Help with conductor derating

Status
Not open for further replies.
Jpflex

Jpflex

Electrician big leagues
Location
Victorville
Occupation
Electrician commercial and residential
I just wanted to be sure I’m doing this correct

I have a spectrometer 240 VAC single phase 60hz nominal power 2.9 KVA 0.99 power factor, max 15 amps requiring a NEMA 6-15 receptacle

In determining the optimum wire gauge I’m thinking either #12 or #10 THHN

While using copper.org chart for area ambient temperatures I find BISHOP, CA (closest area to site) with a design temp at 100 degrees

With NEC 310.15 A requirement I proceed to derate THHN #12 AWG wire,

Table 310.15b1 with ambient temp found at 100 degrees with an insulation rated at 90 degrees my conductor derating is x 0.91

Table 310.16 #12 AWG at 60 degree (60 degree for under a 100 ampere circuit or within 14 to 1 AWG conductors ) rating is 20 amperes?

Therefore 20 amperes x 0.91 = 18.2 ampacity for # 12 AWG after correction

However, receptacle circuits are not to be loaded over 80 percent.

Therefore 18.2 i x 0.80 percent = 14.56 amps usable on circuit, which is under 15 amps target

Therefore am I overkill to go up to next size #10 AWG?
 

Attachments

  • 22A0BE22-8EBB-47BF-9AE6-AD33C908B58D.jpeg
    22A0BE22-8EBB-47BF-9AE6-AD33C908B58D.jpeg
    112.9 KB · Views: 8
This will be a dedicated circuit receptacle with an isolated ground
 
Is your conductor outside on a roof top?
How long is the circuit?
2.9 kw is about 13 amps
Is this device on for a long time?
 
Jpflex said:
Table 310.15b1 with ambient temp found at 100 degrees with an insulation rated at 90 degrees my conductor derating is x 0.91

Table 310.16 #12 AWG at 60 degree (60 degree for under a 100 ampere circuit or within 14 to 1 AWG conductors ) rating is 20 amperes?

Therefore 20 amperes x 0.91 = 18.2 ampacity for # 12 AWG after correction
Click to expand...
No, stop there. When applying the 0.91 you use the 90C rating of 30A, to get 27A. Then if your equipment has 60C rated terminals, you take the lesser of 20A (the 60C unadjusted rating) and 27A (the 90C adjusted rating), and you get 20A.

Cheers, Wayne
 
Jpflex said:
I just wanted to be sure I’m doing this correct

I have a spectrometer 240 VAC single phase 60hz nominal power 2.9 KVA 0.99 power factor, max 15 amps requiring a NEMA 6-15 receptacle

In determining the optimum wire gauge I’m thinking either #12 or #10 THHN

While using copper.org chart for area ambient temperatures I find BISHOP, CA (closest area to site) with a design temp at 100 degrees

With NEC 310.15 A requirement I proceed to derate THHN #12 AWG wire, THHN is a 90° conductor so you use the 90° ampacity of 30 amps

Table 310.15b1 with ambient temp found at 100 degrees with an insulation rated at 90 degrees my conductor derating is x 0.91

Table 310.16 #12 AWG at 60 degree (60 degree for under a 100 ampere circuit or within 14 to 1 AWG conductors ) rating is 20 amperes?

Therefore 20 amperes x 0.91 = 18.2 ampacity for # 12 AWG after correction You can use the 30 amps X .91 = 27.3 amps

However, receptacle circuits are not to be loaded over 80 percent. What Code section is that ?

Therefore 18.2 i x 0.80 percent = 14.56 amps usable on circuit, which is under 15 amps target

Therefore am I overkill to go up to next size #10 AWG?
Click to expand...
Your load is actually 12 amps. A #14 THHN under the same parameters as above would be 25 X .91 or 23 amps so you can use a #14 on a 15 amp breaker {240.4(D) exclusion } so #12 is actually overkill
 
wwhitney said:
No, stop there. When applying the 0.91 you use the 90C rating of 30A, to get 27A. Then if your equipment has 60C rated terminals, you take the lesser of 20A (the 60C unadjusted rating) and 27A (the 90C adjusted rating), and you get 20A.

Cheers, Wayne
Click to expand...
Ok thanks
 
augie47 said:
Why are you using the 100 deg adjustment... is the lab temp 100 deg ?? :)
Click to expand...
No but from my understanding NEC requires conductor ampacity adjustment/ correction for geographic locations other than 85 degrees Celsius.

Based on copper.org chart, Bishop, California is rated with a “design” ambient temperature 100 degrees Celsius. This temp exceeds the NEC conductor ampacity table with 3 conductors or less based on an ambient temperature of 85 degrees celcius

Some people would probably just tell me to skip this step, but I do it for my knowledge especially to take future journeyman license testing
 

Attachments

  • 483A19E3-956B-4B2D-91BF-DC6F701CE881.png
    483A19E3-956B-4B2D-91BF-DC6F701CE881.png
    1.3 MB · Views: 0
Jpflex said:
Ok thanks
Click to expand...
What about NEC requirements not not load up a circuit or receptacle over 0.80 percent?

20. X 0.80 = 16 amperes? (Which would still work) but when does 80 percent rule work? Residential/ commercial?
 
Jpflex said:
No but from my understanding NEC requires conductor ampacity adjustment/ correction for geographic locations other than 85 degrees Celsius.

Based on copper.org chart, Bishop, California is rated with a “design” ambient temperature 100 degrees Celsius. This temp exceeds the NEC conductor ampacity table with 3 conductors or less based on an ambient temperature of 85 degrees celcius

Some people would probably just tell me to skip this step, but I do it for my knowledge especially to take future journeyman license testing
Click to expand...
The adjustment is for the ambient temperature for the environment where the conductor is installed not the outside temperature.
 
Jpflex said:
What about NEC requirements not not load up a circuit or receptacle over 0.80 percent?

20. X 0.80 = 16 amperes? (Which would still work) but when does 80 percent rule work? Residential/ commercial?
Click to expand...
and where is that requirement ??
(hint: read 210.21)
 
augie47 said:
and where is that requirement ??
(hint: read 210.21)
Click to expand...
Ok it says the equipment with cords and plugs not fastened into place shall not exceed 80 percent of the branch circuit ampere rating.

Therefore, those branch circuit conductors may have to be enlarged as I was suggesting?

Therefore for this 15 ampere load at approx 240 volts

#12 AWG rated at 90 degrees prior to derating = 25i

25i x 0.91 temperature correction for Bishop, CA = 22.75i amperes

22.75i x 0.80 percent (conductor bundling)= 18.2i

18.2i x 0.80 max load on branch circuit = 14.56 i able to be used on circuit

Therefore , again it appears I should go with #10 AWG THHN?
 
Jpflex said:
Ok it says the equipment with cords and plugs not fastened into place shall not exceed 80 percent of the branch circuit ampere rating.

Therefore, those branch circuit conductors may have to be enlarged as I was suggesting?

Therefore for this 15 ampere load at approx 240 volts

#12 AWG rated at 90 degrees prior to derating = 25i

25i x 0.91 temperature correction for Bishop, CA = 22.75i amperes

22.75i x 0.80 percent (conductor bundling)= 18.2i

18.2i x 0.80 max load on branch circuit = 14.56 i able to be used on circuit

Therefore , again it appears I should go with #10 AWG THHN?
Click to expand...
My bad I had not placed a ruler under chart so I got 25i instead of 30i for #12 AWG THHN wires

New calculation hopefully is 30i x 0.91 (temp derating) = 27.3i

27.3i x 0.80 (conductor bundling over 3 conductors 2 designated circuits in one 3/4” EMT) = 21.84i

21.84i x 0.80 percent rule for branch circuit = 17.47i

Looks like I can go #12 AWG THHN like you said and hopefully I did this correct at last?
 
(a) The 80% you reference is in 210,23 "multi-outlet branch circuits". Since you have 1 piece of equipment you can make it a single circuit and use the receptacle at full load
(b) IF you want to apply the 80% rule, your load is 12 amps (2.9kw/240) which is only 80% of a 15 amp circuit.
(c) #12 is fine but #14 (25 X .8 = 20 amps) on a 15 amp OCP device (240.6) would be Code compliant
 
augie47 said:
(c) #12 is fine but #14 (25 X .8 = 20 amps) on a 15 amp OCP device (240.6) would be Code compliant
Click to expand...
Just to reiterate what Augie said, for an individual branch circuit, in conditioned spaces, neither the 80% limit nor the temperature correction apply. It's as simple as "non-motor load <= 15A; use a 15A breaker with #14 wire."

Cheers, Wayne

P.S. In the OP you have highlighted the lines for both the spectrometer and the water chiller. These are each getting separate circuits? Does either one use a motor?
 
augie47 said:
(a) The 80% you reference is in 210,23 "multi-outlet branch circuits". Since you have 1 piece of equipment you can make it a single circuit and use the receptacle at full load
(b) IF you want to apply the 80% rule, your load is 12 amps (2.9kw/240) which is only 80% of a 15 amp circuit.
(c) #12 is fine but #14 (25 X .8 = 20 amps) on a 15 amp OCP device (240.6) would be Code compliant
Click to expand...
If yoke has two receptacles on one strap even if hooked up to a single gang box, could this be considered “multi-outlet?” Yes the branch circuit only goes to a single box to from a single breaker circuit to be dedicated
 
Status
Not open for further replies.
Top