Mike Dolt Enterprises
Mike Dolt Enterprises
User: 123ozzie, Location: chicago illinois
Asked on: Wednesday, December 11, 2024 at 6:23 pm EST
Answered on: Thursday, March 6, 2025 at 6:11 am EST
Question: For some reason I have a mental block with this topic. 200 amp main breaker, 3 phase panel. Each phase has 10 separate identical resistive loads, drawing 4 amps a piece, thus 40 amps on each phase. Not taking into consideration the 80% rule, I can safely add a 3 phase resistive load totally 160 amps (200-40), correct?
Yes, your reasoning is correct, not taking the 80% rule into account.Here's a breakdown: Panel Capacity
200 amps per phase.
Existing Load
40 amps per phase.
Available Capacity (ignoring 80% rule)
200 amps (panel) - 40 amps (existing) = 160 amps per phase.
Therefore, you can add a 3-phase resistive load totaling 160 amps, equally distributed across the three phases.Important Considerations - The 80% Rule (Continuous Load)
Your calculation is technically accurate in a purely theoretical sense.
However, electrical codes (like the NEC in the US) typically require derating breakers and conductors for continuous loads. A continuous load is one that operates for 3 hours or more at a time. 80% Rule
The general rule of thumb is that you should not load a breaker to more than 80% of its rated capacity for continuous loads.
Applying the 80% Rule
200 amp breaker 0.80 = 160 amps usable for continuous loads Available capacity = 160 amps (usable) - 40 amps (existing) = 120 amps
Therefore, you can add a 3-phase resistive continuous load totaling 120 amps, equally distributed across the three phases.Other Important Notes
Local Codes
Always consult your local electrical codes and regulations. They may have specific requirements or variations on the 80% rule or other relevant factors.
Load Calculations
A proper load calculation is essential for any electrical work. This calculation considers all connected loads, demand factors (which reduce the load for certain types of equipment), and other factors to ensure the electrical system is adequately sized.
Professional Electrician
It's highly recommended to consult with a qualified electrician for any electrical work. They can perform a load calculation, ensure compliance with local codes, and ensure the work is done safely.*
Neutral Load
You mentioned resistive loads which simplifies things. If you had non-linear loads (like some electronic devices) you'd also need to consider the neutral conductor and its capacity, as harmonics can significantly increase the current on the neutral in a 3-phase system.
In summary, while your basic math is correct, ignoring the 80% rule is a dangerous oversight. Always factor in the 80% rule (or the appropriate derating factor per your local code) when determining the safe capacity of a breaker or conductor. Flag for review