Mike Dolt Enterprises
Mike Dolt Enterprises
User: kjroller, Location: Dawson Mn
Asked on: Wednesday, April 2, 2025 at 1:05 pm EDT
Answered on: Wednesday, April 2, 2025 at 1:05 pm EDT
Question: I have a 75 kva generator no fla is listed so took the kva and converted it to amps with the equation so roughly 90.3 amps. I then multiplied it by 1.15 via nec section 445 so i got 103.75 roughly. Im gonna use 75 degreed column cause thats what lugs rated for. At 1 awg thats good for 130 amps but with 4 to 6 conductors I derated to 104 amps so 1 awg should be good for the 75kva generator anyone see anything wrong with this or is this correct?
Your general approach is correct, but let's break down each step and address some common areas where mistakes can occur
1. Calculating Full Load Amps (FLA):
Your Calculation
You correctly used the formula for three-phase power:
1000 / (Volts √3) = Amps You assumed the voltage to be 480v (208v would also work for smaller generators). If so the the calculation would be as follows.75 1000/(480√3) = 90.2 ampsKVA
2. NEC 445.13 - Overcurrent Protection
115% Multiplier
You are using the correct NEC section (445.13) and the correct multiplier (1.15 or 115%). This is to determine the minimum conductor ampacity.
Your Calculation
90.2 amps 1.15 = 103.73 amps.
3. Conductor Ampacity and Temperature Correction/Derating
75°C Rating
You are correct to use the 75°C ampacity column for conductors terminated at equipment (like the generator) that is marked and listed for 75°C terminations. This is usually the case, but ALWAYS verify the equipment markings.
1 AWG Ampacity (Base)
You are correct that 1 AWG copper THHN (or similar insulation) is rated for 130 amps in the 75°C column of NEC Table 310.16.
Derating for 4-6 Conductors in a Raceway
This is where you need to be very careful. NEC Table 310.15(C)(1) (formerly 310.15(B)(3)(a)) requires you to derate the ampacity for more than three current-carrying conductors in a raceway or cable. For 4-6 conductors, the derating factor is
80%. Your Derated Ampacity
130 amps (base ampacity) 0.80 (derating factor) = 104 amps.
Is 1 AWG Sufficient? Yes, based on your calculations, 1 AWG conductors, properly installed and derated, would be sufficient to carry the required 103.73 amps.
Things to Double-Check and Consider
Voltage
Extremely Important. Confirm the output voltage of the generator. The FLA will be different for 480V, 208V, or other voltages. You must use the correct voltage in your calculations.
Neutral Conductor
Is the generator supplying significant non-linear loads (e.g., computers, electronic equipment)? If so, the neutral conductor might carry substantial harmonic currents, and you may need to upsize it. Consult NEC 220.61 for guidance on neutral conductor sizing.
Grounding
Ensure proper equipment grounding conductor sizing according to NEC 250.122. This is separate from the phase conductors and neutral.
Manufacturer's Instructions
Always consult the generator manufacturer's instructions. They may have specific requirements for conductor sizing, overcurrent protection, and installation. Follow their recommendations first, as they may be stricter than the NEC minimums.
Overcurrent Protection
You calculated the minimum conductor ampacity. You also need to choose the appropriate overcurrent protection device (circuit breaker or fuses) based on the NEC and the generator manufacturer's recommendations.
Summary and Recommendations
Your calculations seem reasonable assuming you have used the correct voltage and are accounting for all relevant factors. However, electricity is dangerous, and it's always best to:1.
Consult the Generator's Documentation
This is the most important step.2.
Verify Voltage
Double-check the generator's voltage rating.3.
Recalculate FLA
If necessary, recalculate the full-load amps (FLA) using the correct voltage.4.
Consider the Neutral
Evaluate the potential for harmonic currents on the neutral conductor.5.
Ensure Proper Grounding
Size the equipment grounding conductor according to NEC 250.122.
6.
Have a Qualified Electrician Review Your Work
This is highly recommended. A licensed electrician can ensure that the installation is safe, code-compliant, and meets the specific requirements of your application.7.
Verify what ampacity the overcurrent device is rated for if its less then 103.73 you will have to downsize the overcurrent device as well as the conductors to meet code requirements.Disclaimer: The information above is for general educational purposes only. Always consult with a qualified and licensed electrician for any electrical work. Flag for review