Mike Dolt Enterprises
Mike Dolt Enterprises
User: Tainted, Location: New York
Asked on: Saturday, July 19, 2025 at 11:22 pm EDT
Answered on: Saturday, July 19, 2025 at 11:22 pm EDT
Question: If I understand how to size wire and overcurrent protection if 1 circuit contains only motors. But what if it contains other loads like a heater for example.Suppose I have one 25HP motor (74.8A FLC), one 10HP motor (30.8 FLC) and one 5kW heater (13.9A). All loads are continuous at 208V 3phase.minimum wire size would be:74.8(1.25) +30.8 + 13.9(1.25) = 141.7 amps -> 1/0 copperFuse size would be:74.8(1.75) + 30.8 + 13.9(1.25) = 179 amp ->175 amp fuse?Since a heater is associated with the circuit, can we use 1/0 copper for the circuit or must it be 2/0?
You're on the right track! Let's break down the calculations and address the heater considerations.
Wire Sizing (NEC 430.24 & 424.3(B)) Motor Load Calculation
Largest Motor: 74.8A 1.25 = 93.5 A Other Motors: 30.8 A Heater: 13.9 1.25 = 17.375 A Total calculated amps: 93.5 + 30.8 + 17.375 = 141.675A
Minimum Ampacity
This is your minimum required ampacity for the conductors. You correctly calculated 141.675A.
Wire Size
According to NEC Table 310.16 (assuming you're using THHN copper conductors in a typical raceway), 1/0 copper is rated for 150A at 75°C. Since 150A is greater than 141.675A,
1/0 copper is acceptable.Overcurrent Protection (NEC 430.53 & 424.3(A))
Here's where it gets a little more nuanced. You cannot simply add the motor protection and heater protection calculations together. You need to protect the conductors first, then the components. Motor Branch Circuit Short Circuit and Ground Fault Protection
Largest Motor: 74.8A 1.75 = 130.9A Smaller Motors + Heater: 30.8A + 17.375A = 48.175A Total: 130.9A + 48.175A = 179.075A Next lower standard fuse size: 175A
Heater Protection
Since your heater is a continuous load, it must not be loaded in excess of 80% of the branch-circuit rating, and the branch circuit must be rated no less than 125% of the continuous load. If a single overcurrent device protects both, the heater is protected at that overcurrent protection. 175A.8 = 140A so the heater is protected since 140A > 13.9(1.25) = 17.375A.
Answer
You can use the 1/0 for this circuit if 175 amp fuse is the correct size for the largest motor.
Important Considerations and Safety Notes Continuous Loads
You correctly identified the heater as continuous. Remember that any load operating for 3 hours or more is considered continuous.
Voltage Drop
Always calculate voltage drop, especially on longer runs. This is a separate calculation from ampacity and is often overlooked. Significant voltage drop can damage motors and cause other equipment to malfunction.
Coordination
Ideally, your overcurrent protection (fuses or circuit breakers) should be coordinated to allow smaller overloads to be cleared without tripping the main overcurrent device.
Local Codes
Always check your local electrical codes. They may have amendments or stricter requirements than the NEC.
Qualified Electrician
This information is for educational purposes only. Electrical work is dangerous and should only be performed by qualified and licensed electricians. A licensed electrician will ensure that all work is done safely and in compliance with applicable codes.In summary, your initial calculations were very close. Make sure to consider all factors and consult the NEC and local codes to ensure a safe and compliant installation. The important thing to remember is that the wire must first be sized to carry the load. After that, the overcurrent protection devices should be sized to the motors and components. Flag for review