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.

 

[kwired]

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

Code has always called it ampacity adjustments, at least for the past 40- 60 years anyway. "Derating" is basically the effect you have on the conductor ampacity though. For most part you don't end up needing to increase conductor size for bundling type issues until you reach that 50% adjustment factor.

With temperature related adjustments you can have instances where you increase conductor ampacity instead of lowering it.

 

[garbo]

Code has always called it ampacity adjustments, at least for the past 40- 60 years anyway. "Derating" is basically the effect you have on the conductor ampacity though. For most part you don't end up needing to increase conductor size for bundling type issues until you reach that 50% adjustment factor.

With temperature related adjustments you can have instances where you increase conductor ampacity instead of lowering it.

Bet 50 years from now old timers will be still calling the grounded conductor a neutral and a luminare a light fixture. Since the early 1970's have been calling it derating.

 

[TG3]

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.

no bundling at the terminal… thank you for that

 

[TG3]

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.

TY

 

[TG3]

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

TY

 

[TG3]

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

TY

 

[WD40]

An electrical contractor got mad when I asked him if they make 120 volt bolt on circuit beakers in 5 or 7 amps

I ran into a situation recently where 10 Amp is now all of a sudden a 'standard NEC size' breaker for dedicated equipment circuits, we used to get away with a 15 now its a 10. You cant use if for residential (I dont think)

 

[garbo]

I ran into a situation recently where 10 Amp is now all of a sudden a 'standard NEC size' breaker for dedicated equipment circuits, we used to get away with a 15 now its a 10. You cant use if for residential (I dont think)

Read that 16 Guage copper NMB will be allowed in houses to feed LED luminares on 10 amp circuit breakers. I can see DIY Johnny homeowner purchasing 16 guage NMB cable at big box stores and using to feed receptacles. 50 years ago thought they should have made 3 amp single pole circuit breakers to replace the little red one pole safety switch that had a 3 amp fuse to fed Manuel fire pull stations that feed 120 volt fire bells in triplex apartments.

 

[kwired]

I ran into a situation recently where 10 Amp is now all of a sudden a 'standard NEC size' breaker for dedicated equipment circuits, we used to get away with a 15 now its a 10. You cant use if for residential (I dont think)

What Garbo said. Residential lighting I believe was where this was mostly intended for I think.

I tend to run lighting circuits like I have for many years, although I'd bet I could put all the lighting in average house on one 15 amp circuit and never get a call back. I usually end up with at least 3 or 4 like i always have had. I know one the last homes I did I made two home runs to basement lights, but tied together and put on single breaker. Can be separated someday if needed I guess without running any new cables.

 

[Dsg319]

310.14(A)(2)

This is probably often forgot about