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My Students are having trouble with this

Merry Christmas
Cindysparky

Cindysparky

Member
Location
Milwaukee
Occupation
Electrician
I’m teaching from this book. In this particular chapter, the book talks about choosing conductors with two things happening: continuous load, and adjustment factors of temperature and current carrying conductors. Many people in the class are very knowledgeable electricians. They are a smart bunch. Two of them had a fit that the book was wrong on this topic. They said that the conductors should be sized up from 12 gauge to 10 gauge.

I did mention that the book warns that oversizing conductors (per code requirement ) is a common misconception. That the true sizing the largest of the two observations: continuous load vs heat/current carrying adjustment.

I told them that I would take the time to pose this question to the forum and get some expert advice.

I think the book is correct. What do you think???💜
IMG_3500.jpeg
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
I have found many 'electricians' believe what they where taught by the 'do as I told you' mentorship program to be gospel and never questioned. When they change employers or take code classes they sometimes find they were being taught shirt pocket code rules.
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
The book has the correct answer, but "Comparison 3" is not required, as (a) it is redundant with "Comparison 1" and (b) this is not a feeder so Article 215 does not apply. Also, I question the assumption that "electric discharge lighting" is a nonlinear load, is that really true?

Cheers, Wayne
 
Cindysparky

Cindysparky

Member
Location
Milwaukee
Occupation
Electrician
Ok! Thanks and Thanks. The whole 3 comparisons was the way the book chose to teach it and it was driving me bananas. I was like: there’s only 2.

Also, yes electric discharge lighting is nonlinear 🩷 I’m sure you’ve looked it up by now. But I’ll attach a quick photo.

Good catch on the feeder
 
Cindysparky

Cindysparky

Member
Location
Milwaukee
Occupation
Electrician
electrofelon said:
I was going to say the same thing. I would not be counting the neutral as a CCC. IMO the nonlinear load thing is mostly not applicable anymore and should be deleted from the code.
Click to expand...
Are you saying it doesn’t apply anymore because of LEDs? I guess it would still apply is an old installation if you were adding something to an existing conduit…
 
tom baker

tom baker

First Chief Moderator & NEC Expert
Staff member
Location
Bremerton, Washington
Occupation
Master Electrician
Cindysparky said:
Are you saying it doesn’t apply anymore because of LEDs? I guess it would still apply is an old installation if you were adding something to an existing conduit…
Click to expand...
Non linear loads were older computers but non longer an issue with switching power supplies.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Cindysparky said:
Ok! Thanks and Thanks. The whole 3 comparisons was the way the book chose to teach it and it was driving me bananas. I was like: there’s only 2.

Also, yes electric discharge lighting is nonlinear 🩷 I’m sure you’ve looked it up by now. But I’ll attach a quick photo.

Good catch on the feeder
Click to expand...
Cindysparky said:
Are you saying it doesn’t apply anymore because of LEDs? I guess it would still apply is an old installation if you were adding something to an existing conduit…
Click to expand...

Where are you hearing that "electric discharge lighting" is nonlinear? Even in the old days, I dont believe say a magnetic fluorescent ballast would be nonlinear. Certainly inductive, but no non-linear.

It doesnt apply anymore because electronic power supplies are pretty good now and are power fator corrected and designed to be generally linear loads. A simple rectifier (or crude power supply contain one) would be non-linear because it only draws current at certain times in the cycle.
 
R

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
wwhitney said:
The book has the correct answer
Click to expand...
The book can be checked by NEC equation 310.15(B) without engineering supervision

I'-I*Sqrt((Tc-T'a)/(Tc-Ta))

I′ = ampacity corrected for ambient temperature
I = ampacity shown in the tables
Tc = temperature rating of conductor (°C)
T'a = new ambient temperature (°C)
Ta = ambient temperature used in the table (°C)

for #12 THHN @ 49°C or 120°F

I' = 25*Sqrt((75-49)/(75-30)) = 19 Amps

Teacher's book agrees 210.19(A)(1)(a) requires (15.5 * 1.25) = 19.38, which exceeds 19A

Teacher's book errors using 25A from T 310.16 based on 30°C. Table 310.16 is not valid for 49°C ambient.
 
Last edited:
T

Tulsa Electrician

Senior Member
Location
Tulsa
Occupation
Electrician
I think what the students are missing is the verbage of 210.19 (A) (1) for branch circuit rating,conductor.

Must be sized to carry the largest of (a) and (b).

(a): 15.5 *1.25=19.375

(b): 15.5/.82/.70= 27
This short cut allows to find the conductor.
In this case 90c conductor is used. Go to that column and find the one the is greater than 27.

Use copper
Use 90c conductor for derating
#12 ,30 amp@90c

For OCPD:* applies
19.375.amp load
#12 THHN copper @75c 25amp
20 amp OCPD.

I would start by having them go to article 100 and read the definition of ampacity. Then read the table header in 310 ampacity table.
The conductors ampacity (continues use) with an ambient of 30c.

This will help them understand why largest of the two in 210.
Then they can see both (a) and (b) work.

For conductor sizing use 27
For OCPD use 19.375
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
ramsy said:
for #12 THHN @ 49°C or 120°F

I' = 25*Sqrt((75-49)/(75-30)) = 19 Amps
Click to expand...
THHN has 90C rated insulation, so in any situation where you are required to do ampacity correction for ambient temperature, you can use the 90C version of your formula: I' = 30 * sqrt((90-49)/90-30) = 24.8A is the corrected (but not adjusted) ampacity.

The book's comparison 1 and comparison 2 are correct (if you accept that there are 8 CCCs rather than 6 CCCs). Note that comparison 1 (210.19(A)(1)(b)) considers ampacity adjustment and correction, but ignores terminal temperature limits and any continuous use factor. While comparison 2 (210.19(A)(1)(a)) ignores ampacity adjustment and correction, but includes terminal temperature limits and the continuous use factor.

Cheers, Wayne
 
R

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
electrofelon said:
Where are you hearing that "electric discharge lighting" is nonlinear?
Click to expand...
NEC Definition of Nonlinear in Art. 100 also describes "electric discharge lighting".

NEC 220.18(B) departs from incandescent Watts by requiring Nameplate Amps for lighting, since Watts ignores nonlinear power factor.

NEC 530.2 also informs us that some dimmers are nonlinear.

NEC Informative Annex, Example D3(a) has always considered HID lighting nonlinear.

"Note that the neutral conductors are counted as current-
carrying conductors [see 310.15(E)(3)] in this example because
the discharge lighting has substantial nonlinear content."
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
ramsy said:
NEC Definition of Nonlinear in Art. 100 also describes "electric discharge lighting".

NEC 220.18(B) departs from incandescent Watts by requiring Nameplate Amps for lighting, since Watts ignores nonlinear power factor.

NEC 530.2 also informs us that some dimmers are nonlinear.

NEC Informative Annex, Example D3(a) has always considered HID lighting nonlinear.

"Note that the neutral conductors are counted as current-
carrying conductors [see 310.15(E)(3)] in this example because
the discharge lighting has substantial nonlinear content."
Click to expand...
Fair enough, but note the definition says "may be". I think its a bad question to assume that, they should just tell you, or at the very least say "with electronic ballasts".
 
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