This calculator helps you derate your cable to properly determine the wire size needed for a given ampacity (MCA). Enter the values below and click “Calculate” to see the derated ampacity. Use this value in our Wire Size Calculator to get the recommended wire size as per the Canadian Electrical Code (2021 CEC), CSA C22.1:21.

Wire Size Calculator

Ampacity Derating Calculator

Ampacity (A): *

Ambient Temperature:

Select Configuration:

Termination Rating:

Number of Conductors:

Free Air Conductor Spacing (Ignore if in a Raceway):

Ambient Temperature Correction Factor:

Number of Conductors Correction Factor:

Derated Ampacity (A):

Derating

The term “derating” refers to the reduction in the current-carrying capacity of conductors (often electrical wires) under specific conditions. One primary reason for derating conductors is to account for elevated ambient temperatures. Electrical conductors, when in use, generate heat. If the environment in which they operate is already hot, they may not dissipate this heat effectively, which can lead to a dangerous rise in conductor temperature. Overheated conductors not only degrade insulation faster but also pose a serious fire risk.

Another factor that necessitates derating is conductor bundling or the number of current-carrying conductors in a conduit. When multiple conductors run closely together, they don’t dissipate heat as efficiently as they would individually. As a result, the CEC and the National Electrical Code (NEC) requires conductors to be derated when more than three are bundled together, ensuring that the heat they generate doesn’t exceed safe levels. It’s essential for engineers and electricians to be knowledgeable about these derating factors to guarantee the safe and efficient operation of building electrical systems.

How to derate cables?

The CEC has a few tables used to derate your conductors:

Table 5A – Ambient temperature above 30°C
Table 5B – 4 or less free air cables spaced less than 25% of the largest cable diameter
Table 5C – 4 or more cables in a raceway
Table 5D – Free air cables with spacing between 25% and 100% of the largest cable diameter.

It’s important to note that if you have free air conductors and the spacing is above 100% of the largest cable diameter, than no correction factor is required. Simply use the wire size calculator as if it was single conductor.

Another item to note, is that if you have 5 or more free air conductors spaced less than 25% apart, you must use the ratings for conductors in raceways (Table 2 for copper or Table 4 for aluminum). For instance, you have 10-100A conductors installed in free air and spaced less than 25% apart. In our wire size calculator, simply select raceway configuration instead of free air.

Ambient Temp \ Termination Temp	60	75	90	105	110
35	0.91	0.94	0.96	0.97	0.97
40	0.82	0.88	0.91	0.93	0.94
45	0.71	0.82	0.87	0.89	0.9
50	0.58	0.75	0.82	0.86	0.87
55	0.41	0.67	0.76	0.82	0.83
60		0.58	0.71	0.77	0.79
65		0.47	0.65	0.73	0.75
70		0.33	0.58	0.68	0.71

Number of conductors	Correction Factor
1	1
2	0.9
3	0.85
4	0.8

Number of Insulated conductors	Ampacity correction factor
1-3	1
4-6	0.8
7-24	0.7
25-42	0.6
43 and up	0.5

Number of horizontal conductors:	1	2	3	4	5	6
1 Vertical Layer	1.00	0.93	0.87	0.84	0.83	0.82
2 Vertical Layers	0.89	0.83	0.79	0.76	0.75	0.74

Derating

How to derate cables?

Need to calculate the wire size? Click on the link below for a Wire Size calculator: