Cable Ampacity: What is it all about?

FACTS CHECKED BY  Jose George​

Cable selection comes with a lot of factors to consider. The key factor is the ampacity rating of the wire. Establishing the highest current power cables can take up without destroying their electrical properties. If you want to select a wire and are unsure of its ampacity, the NEC (National Electrical Code) has you sorted. It has the recommended ampacity tables that share the allowable ampacity of insulated conductors. That said, we will go further and find out what the cable ampacity is all about.

Table of Contents

What is Cable Ampacity?

Cable ampacity is the capacity of current that a conductor safely carries without exceeding its temperature rating. In other words, it’s the current-carrying capacity. The calculation of ampacity is quite simple, and measurement is in Amperes or Amps.

There are two primary components of wires:

  • Copper conductor 
  • Wire insulation.

As currents maneuver through the conductor, they cause a voltage drop and generate heat. If the heat overpowers the cable rating, it could fail. Therefore, it’s best to dissipate the heat outside the cable. 

If you don’t dissipate the heat outside the cable, the cable’s temperature will continue to increase, exceeding its temperature rating. In the end, the cable would deteriorate. 

The National Electrical Code(NEC) published cable ampacity tables to keep electrical cables safe from failures to show the maximum currents allowed for different wire sizes and applications. The Institute of Electricals and Electronics Engineers (IEEE) also provides the same. 

Tip: IEEE 399, ICEA P-54-440, NEC NFPA 70 current derating ensures cables have the maximum operation and are secure

cable tray with electrical wiring arranged

cable tray with electrical wiring arranged

Temperature Ratings of Cables

There are three most common temperature ratings. They are 60ºC, 75ºC, and 90ºC. However, 90ºC is the most common conductor rating temperature. Surprisingly, the conductors can rate up to higher temperatures of 1200ºC for special wires and larger power cables. 

The material used for insulation and the jacket of the cable will affect the temperature rating. A material with high heat resistance will survive in higher temperatures.

According to the NEC 2014 Article 310 ampacity tables, electrical engineers use a 75ºC temperature rating because many connectors rate 75ºC. 

Derating Factors of Cables

Electrical engineers perform derating to reduce the cable’s current carrying capacity. They do this by accounting for all factors that could increase temperatures during power cable installation. The idea is to prevent cable damage and system loss. 

There are two major requirements for derating ampacity in line with NEC. First is the ambient temperatures, and second is the number of current-carrying conductors. 

If more than three current-carrying conductors are in a raceway, the ampacity rating needs derating to control heat flow. When these cable types are closer, they cannot dissipate heat. As a result, there will be an increase in ambient temperature.

In hot climatic regions, the soil has high ambient heat. Installing a cable in these soils reduces heat dissipation. As a result, the cables will become hotter. As such, the cables will require derating to minimize current-carrying capacity. In simpler terms, current rating.

Not all conductors carry current. Therefore, carry out derating depending on the number of current-carrying conductors.  

Maximum Ampacities for Wire

The table below shows the allowable ampacities of insulated conductors at rates between 0 to 2000 volts, 60 degrees celsius to 90 degrees Celsius, and not more than three carrying conductors in raceway, cable, or earth. 

Based on ambient temperatures of 30 degrees Celsius

Wire Size (AWG or Kcmil)Copper Conductor temp rating Aluminum Conductor Temp rating
Types TW, UFTypes RHW, THHW, THW, THWN, XHHW, USE, ZWTypes TBS, SA, SIS, FEP, FEPB, MI, RHH, RHW-2, THHN, THHW, THW-2,  THWN-2, USE-2, XHH, XHHW, XHHW-2, ZW-2Types TW, UFTypes RHW, THHW, THW, THWN, XHHW, USE, ZWTypes TBS, SA, SIS, FEP, FEPB, MI, RHH, RHW-2, THHN, THHW, THW-2,  THWN-2, USE-2, XHH, XHHW, XHHW-2, ZW-2
60ºC (140ºF)75ºC (167ºF)90ºC (194ºF)60ºC-(140ºF)75ºC (167ºF)90ºC (194ºF)
14*202025
12*252530202025
10*303540253035
8405055304045
6556575405060
4708595556575
385100110657585
2951151307590100
111013015085100115
1/0125150170100120135
2/0145175195115135150
3/0165200225130155175
4/0195230260150180205
250215255290170205230
300240285320190230255
350260310350210250280
400280335380225270305
500320380430260310350
600355420475285340385
700385460520310375420
750400475535320385435
800410490555330395450
900435520585355425480
1000  455545615375445500
1250495590665405485545
1500520625705435520625
1750545650735455545615
2000560665750470560630

According to NEC, no overcurrent protection device shall exceed 15A for 14 AWG copper, 20A for 12 AWG copper, and 30A for 10 AWG copper.

Ambient temperatures exceeding 30 degrees celsius

Multiply the allowable ampacity as shown in the above table with the correction factors below. 

Ambient Temperatures Copper Conductors Aluminium Conductors 
21-25ºC
69-77ºF
1.081.051.041.081.051.04
26-30ºC
78-86ºF
111111
31-35ºC87-95ºF0.910.940.960.910.940.96
36-40ºC96-104ºF0.860.880.910.860.880.91
41-45ºC
105-113ºF
0.710.820.870.710.820.87
46-50ºC114-122ºF0.580.750.820.580.750.82
51-55ºC
123-131ºF
0.410.670.760.410.670.76
56-60ºC
132-140ºF
0.580.71 0.580.71 
61-65ºC
141-149ºF
0.470.650.470.65
66-70ºC
150-158ºF
0.330.580.330.58
71-75ºC
159-167ºF
0.500.50
76-80ºC
168-176ºF
0.410.41
81-85ºC177-185ºF0.290.29

These ampacity charts guide electrical professionals toward the right cable gauge and length. These charts are a plus for professionals in the welding sector. They must use proper cable gauge size for their safety and deliver high-quality welding.

So, cable experts advise choosing the required welding cable to achieve this. Failure to do so will cause excessive heat absorption, further damaging the welding equipment. 

high voltage wire for electrodes and welding

high voltage wire for electrodes and welding

Conclusion

Knowing cable ampacity is essential when choosing a cable. It will help you know the cable size during installation. On that note, always check each conductor’s NEC and IEEE allowable ampacity before proceeding. You can also read the electrical cable gauge chart to get more details on wire sizes.

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