Correctional factors and adjustment factors
- Thread starter elecold
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General conductor sizing is a three-part determination:
- Determine minimum size for terminal temperature limitation (continuous padded 25%).
- Determine minimum size using adjusted and corrected ampacity (no continuous padding).
- Determine minimum size protected by ocpd (at adjusted and corrected ampacity).
Determinations #1 and #2 are in compliance with:
- 210.19(A)(1)(a) and (b)
- 215.2(A)(1)(a) and (b)
- 230.42(A)(1) and (2)
Determination #1 is the "selected and coordinated" part of 110.14(C) requirement and uses the Table 310.15(B)(16) column equivalent to the terminal temperature limit: 60°C up to 100A if not listed and identified for higher, 75°C over 100A. Most new equipment is listed and identified for 75° terminations, but don't assume.
Last edited:
JFletcher
Senior Member
- Location
- Williamsburg, VA
Also see 240.4, 240.4(D).
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
two major factors determine minimum size of conductor needed for a circuit.
termination temperature rating.
Conductor must be large enough to not cause excessive heating of the termination method. You can not have a conductor smaller then this part of the calculation requires.
insulation temperature rating.
Away from the termination you may encounter multiple conductors in a raceway (which provides the opportunity for heat to build up in the raceway) or high ambient temperatures can exist as well. This is where the 90C rating of the conductor can be used as the starting point for ampacity adjustments - all that matters here is insulation temperature, termination temperature is in a different location and (possibly) a different environment.
You can have different ampacity apply to different sections of a circuit based on the conditions in that section.
termination temperature rating.
Conductor must be large enough to not cause excessive heating of the termination method. You can not have a conductor smaller then this part of the calculation requires.
insulation temperature rating.
Away from the termination you may encounter multiple conductors in a raceway (which provides the opportunity for heat to build up in the raceway) or high ambient temperatures can exist as well. This is where the 90C rating of the conductor can be used as the starting point for ampacity adjustments - all that matters here is insulation temperature, termination temperature is in a different location and (possibly) a different environment.
You can have different ampacity apply to different sections of a circuit based on the conditions in that section.
Carultch
Senior Member
- Location
- Massachusetts
In general, the 60C column rule is more academic than practical, as most equipment you'll find in practice is listed and labeled otherwise for 75C. Still, you do have a burden of proof on both ends of the circuit, to determine if you can use the 75C column, for 100A and less. The default for 100A and less is 60C rated terminations, and the default for over 100A is 75C rated terminations. When taking an exam, you have to go by this default, unless the exam specifies otherwise. It might do so in the problem, or as a general beginning statement like "all terminations on this exam are 75C, unless the question specifies otherwise".
No matter what insulation rating you have, you have to have at least the right size wire for the terminations at the required ampacity. Given 75C terminations and 90C wire, you pick your wire size to fit the 75C column for the terminations first. You might find lugs that appear to be marked for 90C, but take this with a grain of salt. It is not the lug that matters, but the equipment it is within, when it is part of manufactured product. Little, if any, manufactured equipment is listed for 90C, but it is very common for 75C rated equipment to have lugs that are marked for 90C.
So what is the value of 90C wire? Most wire available today is 90C rated, like THWN-2 and XHHW-2.
When temperature correction factors and bundling adjustments apply, the factors apply to the 90C rating of the wire, even if the terminations are 75C or even 60C rated. This is the most common reason you take credit for the 90C rating of your wire. Temperature correction and bundling adjustment factors do not apply to the terminations. Instead, the 125% continuous load factor does, when the load is continuous. Else, it is 100% non-continuous + 125% continuous, when you have mixed loads.
Another application of the 90C rating of wire, is that when you terminate on separately installed 90C rated connectors that are not part of manufactured equipment. An example is a Polaris insulated tap connector, that you might put in an adjacent enclosure. If you terminate in this manner on both ends, and then connect another 90C rated conductor with 75C sizing to make the final connection to the equipment, then the majority of the run can be installed with 90C sizing. You might do this if you screw up, or if it is a value engineering choice.
Last edited:
elecold
Member
In general, the 60C column rule is more academic than practical, as most equipment you'll find in practice is listed and labeled otherwise for 75C. Still, you do have a burden of proof on both ends of the circuit, to determine if you can use the 75C column, for 100A and less. The default for 100A and less is 60C rated terminations, and the default for over 100A is 75C rated terminations. When taking an exam, you have to go by this default, unless the exam specifies otherwise. It might do so in the problem, or as a general beginning statement like "all terminations on this exam are 75C, unless the question specifies otherwise".
No matter what insulation rating you have, you have to have at least the right size wire for the terminations at the required ampacity. Given 75C terminations and 90C wire, you pick your wire size to fit the 75C column for the terminations first. You might find lugs that appear to be marked for 90C, but take this with a grain of salt. It is not the lug that matters, but the equipment it is within, when it is part of manufactured product. Little, if any, manufactured equipment is listed for 90C, but it is very common for 75C rated equipment to have lugs that are marked for 90C.
So what is the value of 90C wire? Most wire available today is 90C rated, like THWN-2 and XHHW-2.
When temperature correction factors and bundling adjustments apply, the factors apply to the 90C rating of the wire, even if the terminations are 75C or even 60C rated. This is the most common reason you take credit for the 90C rating of your wire. Temperature correction and bundling adjustment factors do not apply to the terminations. Instead, the 125% continuous load factor does, when the load is continuous. Else, it is 100% non-continuous + 125% continuous, when you have mixed loads.
Another application of the 90C rating of wire, is that when you terminate on separately installed 90C rated connectors that are not part of manufactured equipment. An example is a Polaris insulated tap connector, that you might put in an adjacent enclosure. If you terminate in this manner on both ends, and then connect another 90C rated conductor with 75C sizing to make the final connection to the equipment, then the majority of the run can be installed with 90C sizing. You might do this if you screw up, or if it is a value engineering choice.
So below 100 amps I use the 60 degree column of 310.15 (b)(16).
An example would be I'm feeding 100 amps. Three conductors...No derating needed.. With an ambient temperature of 105°f the correctional of .87.
column for #1 copper @ 90 degrees is 145 amps.
Multiply 145 by .87 and it comes out to roughly 126 amps....
Regardless I would have to go off the 60 degree column which would put #1 at 110 amps
Correct for below 100 amps?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
If you have 60C terminals - depends on a couple other things.
2 AWG copper is good for 95 amps @ 60C - can be protected with next standard size overcurrent device of 100 amps if the load is 95 amps or less. If the load is between 96 and 100 amps then you would need a 1 AWG conductor.
Otherwise yes you make ampacity adjustments for ambient temp or number of conductors in raceway on the 90C ampacity if you have 90C insulation. Which ever result, termination or insulation, yields a larger conductor is what you must use.
Also keep in mind the continuous load 125% factor. If you had a circuit with a 100 amp continuous load you end up needing a 125 amp conductor minimum - you also end up needing a 125 amp overcurrent device minimum.
Unless you are landing on some old equipment, we are talking over 25 years old maybe even 35-40 years old, you likely will have 75C terminations.
Carultch
Senior Member
- Location
- Massachusetts
Yes. For a fully utilized 100A circuit with the default 60C terminations, and derate factors that are not enough to govern the situation, #1 Cu is correct. Derating applies to the wire rating, rather than the termination rating.
In some applications, you could use #2 Cu for this circuit. See 240.4(B) for details.
Carultch
Senior Member
- Location
- Massachusetts
Here's one example of a modern product, where you cannot count on the 75C rating. Observe that #6 is rated for 63A rather than 65A. So for #8, you have to assume 40A rather than 50A.
https://www.mcmaster.com/#terminal-blocks/=164nwds
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
When determining that the wire is protected by OCPD, you use the conditions of use derate for the wire you are using, e.g., the COU derated 90 degree ampacity if it is THWN-2. The derate for terminal temp is not part of that equation.
Carultch
Senior Member
- Location
- Massachusetts
I thought both the non-corrected terminal ampacity, and the corrected conductor ampacity, must each "round up" to the actual standard size OCPD, in order to qualify for 240.4(B). And for cases where 240.4(B) does not apply, both of the above must be equal to or greater than the OCPD, such that the OCPD is the "weak link".
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
The test is to see if the wire is protected, i.e., to see if the insulation will hold up under the conditions of use. Use the COU calc for the type of insulation used. The result must be more than the next size down OCPD.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Correct, did I say something that was taken that way?
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