derating conductors and one more question

[Ivan Young]

Gentlemen, I have a question about a situation on the job which the journeyman simply did not explain clearly to me, so just for my information I would like to get some help.
We are running three different 120v 20amp circuits in the same conduit, each circuit has its own neutral and all three share one ground. Each circuit feeds five 20 amp receptacles in one large room. The first room is 100ft from the panel, the second is 110ft, and the third is 120ft. Based on distance alone we are using #8 THHN wires.
My question is, should the neutrals be counted as cc wire? Just so I would know in the future, what if we had to adjust for temperature and say 10 cc wires in the conduit so now our 20 amp circuit in only good for say 10 amps, what needs to be done to keep the circuit at 16 amps, increase the size of the wires?
My other question has to do with two outdoor flood lights controlled by two 3 way switches. These are high-end lights so the home owner wants to add motion sensors to these lights or if this is not possible he would buy lights with the sensors attached to the lights. Each light has two bulbs and is at opposite ends of the house. The problem is he also wants to be able to control these lights with the 3 way switches as well as by the sensors. Can this be done, and how?
Any help would be greatly appreciated.
Thanks,
Ivan

 

[ActionDave]

Gentlemen, I have a question about a situation on the job which the journeyman simply did not explain clearly to me, so just for my information I would like to get some help.
We are running three different 120v 20amp circuits in the same conduit, each circuit has its own neutral and all three share one ground. Each circuit feeds five 20 amp receptacles in one large room. The first room is 100ft from the panel, the second is 110ft, and the third is 120ft. Based on distance alone we are using #8 THHN wires.

Seems a bit overkill to me. 12AWG likely would have been ok, 10AWG certainly. That's another discussion though.

My question is, should the neutrals be counted as cc wire?

Yes. You do not count the neutral when it carries the imbalance. There is no way for the neutral of a two wire circuit to carry any imbalance, it has to carry the same current as the hot in your case.

Just so I would know in the future, what if we had to adjust for temperature and say 10 cc wires in the conduit so now our 20 amp circuit in only good for say 10 amps, what needs to be done to keep the circuit at 16 amps, increase the size of the wires?

For 15 and 20A circuits, general rule is once you go more than nine CCC in a pipe you have to upsize the wire.

Don't forget you can use the ninety degree scale for derating.

My other question has to do with two outdoor flood lights controlled by two 3 way switches. These are high-end lights so the home owner wants to add motion sensors to these lights or if this is not possible he would buy lights with the sensors attached to the lights. Each light has two bulbs and is at opposite ends of the house. The problem is he also wants to be able to control these lights with the 3 way switches as well as by the sensors. Can this be done, and how?
Any help would be greatly appreciated.
Thanks,
Ivan

Yes, it is possible. Need some more info. If you want to leave the lights energized and let the motion sensors take over, that's easy. If you want to keep the motion sensors active no matter the position of the three ways, that's a bit more involved.

 

[GoldDigger]

Many motion detector lights allow you to turn them on (override the motion detector) by simply toggling them rapidly off then on again.
Again usually, this will leave the light on after the motion detector timeout (but not always).
And turning the light off for a few seconds then back on will restore normal motion-only actuation.

I do not see this behavior being any different if the on-off control comes from a three way control circuit rather than just a single switch.

But without the cooperation of the logic of the motion detector-lamp fixture you will not be able to both override on and override off from any switches.

 

[Dennis Alwon]

I am confused-- you have three circuits with each having it's own neutral and then an equipment grounding conductor. I count that as 6 current carrying conductor's. Based on that you would generally derate from the 90C ampacity column at 70%. For instance a #8 is rated 55 amps. 55 amps at 70% is 38.5 amps. I don't see an issue.

BTW the equipment grounding conductor needs to be upsized to a #8 based on 250.122(B)

I also agree that 8 is overkill and 10 or even #12 is sufficient.

 

[kingpb]

We are running three different 120v 20amp circuits in the same conduit, each circuit has its own neutral and all three share one ground. Each circuit feeds five 20 amp receptacles in one large room. The first room is 100ft from the panel, the second is 110ft, and the third is 120ft. Based on distance alone we are using #8 THHN wires.
My question is, should the neutrals be counted as cc wire? Just so I would know in the future, what if we had to adjust for temperature and say 10 cc wires in the conduit so now our 20 amp circuit in only good for say 10 amps, what needs to be done to keep the circuit at 16 amps, increase the size of the wires?

Even if you derate 50% due to 10 CCC, a #10AWG is good for 40A at 90 deg C, and therefore 20A after derate. A #10 AWG is also going to keep you at just over 3% VD (fully loaded at 16A so you should be fine) #8 will work but overkill.

My other question has to do with two outdoor flood lights controlled by two 3 way switches. These are high-end lights so the home owner wants to add motion sensors to these lights or if this is not possible he would buy lights with the sensors attached to the lights. Each light has two bulbs and is at opposite ends of the house. The problem is he also wants to be able to control these lights with the 3 way switches as well as by the sensors.

All you have to do is make sure the sensor and switches are wired in parallel, so either will make the lights come on. Doesn't matter where they are located, just may take more wire(s) or conduit.

 

[Ivan Young]

Thanks for the education guys. The journey man sized his conductors based on the distance according to him but he did not count the neutrals and did not upsize the grounding conductor. Well he runs the show. Thanks again guys.

 

[Ivan Young]

Gentlemen, I have a follow-up question. I asked my jm today if he could explain to me if you have some receptacles on a 20 amp circuit and because of distance, temperature, and the number of cc wires your derating calculations indicate that you need a bigger than #10 wire and the only wire you have is THHN. The code says you can use the 90 degree column for derating purposes only but must use the ampacity of the 60 or 70 degree ampacity. He told me he has no clue what that means and how to go about accomplish that. Can you guys explain it to me please.
In regards to the two outdoor motion sensor floodlights, each light carries two bulbs and are controlled by two 3 way switches. Someone mentioned that if I wanted to use these switches to over-ride the sensors that I would have to wire them up in parallel. Please tell me how.
Thanks in advance.

 

[Smart $]

Gentlemen, I have a follow-up question. I asked my jm today if he could explain to me if you have some receptacles on a 20 amp circuit and because of distance, temperature, and the number of cc wires your derating calculations indicate that you need a bigger than #10 wire and the only wire you have is THHN. The code says you can use the 90 degree column for derating purposes only but must use the ampacity of the 60 or 70 degree ampacity. He told me he has no clue what that means and how to go about accomplish that. Can you guys explain it to me please.
In regards to the two outdoor motion sensor floodlights, each light carries two bulbs and are controlled by two 3 way switches. Someone mentioned that if I wanted to use these switches to over-ride the sensors that I would have to wire them up in parallel. Please tell me how.
Thanks in advance.

Derating is ampacity "adjustment" for number of current carrying conductors and "correction" for ambient temperature. The adjustment and correction are implemented by factors, i.e. multiplying the value for the conductor type (e.g. 90°C-rated THHN) given in the allowable ampacity table by the prescribed percentage and/or a number, respectively. The result must equal or exceed the calculated load on that circuit and also be considered protected by the OCPD per 240.4.

The 60° or 75° rating comes into play for compliance with terminal temperature limitations covered by 110.14(C). What this means is the maximum circuit ampacity is limited to the 60° or 75° allowable ampacity value in Table 310.15(B)(16) for the size wire you are using... and it must equal or exceed the noncontinuous load plus 125% of the continuous load.

The wire type and size used must meet both the preceding requirements at a minimum.

 

[Smart $]

....
In regards to the two outdoor motion sensor floodlights, each light carries two bulbs and are controlled by two 3 way switches. Someone mentioned that if I wanted to use these switches to over-ride the sensors that I would have to wire them up in parallel. Please tell me how.
Thanks in advance.

If you want independent motion turn on for each "head" you will need motion detectors with an override function. Check the literature very carefully to make sure the override function performs like you (or the owner) wants.

If parallel operation of both heads on motion is okay, simple motion detectors are all that are needed, but you'll have to make sure they are rated for all four lamps being powered. To wire, you run full time hots to each sensor and one 3-way, travelers to the second 3-way, and a switch leg to both heads and wire in the motion sensor switch legs at each head. Make sure your neutral routing is compliant.

 

[degupita]

Gentlemen, I have a follow-up question. I asked my jm today if he could explain to me if you have some receptacles on a 20 amp circuit and because of distance, temperature, and the number of cc wires your derating calculations indicate that you need a bigger than #10 wire and the only wire you have is THHN. The code says you can use the 90 degree column for derating purposes only but must use the ampacity of the 60 or 70 degree ampacity. He told me he has no clue what that means and how to go about accomplish that. Can you guys explain it to me please.
In regards to the two outdoor motion sensor floodlights, each light carries two bulbs and are controlled by two 3 way switches. Someone mentioned that if I wanted to use these switches to over-ride the sensors that I would have to wire them up in parallel. Please tell me how.
Thanks in advance.

All that needs to be done is the switches are closer to the source then the sensors. If the sensors are by the lights, or attached to the lights, as they usually are. Then you don't have to do anything extra. You can shut off the lights by the switches, or leave the switch on and have the sensor do it. Doing parallel circuits, I am not sure how that would be practicable.

 

[Ivan Young]

Thanks gentlemen fall all the help. I have a better understanding now.
Thanks again.