To calculate the load in order to find the minimum cable ampacity for loads with a lot of harmonics, you can use the following steps:1. Determine the harmonic content of the load. This can be done using a power quality analyzer or by consulting the manufacturer's specifications.2. Calculate the RMS current for each harmonic. The RMS current is the square root of the mean of the squared values of the current over a period of time.3. Multiply the RMS current for each harmonic by the harmonic order. This will give you the harmonic current contribution for each harmonic.4. Sum the harmonic current contributions for all harmonics. This will give you the total harmonic current (THC).5. Calculate the load current. The load current is the square root of the sum of the squares of the fundamental current and the THC.6. Apply a diversity factor. A diversity factor is a number that is used to account for the fact that not all of the loads will be operating at the same time. The diversity factor for VFDs is typically 0.8.
7. Multiply the load current by the diversity factor. This will give you the minimum cable ampacity.It's important to note that the NEC does not provide specific guidelines for calculating the minimum cable ampacity for loads with harmonics. However, the NEC does require that the neutral conductor be sized to carry the THC.## ExampleLet's say you have a VFD load with the following harmonic content:| Harmonic Order | RMS Current (A)

---|---

1 | 10

3 | 5

5 | 3

7 | 2

9 | 1 |To calculate the minimum cable ampacity, we would do the following:1. Calculate the THC: $$THC = \sqrt{5^2 + 3^2 + 2^2 + 1^2} = 6.71 \ A$$2. Calculate the load current: $$Load \ Current = \sqrt{10^2 + 6.71^2} = 11.9 \ A$$3. Apply a diversity factor: $$Minimum \ Cable \ Ampacity = 11.9 \ A \times 0.8 = 9.52 \ A$$Therefore, the minimum cable ampacity for this VFD load would be 9.52 A. Flag for review