**Ambient Temperature**

Table 310.15(B)(16) of the National Electrical Code lists the allowable ampacities of insulated general use conductors. This makes it a very useful table, but, we must keep in mind that the conductor ratings of this table are based on certain limitations. First, the ambient temperature, or temperature surrounding the conductors, are to be a minimum of 30 ̊C. (86 ̊F.). Secondly, there’s a maximum limit of only three conductors per raceway, cable or earth.

To deal with these limitations we have to make adjustments. One adjustment has to do with heat. Ambient temperature corrections adjusts for the heat surrounding a conductor. Remember, heat increases the resistance of conductors. Article 310.15(A)(3) defines the temperature limitations of conductors...

Let’s examine a #10* THW conductor. The number 10, as we know, is the AWG (American Wiring Gauge) or size. Table 310.15(B)(16) states that a #10* THW conductor has an ampacity of 35 amps, but there’s more regarding the insulation rating. Do you know what THW stands for ? The “T” stands for thermoplastic insulation, the “H” stands for heat resistant up to 75°C., and the “W” stands for water resistant, allowing us to use the wire outdoors or underground or anywhere it gets wet.

A lot of good information regarding insulation rating of conductors can be found in Table 310.104(A). Neat stuff like a description of what the insulation is made of, what it can be used for, where it can be used, the mills thickness and the conductors temperature limitation. Here are some of the descriptions of different types of insulations used these days...

*310.15(A)(3): Temperature Limitation of Conductors. No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved. In no case shall conductors be associated together in such a way, with respect to type of circuit, the wiring method employed, or the number of conductors, that the limiting temperature of any conductor is exceeded.*Let’s examine a #10* THW conductor. The number 10, as we know, is the AWG (American Wiring Gauge) or size. Table 310.15(B)(16) states that a #10* THW conductor has an ampacity of 35 amps, but there’s more regarding the insulation rating. Do you know what THW stands for ? The “T” stands for thermoplastic insulation, the “H” stands for heat resistant up to 75°C., and the “W” stands for water resistant, allowing us to use the wire outdoors or underground or anywhere it gets wet.

A lot of good information regarding insulation rating of conductors can be found in Table 310.104(A). Neat stuff like a description of what the insulation is made of, what it can be used for, where it can be used, the mills thickness and the conductors temperature limitation. Here are some of the descriptions of different types of insulations used these days...

__Letter Description__

T Thermoplastic

R Rubber

X Cross-Linked Polymer

H Heat Resistant (75°C.)

HH Heat Resistant (90°C.)

W Water Resistant

N Nylon Outer-Cover

Let’s now look up a #10* THHN. That’s a #10* AWG sized wire with, according to Table 310.15(B)(16), a thermoplastic insulated, heat resistant to 90°C., with a nylon outer-covering, which helps dissipate heat. The table lists this conductor at 40 amps. That’s fine but what does that asterisk (*) stand for ? Take a look at Section 240-4(D)...

240.4(D): Unless specifically permitted the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper , or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

O.K., so the conductor is rated at 40 amps but the circuit breaker is rated at 30 amps. What is going on here ? Let’s call the 40 amp rating the conductor ampacity. Of course that’s before any correction factors are applied, but the limit is 30 amps after correction factors are applied.

Remember, the circuit protection is rated at 30 amps and will trip when more than 30 amps are applied. But, what is a correction factor ?

When the ambient temperature, or temperature surrounding the wire is not 30°C (86°F), or, when there are more than three conductors in a raceway, we have to make a correction to compensate for the extra heat generated.

240.4(D): Unless specifically permitted the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper , or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

O.K., so the conductor is rated at 40 amps but the circuit breaker is rated at 30 amps. What is going on here ? Let’s call the 40 amp rating the conductor ampacity. Of course that’s before any correction factors are applied, but the limit is 30 amps after correction factors are applied.

Remember, the circuit protection is rated at 30 amps and will trip when more than 30 amps are applied. But, what is a correction factor ?

When the ambient temperature, or temperature surrounding the wire is not 30°C (86°F), or, when there are more than three conductors in a raceway, we have to make a correction to compensate for the extra heat generated.

Ambient Temperature Correction Factors

If we are using our #10 THHN* conductor in a boiler room with a temperature of 36°C. or (96°F.). That’s hot enough to break a sweat. We can use a #10 THHN* at 36°C, we can actually use it up to 90°C. or (194°F.), according to the absolute limit of the conductor, check Table 310.15(B)(2)(a).

Anyway, the Correction Factor Table states that “For ambient temperatures other than 30°C. (86°F.), multiply the allowable ampacities shown above by the appropriate factor shown below.”

The ambient temperature correction factor for 36°C. in the THHN column is .91. Now, we can use the 40 amp rating of the #10* THHN conductor, before derating, but we must multiply it by .91 to correct for the 36°C. ambient temperature.

But the asterisk (*) note at the bottom of the table still limits the overcurrent protection to 30 amps after corrections. So, we still are limited to 30 amps on the conductor.

Here’s what a #10* THHN conductor is limited to at various temperatures...

Anyway, the Correction Factor Table states that “For ambient temperatures other than 30°C. (86°F.), multiply the allowable ampacities shown above by the appropriate factor shown below.”

The ambient temperature correction factor for 36°C. in the THHN column is .91. Now, we can use the 40 amp rating of the #10* THHN conductor, before derating, but we must multiply it by .91 to correct for the 36°C. ambient temperature.

**40 x .91 = 36.4 amps**But the asterisk (*) note at the bottom of the table still limits the overcurrent protection to 30 amps after corrections. So, we still are limited to 30 amps on the conductor.

Here’s what a #10* THHN conductor is limited to at various temperatures...

Your probably still wandering with this asterisk (*) business is all about ? It actually refers to the overcurrent protection on smaller conductors (#14, #12, and #10 copper, #12 and #10 aluminum) These conductors can easily be reduced to such a small ampacity that they would be unusable. So, the asterisk gives them a little head start (5 to 10 amps).

Also, you might wander why the correction factors never reach the true temperature limits of the conductors. Take #10 THHN* for instance. It’s rated up to 90°C. but the correction factors only go up to 80°C. The 1975 Code had a note which restricted conductors from being used within 10°C. of its maximum insulation rating. The note was removed in 1978 but there still seems to be a built-in limitation by the absence of correction factors within 5° to 10°C. of a conductors insulation rating.

Number Of Conductors

Not only does Table 310.15(B)(16) limit the ampacity on conductors due to ambient temperature, there is also a correction factor for more than three conductors. Remember, adjacent conductors have the dual effect of raising the ambient temperature and impeding heat dissipation. Those bright people who write the Code feel that when more than three conductors get together there’s a problem with heat build-up. Table 310.15(B)(3)(a) lists the correction factors based on number of conductors either in a conduit or grouped together..

Conductors That Don’t Count

Table 310.15(B)(3)(a) applies to current carrying conductors only. So is a neutral a current carrying conductor ? Sometimes ! What about grounding conductors ?

310.15(B)(5)(b) In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wye connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(3)(a).

310.15(B)(5)(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.

310.15(B)(5) Grounding or Bonding Conductor. A grounding or bonding conductor shall not be counted when applying the provisions of 310.15(B)(3)(a).

In a single-phase circuit, with two hot conductors and a neutral which carries only the unbalanced current, the neutral would not count. On a three-phase circuit, with three hot conductors and a neutral which carries only the unbalanced current, the neutral also would not count. If, on the other hand, we are using only two hot conductors out of the three available on a four-wire three-phase (wye connected) system, then the neutral would count as a current carrying conductor.

*310.15(B)(5)(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(3)(a).*310.15(B)(5)(b) In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wye connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(3)(a).

310.15(B)(5)(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.

310.15(B)(5) Grounding or Bonding Conductor. A grounding or bonding conductor shall not be counted when applying the provisions of 310.15(B)(3)(a).

In a single-phase circuit, with two hot conductors and a neutral which carries only the unbalanced current, the neutral would not count. On a three-phase circuit, with three hot conductors and a neutral which carries only the unbalanced current, the neutral also would not count. If, on the other hand, we are using only two hot conductors out of the three available on a four-wire three-phase (wye connected) system, then the neutral would count as a current carrying conductor.

Also, if a branch circuit feeds non-linear loads like electric discharge lighting, data processing equipment, and other types of electronic equipment, there may be harmonic currents. Normal circuits in the United States are at a frequency of 60 hertz, (60 cycles per second). Some electronic equipment, like computers, run in the kilohertz (thousands) or megahertz (millions) of cycles per second. This causes a distortion in the normal sine-wave shaped wave form.

An increase in frequency causes overheating in conductors due to the fact that higher frequency currents tend to flow on the surface of the conductor rather than utilizing the entire conductive area. Because of this effect, a neutral carrying harmonic currents must be counted when calculating Table 310.15(B)(3)(a).

There is some good news, though. According to 310.15(B)(6) grounding and bonding conductors are not considered current carrying, and need not be counted in Table 310.15(B)(3)(a). Neither are conductors for control wiring, cable trays, or conduit nipples 24” or less.

Remember Table 310.15(B)(16) is based on up to three current-carrying conductors in a raceway or cable or earth. We make our corrections for more than three current-carrying conductors by multiplying the allowable ampacities by the correction factors from Table 310.15(B)(3)(a).

For example: we have four #10 THHN* conductors in a conduit, all of which are current-carrying (no neutrals). Now, we can still use the 40 amp rating of the #10 THHN* conductor (before derating), don’t forget the note on the bottom of the table. Looking up four conductors from Table 310.15(B)(6) we find an 80% derating factor for four conductors.

There is some good news, though. According to 310.15(B)(6) grounding and bonding conductors are not considered current carrying, and need not be counted in Table 310.15(B)(3)(a). Neither are conductors for control wiring, cable trays, or conduit nipples 24” or less.

Remember Table 310.15(B)(16) is based on up to three current-carrying conductors in a raceway or cable or earth. We make our corrections for more than three current-carrying conductors by multiplying the allowable ampacities by the correction factors from Table 310.15(B)(3)(a).

For example: we have four #10 THHN* conductors in a conduit, all of which are current-carrying (no neutrals). Now, we can still use the 40 amp rating of the #10 THHN* conductor (before derating), don’t forget the note on the bottom of the table. Looking up four conductors from Table 310.15(B)(6) we find an 80% derating factor for four conductors.

40 x .8 = 32 amps

The asterisk (*) note at the bottom of the table still limits the circuit to 30 amps after corrections. So, we still are limited to 30 amps on the conductor.

Ambient Temperature And Number Of Conductors

What about a situation where where we have to make a correction for ambient temperature and more than three conductors in the same problem. It may be a little more complicated but it can be done. Check out this formula...

Where "Allowable Amps" are the amount of amps allowed on a conductor, the "Conductor Amp Rating" is based on the Table 310.15(B)(16) ampacity rating for the conductor. The "Ambient Temperature Correction Factor" is the adjustment factor at the bottom of Table 310.15(B)(16), and the "Number of Conductors Correction Factor" is the percentage from Table 310.15(B)(3)(a) for number of conductors.

Let’s try combining our last couple of examples. Let’s take our four #10 THHN*’s and apply them to a 36°C. ambient temperature situation and see how many amps are allowed on our circuit.

Let’s try combining our last couple of examples. Let’s take our four #10 THHN*’s and apply them to a 36°C. ambient temperature situation and see how many amps are allowed on our circuit.

Conductor Amp Rating = 40 amps

Ambient Temperature Correction Factor = .91

Number Of Conductors Correction Factor = .8

Ambient Temperature Correction Factor = .91

Number Of Conductors Correction Factor = .8

Allowable Amps = 40 amps x .91 x .8 = 29.1 amps

Now, you might be thinking ! If we’re only allowed 29.1 amps on this circuit what size circuit breaker are we gonna use ? They don’t make a 29.1 amp breaker. This is true, but, those geniuses who write the Code included Section 240.4(B), which basically says that if your ratings do not correspond with the standard size ratings for breakers or fuses (Article 240.6), you can go up to the next larger standard size. So, after all that work we’re still using a 30 amp breaker !

Finding A Conductor

O.K. ! We now know how to determine how many amps we can put on a wire. What if we don’t know which wire to use ? Guess what ! We have a formula for that too !

Say we got a set of lights totaling 20 amps located in a building which reaches temperatures of about 55°C. for a prolonged time period. Let’s say we have a total of seven conductors in our raceway. We need to find a conductor suited for the set of lights with an insulation that has to be able to handle 55°C. Let’s go with THW insulation, it's good up to 75°C.

So, we need a THW conductor from Table 310.15(B)(16). How about a #8 THW ?

Here’s a simplified way of determining the above formulas...

Here’s a simplified way of determining the above formulas...

Just put your thumb over "Amps" to find how many amps you can put on your wire...

“Amps” (Allowable Amps) = Wire (Amps) x ATCF x NC

"ATCF" stands for Ambient Temperature Correction Factor from Table 310.15(B)(2)(a) and

"NC" stands for Number Of Conductor Correction Factor from Table 310.15(B)(3)(a).

"NC" stands for Number Of Conductor Correction Factor from Table 310.15(B)(3)(a).

Ambient Temperature & Number Of Conductor Sample Problems...

**Problem #1:**What size THHN conductor is required for a (120/240 volt) seven conductor branch circuit, which supplies power to fluorescent lights in a boiler room. (Ambient Temperature of 140°F. and 15 amps per circuit, continuous load) ?

Amps = 15

Ambient

Temperature

(ATCF) = .71 (T310-15 (b)(16)) THHN @ 140°F = 71%

Number Of

Conductors (NC) = .7 (T310-15(B)(3)(a)) 7 conductors = 70%

Wire =

.71 x .7 .497

Wire = 30.18 amps (#10 THHN*) good for 40 amps

Problem #2: What is the ampacity of two #10* TW aluminum conductors feeding a 12 amp 230 Volt, single-phase motor, with an ambient temperature of 43°C. ?

Amps = Wire x ATCF x NC

Wire = 25 (T310-15 (b)(16)) #10 auminum = 25 amps

ATCF = .71 (T310-15 (b)(2)(a)) #10 TW aluminum @ 43°C.

NC = does not apply

Amps = 25 x .71

Amps = 17.75 amps (#10 TW aluminum)

Ambient

Temperature

(ATCF) = .71 (T310-15 (b)(16)) THHN @ 140°F = 71%

Number Of

Conductors (NC) = .7 (T310-15(B)(3)(a)) 7 conductors = 70%

Wire =

__15__=__15__.71 x .7 .497

Wire = 30.18 amps (#10 THHN*) good for 40 amps

Problem #2: What is the ampacity of two #10* TW aluminum conductors feeding a 12 amp 230 Volt, single-phase motor, with an ambient temperature of 43°C. ?

Amps = Wire x ATCF x NC

Wire = 25 (T310-15 (b)(16)) #10 auminum = 25 amps

ATCF = .71 (T310-15 (b)(2)(a)) #10 TW aluminum @ 43°C.

NC = does not apply

Amps = 25 x .71

Amps = 17.75 amps (#10 TW aluminum)