Meet code? Designing 120v receptacle circuits?must you consider harmonics?

Status
Not open for further replies.
C

collier

Member
Can you install 6 circuits (9 wires) of #12 thhn in one conduit under the following conditions:

I understand 240.2(B)(1) basically says a 20 amp circuit for receptacles must have a derated ampacity of
at least 20 amps?.So can I choose to use the 80% adjustment factors and meet code or the 70% and not
meet code

Given:
? 20 amp single pole breakers. The Engineers does not want to use 15 amp breakers (not concerned with handle tie code issue for now)
? 6 circuits in one conduit ? phase A,B,C, and N, Phase A,B,C and N, 1 equipment ground)
? Assume voltage at panel is 120/208 3 phase 4 wire
? Assume ambient temperature is 110 degrees F
? Assume there are four 120v duplex receptacles on each circuit.
? Application is high school classroom (not a computer lab)

Which do you do?:
? Do you derate for 4-6 conductors @ 80% (which will make the installation meet code) or
? for 7-9 conductors @ 70% because non-linear loads could theoretically be present at some point in the future. (which will not make the installation meet code

Step 1--------#12 thhn. 90 degree column = 30 amps x .87 for ambient temperature = 26.1 amp

Step 2--------26.1 amp x 80% (4-6 conductors ? neutral only carrying imbalance) = 20.88 amps MEETS
CODE

or

26.1 amp x 70% (7-9 conductors ? neutral carrying more than unbalance because nonlinear/harmonics
present) = 18.27 amps. DOES NOT MEET CODE
 
Pull a #10 neutral,the cost of running a #10 instead of a #12 is nothing, and run a ground wire. Or are you trying to squeeze this into a 1/2" conduit ?
 
Unless the specs say differently, we usually homerun 9 #12 in one 3/4" pipe. My concern is....is this application considered 4-6 current carrying conductors at 80% or 7-9 at 70% (non linear load). Is it just a judgement call on what could be plugged into these general purpose receptacles in the future.

If the answer is 70% (30 amps x .7 = 21 amps, then 21 amp x .87 for 110 degree ambient, you now have a derated ampacity of less than 20 amps, which does not meet code for receptacle circuits on a 20 amp breaker.

I don't see what pulling a #10 neutral would do, because the #12 hot conductors would still have an ampacity less than 20

the Engineers always show 20 amp breakers on their drawings, so switching to 15 amp breakers is not an option
 
I'd call it 6 conductors. I don't think you have to worry about what someone might plug in these, unless they are specifically for loads that will be more than 50% harmonics.

Anyway, if you are using conductors rated for 90 deg. C, the ampacity of #12 is 30 amps. That is the number you should derate from.

Steve
 
My mistake, I didn't notice you started at 30 amps and used an ambient adj. factor.

Why do you need to derate for ambient?

Steve
 
I am derating for ambient because our work is in the deep south and it doesn't take much to get really hot between the ceiling and the barjoists, especially when the conduit is up near the ceiling in the barjoists
 
it just seems pretty unlikely you will have all 6 circuits heavily loaded at the same time, all with high harmonic loads, and a high ambient at the same time.
 
Status
Not open for further replies.
Top