lot of guys derate the #10 THHN from the 90Degree column @ 50% for the 10-20CCC and that equates to a final ampacity rating of 20A - and that math works at that point.

However, and please correct me if my understanding is wrong. But you must derate @ the weakest link amongst the circuit - which comes to be the termination points. Most terminals on 20A Breaker, 20A receptacle, the terminals inside the 20A FCU are rated at 75degrees. Those 12-#10 THHN derated @ 50% in the 75degree column is roughly 17.5A now.

I may be wrong but I think - going back to those specific loads I stated above - because the FCU is a fixed-load, the above installation would be NEC/NYC compliant. But the general purpose receptacle circuits would not be. The general purpose circuits/conductors feeding that circuit, must be able to handle the full 20A load. Therefore whichever of those 12-CCC are feeding the general purpose receptacles must be placed on a 15A breaker to become compliant.

 

[suemarkp]

The terminal doesnt get a 50% hit because there is no bundling at the terminal. So yes you use the weakest link, but derating from bundling is separate from termination limits.

 

[Master Nater]

you are derating the conductor, which is 90°C rated. that brings it down to 20a. the 75°C terminations determine that the circuit ampactiy will not end above the 75°C rating. which is 35a. so, realizing that the CONDUCTOR was derated to 20a. the terminations have a limit of 35a. your circuit ampacity is the least of the two. 20a.

 

[infinity]

Most terminals on 20A Breaker, 20A receptacle, the terminals inside the 20A FCU are rated at 75degrees. Those 12-#10 THHN derated @ 50% in the 75degree column is roughly 17.5A now.

The 90° C, #10 conductor after derating is a 20 amp conductor. The terminal value doesn't change that.

 

[darekelec]

90 deg column.
In the new code you have to zip tie every circuit together in EVERY BOX now (with some exceptions) . 200.4 and 210.4 D

 

[infinity]

90 deg column.
In the new code you have to zip tie every circuit together in EVERY BOX now (with some exceptions) . 200.4 and 210.4 D

Zip ties aren't the only method. You can use in number markers too in lieu of zip ties or tape.

 

[garbo]

An electrical contractor got mad when I asked him if they make 120 volt bolt on circuit beakers in 5 or 7 amps because he used several existing 2" conduits from panels to a 12 by 12" 10' long existing ceiling trough after he pulled over 50 #10 THHW/THWN wires. Conduit was 12' long. It was for patient rooms in a large hospital and each room usually went overboard with 5 20 amp normal power receptacle circuits and at least three emergency npower 20 amp circuits. Months later I used my FLIR camera to check temperature of conduits. Was always within one degree of climate controlled electrical room.

 

[darekelec]

Nowadays we call it ‘Applying adjustment factors’
using the word ‘Derating’ is so 90’s

 

[electrofelon]

An electrical contractor got mad when I asked him if they make 120 volt bolt on circuit beakers in 5 or 7 amps because he used several existing 2" conduits from panels to a 12 by 12" 10' long existing ceiling trough after he pulled over 50 #10 THHW/THWN wires. Conduit was 12' long. It was for patient rooms in a large hospital and each room usually went overboard with 5 20 amp normal power receptacle circuits and at least three emergency npower 20 amp circuits. Months later I used my FLIR camera to check temperature of conduits. Was always within one degree of climate controlled electrical room.

I once did a grow house and had 3/4 emts with 9 #12 CCC running at 16 amps. The conduits were almost too hot to touch, but whatever the temp was was probably well below 194 degrees Fahrenheit.