NEC Derating Q

[MFortie]

I'm rebuilding our vacation home outdoor kitchen and am adding some circuits. The original kitchen had two circuits, 12AWG THHN on a twin 20A breaker running through 3/4" PVC roughly 25' to feed 1) convenience outlets circuit and 2) a lighting circuit. I had an beverage cooler plugged into one of the outlets and didn't have any issues for about seven years.

My new design adds (2) more circuits -- dedicated outlet for the cooler and a dedicated outlet for an undersink 120V Bosch water heater (switched - vacation place that's not occupied all the time). I had planned on two 20A twins to feed the circuits. Still 12AWG (new), same conduit run.

I'm negotiating with an AZ licensed sparky for the work and am getting some pushback. First, he said 44 cu in. fill in a 46 cu in. box wouldn't work (4s, 3.5d, where all the conductors land and includes the H20 switch and an outlet). He also said the conduit fill was exceeded. Per the Southwire calculator, I came up with a >20% fill (9 conductors in 3/4" PVC). He's backed off on those after I sent supporting documentation. Still bitching about the crowded box though...

The biggest hurdle is he's claiming we need to use 15A twins instead of 20A due to derating (I have one 20A twin already existing). The location gets 120F ambient temps during the summer (Colorado River, CA/AZ). He's claiming it will violate the NEC.

My Q is whether the 20A twins would violate the NEC based on the above. I can live with changing out the existing 20A to 15A, but am mostly curious if he's correct.

And, since I work with an engineering firm, I had one of the EEs vet the design and he didn't see any issues.

Thanks in advance.

 

[Bert_]

It sounds like you would have 8 current carrying conductors?

That would mean you need to derate those wires to 70%. But if it's THHN then you get to start with the 90° rating.

12 thhn is good for 30 amps at 90°. 30 x 0.7 = 21
That means you can still put it on a 20 amp circuit.

I wouldn't derate for ambient temperature unless it's on a roof, or somewhere that the temperature is much hotter than outdoors. If the breaker is exposed to the same temperatures, then it's going to take less amps to trip it versus a lower temperature

 

[Dagny]

90 degree terminals?

 

[mm08822]

90 degree terminals?

Not needed and rarely (if ever ) found in residential grade equipment. 75C terminal rating will suffice with the derate from 90C insulation. Max OCP is limited to 60C values anyway.

 

[mm08822]

I'm rebuilding our vacation home outdoor kitchen and am adding some circuits. The original kitchen had two circuits, 12AWG THHN on a twin 20A breaker running through 3/4" PVC roughly 25' to feed 1) convenience outlets circuit and 2) a lighting circuit. I had an beverage cooler plugged into one of the outlets and didn't have any issues for about seven years.

My new design adds (2) more circuits -- dedicated outlet for the cooler and a dedicated outlet for an undersink 120V Bosch water heater (switched - vacation place that's not occupied all the time). I had planned on two 20A twins to feed the circuits. Still 12AWG (new), same conduit run.

I'm negotiating with an AZ licensed sparky for the work and am getting some pushback. First, he said 44 cu in. fill in a 46 cu in. box wouldn't work (4s, 3.5d, where all the conductors land and includes the H20 switch and an outlet). He also said the conduit fill was exceeded. Per the Southwire calculator, I came up with a >20% fill (9 conductors in 3/4" PVC). He's backed off on those after I sent supporting documentation. Still bitching about the crowded box though...

The biggest hurdle is he's claiming we need to use 15A twins instead of 20A due to derating (I have one 20A twin already existing). The location gets 120F ambient temps during the summer (Colorado River, CA/AZ). He's claiming it will violate the NEC.

My Q is whether the 20A twins would violate the NEC based on the above. I can live with changing out the existing 20A to 15A, but am mostly curious if he's correct.

And, since I work with an engineering firm, I had one of the EEs vet the design and he didn't see any issues.

Thanks in advance.

Bert covered you on the OCP values when using THHN with 8 ccc's.

3/4" pvc sch 40 is good for 15 #12 thhn.

A box volume can't be exceeded with equivalent wire fill. The box volume total could include mud rings or extensions if applicable.
#12 conductors each require 2.25 cu in.

8 ccc's + 1 grd = 9 12's
h20 switch and recept = 4 12's
There's at least 1 12 off of the switch.

Do the two other circuits pass directly through the box (without splices or are they wire-nutted to continue elsewhere? This answer will determine the next equivalent wire count to add to the 14 count.

Not sure which box you are using, description confusing, but per your info: 46 cu in / 2.25 cu in/12 wire = 20 12's max

 

[MFortie]

8 current carrying conductors & ground. Ambient temp derating per a table I found (forget the #) was .82 for up to 122F.

The device box (Orbit UMAB) has two devices - a switch & a duplex recep. All conductors are wire nutted to continue. THHN in, NM out.

Don’t have the devices on hand (all Leviton), but I doubt they’re 90C.

The electrician tossed out 315(b).(1).(a) as his source for derating; I quote “Table 315(b).(1).(a) if we weren’t in the 90 degree column we wouldn’t have to derate past 21a” end quote.

I don’t know the table though.

I’m open to adding another 4s box for the switch / recep to free up space in the j-box. Also willing to accept the 15A breakers - I can change them later myself if I choose. I’ve done some electrical work (permitted & inspected), so I’m not totally clueless.

 

[mm08822]

8 current carrying conductors & ground. Ambient temp derating per a table I found (forget the #) was .82 for up to 122F.

The device box (Orbit UMAB) has two devices - a switch & a duplex recep. All conductors are wire nutted to continue. THHN in, NM out.

Don’t have the devices on hand (all Leviton), but I doubt they’re 90C.

The electrician tossed out 315(b).(1).(a) as his source for derating; I quote “Table 315(b).(1).(a) if we weren’t in the 90 degree column we wouldn’t have to derate past 21a” end quote.

I don’t know the table though.

I’m open to adding another 4s box for the switch / recep to free up space in the j-box. Also willing to accept the 15A breakers - I can change them later myself if I choose. I’ve done some electrical work (permitted & inspected), so I’m not totally clueless.

With this additional detail, that single box would have the equiv of 21 #12's since the nm-b splices in the box. Sure add another box to reduce fill. Conduit could have an accessible "T" added to accomplish the split.

If the loads for the 2 new ckts are dedicated and below the (30A x .82 x .70 = 17.22A) derated value then the cb for those 2 ckts could be 20A.
Applying both derating factors to the existing circuits that have variable loads (recepts) should not exceed ocp value of 17.22A, meaning use 15's. This is to the letter of the code, but even the fine print notes within NEC admit there is no load diversity consideration. It's kind of like a double-dip.

Your devices do not have to be 90C rated terminations unless the conductor OCP device value was set for the 90C ampacity. See 110.14.C.

 

[MFortie]

Thanks all!

One correction - the H20 dedicated circuit conductor would land on the switch with a switch leg going out. The N would still be wire nutted, so I presume the count would drop to 20 12s?

And moving the devices (yokes) to another box would take out another 4 12s, correct?

Since this on a half height wall with the box above a countertop I can easily add a box next (not directly next to it) to the j-box for the devices.

And I’ll stick with the twin 15a cbs. Except for the bev cooler & lights (LED), very little power usage. Maybe a fan or charging a wireless speaker. Oh, and the TV to watch the Padres take the series this year!

 

[mm08822]

Count would be 8+1 in, 8 out plus consideration for each yoke remaining.

There wasnt enough detail at the start of thread to be certain what exited the box for conductor count.
Exiting conductors could have been simply pass-throughs, each counting as 1 already considered. Adding the splices causes the output wires to be added into count.

 

[theoldwizard1]

And I’ll stick with the twin 15a cbs.

If your paranoid, use #12 wire with the 15a cbs.

 

[ripperd]

And I’ll stick with the twin 15a cbs. Except for the bev cooler & lights (LED), very little power usage. Maybe a fan or charging a wireless speaker. Oh, and the TV to watch the Padres take the series this year!

Wait till all the ladies bring over their crockpots and air fryers. Might trip those 15a's after all lol.

 

[MFortie]

We don't do air fryers and crockpots stay in the house... The margarita blender however...

Not paranoid, just trying to get the work done. Where we are it's tough to find contractors to do any work. I've already be ghosted by a couple of sparkies. One started the job and totally flaked out and disappeared.

Really just need to get the conduit bent for the lighting -- I have a bender, but it takes me three sticks to get one bend right! LOL! I could do it all, but am coming off two knee replacements and a foot/ankle rebuild and am still gimping.