conductor sizing

[ohmboyz]

Hi all

This is my first post. I have a question about conductor sizing and double derating. Here are the conditions: branch circuit of 30 amp continuous load, thhn copper, 4 wires in a raceway and a terminal temp rating of 75 degrees C. After applying 210.19 for the continuous load, and using the 75 degree column in table 310.15 (B) (16), a number 8 wire is chosen.

Now, if I understand correctly that I do not have to double derate, then when I do my adjustments for conduit fill, I can use the listed load of 30 amps (not 37.5 ie- 30 X 125%) and multiply by 80%.

Is that correct?

 

[david luchini]

You are correct (except that you would divide the load current of 30A by 80%, not multiply.)

Another way to look at it is that you've determined your minimum conductor size will be #8, per 210.19.

#8 THHN adjusted for 4 current carrying conductors will have an ampacity of 44 (55*0.8.) 44 is larger than your 30A load, so the conductor is acceptable.

P.S. Welcome to the Forum.

 

[augie47]

I would do it the same as you...
30 amp continuous per 210.19 requires 37.5 ampacity
#8 THHN at derated fro 4 current carrying would be 55 X .8 or 44 amps
75? termination would allow #8 at 50 amps
#10 would only be good 35 amps.

 

[infinity]

Hi all

This is my first post. I have a question about conductor sizing and double derating. Here are the conditions: branch circuit of 30 amp continuous load, thhn copper, 4 wires in a raceway and a terminal temp rating of 75 degrees C. After applying 210.19 for the continuous load, and using the 75 degree column in table 310.15 (B) (16), a number 8 wire is chosen.

Now, if I understand correctly that I do not have to double derate, then when I do my adjustments for conduit fill, I can use the listed load of 30 amps (not 37.5 ie- 30 X 125%) and multiply by 80%.

Is that correct?

Are you assuming that this means that you have 4 CCC's with a single branch circuit?

Welcome to the Forum.

 

[ohmboyz]

much obliged to you both and thank you for the quip about calendar pages.
was wondering why they no longer gave the whole thing.

roger that, mr luchini on dividing rather than multiplying 80 into the 30 amps.

 

[Carultch]

Hi all

This is my first post. I have a question about conductor sizing and double derating. Here are the conditions: branch circuit of 30 amp continuous load, thhn copper, 4 wires in a raceway and a terminal temp rating of 75 degrees C. After applying 210.19 for the continuous load, and using the 75 degree column in table 310.15 (B) (16), a number 8 wire is chosen.

Now, if I understand correctly that I do not have to double derate, then when I do my adjustments for conduit fill, I can use the listed load of 30 amps (not 37.5 ie- 30 X 125%) and multiply by 80%.

Is that correct?

The 75 C column is usually used for your TERMINALS AND EQUIPMENT rating, and does not receive a derate factor. This is about the amount of mass of wire in the termination, and having enough mass to draw the heat away from the terminal. Read 110.14(C) to understand this requirement. Understand that this is independent of what kind of wire you use, unless you use wire that is rated at a lower temperature than the terminals and equipment. Also understand that for 100A and less, unless listed and marked otherwise, your equipment is by default rated 60C. This is mostly academic, as most new products are listed and marked otherwise for 75C.

The 90 C column, if you use 90C rated wire, is what you use for your derating of conductors, the combination of both bundling and temperature corrections. This is the most common application for which you can take credit for the 90C ampacity of the wire.

30A continuous load gets a 1.25 safety factor because it is continuous. Meaning you need a 40A breaker and 37.5A of wire.
240.4(B) allows us to round our breaker to the next standard size in excess of the ampacity if needed.

37.5A of wire at 75C, is #8 Cu. If you have less than 3 current-carrying wires, and you have 86 F [30C] or less ambient temperature, then you are DONE.

If you have an 80% derate factor, because you are sharing the conduit with another identical circuit, then you apply this 0.8 to the ampacity of your wire at its temperature rating. For THWN-2, that is 90C, and #8 Cu at 90C is rated 55A. 55A*0.8 = 44A, which exceeds the 37.5A that we need.

 

[infinity]

The 75 C column is usually used for your TERMINALS AND EQUIPMENT rating, and does not receive a derate factor. This is about the amount of mass of wire in the termination, and having enough mass to draw the heat away from the terminal. Read 110.14(C) to understand this requirement. Understand that this is independent of what kind of wire you use, unless you use wire that is rated at a lower temperature than the terminals and equipment. Also understand that for 100A and less, unless listed and marked otherwise, your equipment is by default rated 60C. This is mostly academic, as most new products are listed and marked otherwise for 75C.

The 90 C column, if you use 90C rated wire, is what you use for your derating of conductors, the combination of both bundling and temperature corrections. This is the most common application for which you can take credit for the 90C ampacity of the wire.

30A continuous load gets a 1.25 safety factor because it is continuous. Meaning you need a 40A breaker and 37.5A of wire.
240.4(B) allows us to round our breaker to the next standard size in excess of the ampacity if needed.

37.5A of wire at 75C, is #8 Cu. If you have less than 3 current-carrying wires, and you have 86 F [30C] or less ambient temperature, then you are DONE.

If you have an 80% derate factor, because you are sharing the conduit with another identical circuit, then you apply this 0.8 to the ampacity of your wire at its temperature rating. For THWN-2, that is 90C, and #8 Cu at 90C is rated 55A. 55A*0.8 = 44A, which exceeds the 37.5A that we need.

Nice post, one error is that derating doesn't apply at 3 or less CCC's, not less than 3.

 

[Carultch]

Nice post, one error is that derating doesn't apply at 3 or less CCC's, not less than 3.

Thanks. I think that is what I meant to say, but didn't think about the language.

 

[ohmboyz]

thank you both, carultch and infinity for the further clarifications.

 

[Ralph Herge]

#12 THHN for thirty amp protected circuits? (90'c)

#12 THHN for thirty amp protected circuits? (90'c)

Is it true, if conductor fill and terminal ratings are at 90'c, I can now run #12 THHN wire. We have a project, that becomes a bigger project if we change the wire sizing to #10. #12 is existing now in the piping. My understanding is this is allowed in the new code, which I have not seen yet. My electrician did provide documentation quoting that code however from a wire manufacturer.

 

[Dennis Alwon]

Is it true, if conductor fill and terminal ratings are at 90'c, I can now run #12 THHN wire. We have a project, that becomes a bigger project if we change the wire sizing to #10. #12 is existing now in the piping. My understanding is this is allowed in the new code, which I have not seen yet. My electrician did provide documentation quoting that code however from a wire manufacturer.

No this is not true except for certain situations such as a/c units or motors. You cannot use the 90C column except for de-rating purposes. In general #12 must be protected at 20 amps not 30 amps 240.4(D)

 

[iwire]

Is it true, if conductor fill and terminal ratings are at 90'c,

While you will find a lot of terminals marked 90C or more you will not find any 600 volt or less equipment rated more than 75C. That will limit you to the 75 C column.

 

[Ralph Herge]

#12 THHN for thirty amp protected circuits? (90'c)

#12 THHN for thirty amp protected circuits? (90'c)

By equipment you mean the breaker, twist lock plug, and connector?

I would love to see the actual NEC code item referenced from this wire manufacturer, as it just stated "the code now allows", #14 to be used for twenty amp circuits, and #12 to be used for thirty amps, the exceptions you stated were the ones I initially told my electricians, that I was aware of, but I was shocked by the verbiage in the item he printed out. It really, appeared like we could start running those twenty amp kitchen devices on #14 wire. I'll research this more but your answer makes me feel somewhat vindicated, and "my bad" for not having the most recent code book.

I must be developing some "Code Geekness" from having dinner with those Philly Code Inspectors.

 

[Dennis Alwon]

Look at the asterisk that is by those small wire sizes and then go to 240.4(D) as I stated earlier

(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.


(1) 18 AWG Copper. 7 amperes, provided all the following conditions are met:


(1) Continuous loads do not exceed 5.6 amperes.
(2) Overcurrent protection is provided by one of the following:

a. Branch-circuit-rated circuit breakers listed and marked for use with 18 AWG copper wire
b. Branch-circuit-rated fuses listed and marked for use with 18 AWG copper wire
c. Class CC, Class J, or Class T fuses


(2) 16 AWG Copper. 10 amperes, provided all the following conditions are met:
(1) Continuous loads do not exceed 8 amperes.
(2) Overcurrent protection is provided by one of the following:
a. Branch-circuit-rated circuit breakers listed and marked for use with 16 AWG copper wire
b. Branch-circuit-rated fuses listed and marked for use with 16 AWG copper wire
c. Class CC, Class J, or Class T fuses


(3) 14 AWG Copper. 15 amperes


(4) 12 AWG Aluminum and Copper-Clad Aluminum. 15 amperes


(5) 12 AWG Copper. 20 amperes


(6) 10 AWG Aluminum and Copper-Clad Aluminum. 25 amperes


(7) 10 AWG Copper. 30 amperes

 

[Ralph Herge]

#12 THHN for thirty amp protected circuits? (90'c)

#12 THHN for thirty amp protected circuits? (90'c)

Amen and thank you.

 

[electrofelon]

By equipment you mean the breaker, twist lock plug, and connector?

I would love to see the actual NEC code item referenced from this wire manufacturer, as it just stated "the code now allows", #14 to be used for twenty amp circuits, and #12 to be used for thirty amps.....

Manufacturers are often as good with code and electrical theory as they are with designing things that are easy to install and have comfortable wiring compartments

 

[Carultch]

Is it true, if conductor fill and terminal ratings are at 90'c, I can now run #12 THHN wire. We have a project, that becomes a bigger project if we change the wire sizing to #10. #12 is existing now in the piping. My understanding is this is allowed in the new code, which I have not seen yet. My electrician did provide documentation quoting that code however from a wire manufacturer.

It may be very true and very common, that the lugs on a panel are marked "AL9CU", and one might think that this allows use of the 90C column of the NEC. However, it isn't only the terminals that need to be listed and labeled for you to use the 90C column instead of the 75C column, but also the equipment. It is rare that a manufactured piece of equipment is listed for 90C wiring.

The exception, where it is the termination temperature rating, is when you are attaching a "separately installed connector" to a busbar, or whether you are installing a connector in a transition box. In these cases, it is only the conductors' and connector's temperature rating matters. Understand that this is a rare scenario, where these ratings are required on BOTH SIDES of the conductor.

Until then, your starting point is either the 60C or 75C column of the NEC. 60C for 100A and less, unless listed and labeled for 75C (which is very common). 75C for over 100A. This is before you apply adjustment and correction factors.

When you apply adjustment and correction factors, that is when you can use the 90C column of the NEC for 90C wiring. It gives you flexibility for your calculations. You still size worst case scenario for 75C before applying these factors, and then use 90C when applying these factors.

 

[Smart $]

It may be very true and very common, that the lugs on a panel are marked "AL9CU", and one might think that this allows use of the 90C column of the NEC. However, it isn't only the terminals that need to be listed and labeled for you to use the 90C column instead of the 75C column, but also the equipment. It is rare that a manufactured piece of equipment is listed for 90C wiring.

The exception, where it is the termination temperature rating, is when you are attaching a "separately installed connector" to a busbar, or whether you are installing a connector in a transition box. In these cases, it is only the conductors' and connector's temperature rating matters. Understand that this is a rare scenario, where these ratings are required on BOTH SIDES of the conductor.

Until then, your starting point is either the 60C or 75C column of the NEC. 60C for 100A and less, unless listed and labeled for 75C (which is very common). 75C for over 100A. This is before you apply adjustment and correction factors.

When you apply adjustment and correction factors, that is when you can use the 90C column of the NEC for 90C wiring. It gives you flexibility for your calculations. You still size worst case scenario for 75C before applying these factors, and then use 90C when applying these factors.

I'll add a couple distinctions to your post...

For minimum conductor SIZE, i.e. termination temperature coordination with 110.14(C), continuous load portion is factored 125%,.

For minimum conductor AMPACITY, continuous load portion is not factored.

 

[Carultch]

I'll add a couple distinctions to your post...

For minimum conductor SIZE, i.e. termination temperature coordination with 110.14(C), continuous load portion is factored 125%,.

For minimum conductor AMPACITY, continuous load portion is not factored.

Interesting you mentioned that. This is described in PV-specific wording in 690.8 as well, and I found it very counter-intuitive, when I first read it. I also thought that it was something that specifically applied to photovoltaic systems, but I guess it is also a general wiring rule as well.

Another distraction is that overcurrent devices where required must "protect" the wire at conditions of use when sized. I put "protect" in quotation marks, because it is still considered protected when complying with the "next size up rule" 240.4(B).

 

[electrofelon]

I'll add a couple distinctions to your post...

For minimum conductor SIZE, i.e. termination temperature coordination with 110.14(C), continuous load portion is factored 125%,.

For minimum conductor AMPACITY, continuous load portion is not factored.

would you mine elaborating on this maybe with an example?