Hello all. Just did my second mock inspection at a manufactured home. The main exterior service panel is rated at 150 amps with breakers adding up to 110 amps. It’s correctly fed with 2/0 aluminum wires. Neutrals and grounds are bonded at this panel. The breakers amperage total is 110 amps.
The interior sub panel is rated at 200 amps with a 200 amp breaker. It’s correctly fed from the main service panel with 4/0 aluminum wires. The neutrals and grounds are correctly not bonded in this sub panel. Adding up all the breakers in the sub panel is 440 amps.
As far as what I’ve learned in their pre-licensing courses this is a no- no.
Am I missing something?? Help would be much appreciated.
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Yes, don’t add up the amp ratings of each breaker in the panel.
AI Overview
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No, adding up the amperage ratings of individual breakers in a panel does not indicate the panel’s total capacity. The panel’s capacity is determined by its main breaker or the upstream electrical service, and it’s designed to handle a certain maximum load, not the sum of all individual circuits.
Here’s why:
- Panel Capacity:
The panel’s capacity is determined by its main breaker (or the service entrance breaker), which limits the total amount of current that can flow into the panel according to a home improvement forum.
- Individual Breakers:
Individual breakers are sized to protect the wiring of specific circuits. They trip if the current draw on that circuit exceeds their rating.
- Load Calculation:
The total amperage of all individual breakers can easily exceed the panel’s capacity because it’s assumed that not all circuits will be running at maximum capacity simultaneously.
- Example:
A 200-amp panel may have individual breakers that add up to 300 or even 400 amps, but the panel is still limited to 200 amps by the main breaker, according to a Reddit thread.
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As Kevin said, don’t add up the breakers. Would you mind sharing the section or text in their pre-licensing course that advises you to add up the breakers? Let’s take a look at what it says. BTW, who is their?
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This 
You need to look at what the panel is rated for, the info will be on the label inside the panel somewhere, and if the feeders & disconnect for that panel is in line with the manufacturer’s rating..
if the breaker/disconnect at the service panel is rated less than the sub panel’s main breaker/disconnect, the sub would have plenty of over current protection anyway so it shouldn’t be an issue.
Maybe @rmeier2 can explain it better…
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The total of all of the amp ratings of all of the circuit breakers is basically meaningless and cannot be used to determine if the service is properly sized. A load calculation is required to determine the minimum size of a service which is beyond the scope of a home inspection.
Is there a circuit breaker in the service for the sub-panel feeder?
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The main service panel has 150 amp main breaker. The sub panel has a 200 amp main breaker.
Thanks Kevin,
I’m at the end of the internachi training and my brain is overloaded. As soon as I read your response I remember learning about that. That’s what mocked inspections are about to get some of the kinks out.
Thanks a million!
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Their …was a typo. As I stated in another response my brain is in overload from all the training. I’m not sure where adding up all the breakers came from but it’s been deleted from the gray matter.
It’s encouraging to know there’s help when needed. Thanks to all!
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Hey Robert,
The forum has helped me realize my goof. Thanks for the reply!
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Just realize regardless of the #4/0 feeder conductor size or the 200 amp OCPD in that sub-panel that the sub-panel has a capacity no larger than the 150 amp service disconnect.
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Hi Thomas
Adding the total of all breaker ratings is totally wrong. Service mains are sized using a calculation set forth by the NEC. Most of the circuits are derated due to the fact that all circuits in the home will not be on at the same time at 100% rating. I would encourage you to review appendix D of the NEC (Calculation examples for single family homes).
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That statement is not true. It doesn’t make sense. Why would anything be derated based on anything not being used? Derating is done in situations where heat dissipation is impeded, thus current capacity is diminished. Examples of conditions where derating is used are where there is a high ambient temperature, many conductors in a conduit, or inductive coupling.
Demand is calculated based on the likelihood of certain loads being active at any given time. Demand calculations take into account such things as non-coincident loads. A example of non-coincident loads is an electric furnace and an air conditioner. The presumption is that the electric furnace and air conditioner would not be in use simultaneously.
The calculated demand is called the Demand Factor. The standard industry terms are Demand Factor and Permissible Reduction. Demand Factor is a completely different concept from Derating. The concepts of Calculated Demand and Permissible Reduction are the opposite of Derating. instead of the size of a service being increased because of Derating, it is decreased because of the Demand Factor (Permissible Reduction). In other words, when things are Derated, they have to be larger, not smaller, to account for the diminished heat dissipation capacity.
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Whose course, and which course, are you referring to? A service size is not determined by adding up all the connected loads. If the course said that the total of the connected loads exceeding the rating of the service disconnect is wrong then the course is wrong.
I may have used the wrong terminoligy but both derating factors and demand factors are used in determining branch circuit and service sizing in the electrical design.
Applying a demand factor in branch circuit calculations is a method used to effectively “derate” the required capacity of the circuit components based on the realistic expectation that not all connected loads will operate at their full capacity simultaneously. [1, 2]
Derating and demand factors are both used to reduce the calculated electrical load for design purposes, acknowledging that a system will not operate at its maximum capacity. The main similarity is that both are multipliers less than one that reduce a total load to a more realistic value for sizing components like wires and breakers. They are not the same, however; a demand factor accounts for the fact that not all devices are used at once, while a derating factor adjusts for environmental conditions or how a cable is installed. [1, 2, 3, 4, 5]
While the National Electrical Code (NEC) uses the specific term “demand factor” for load calculations (primarily in Article 220) and reserves “derating” or “adjustment factors” for physical conditions affecting conductor ampacity (Article 310.15), the underlying principle for both is a reduction from the theoretical maximum value to a practical, safe working value. [3, 4, 5]
Demand Factor vs. Derating (Adjustment) Factor
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Demand Factor is a ratio (always 1) applied during electrical load calculations to estimate the maximum probable load of a system based on anticipated usage patterns. For example, the NEC allows demand factors for electric ranges because it is unlikely all burners and the oven will be at maximum heat at the same time. This allows the designer to size the feeder and overcurrent protection for a lower, more realistic current than the sum of all nameplate ratings, which in essence is a form of load reduction for design purposes.
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Derating (Adjustment/Correction) Factor refers to the reduction of a conductor’s ampacity (current-carrying capacity) to compensate for adverse physical conditions, such as:
Summary
In the context of design and calculation, the use of a demand factor achieves a result similar to derating: it allows the use of smaller (more cost-effective and optimally sized) conductors and protective devices than would be required if every load had to be supplied at 100% simultaneously. This distinction in terminology is important for adhering to the specific rules in the NEC articles: use demand factors for load calculations (Article 220) and derating/adjustment factors for conductor ampacity considerations (Article 310). [2, 11, 12, 13]
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AI has determined its ‘correct’ information from: a home improvement forum.
. . .and the load calculation from a Reddit thread.
Somehow that doesn’t seem like the best sources!
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Instead of demand factor a better term might be load diversity in this context.
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Load diversity is a phrase commonly used by utility companies. It means exactly the same thing as Calculated Demand, Demand Factor, or Permissible Reduction. The most common size transformer for residential service is 25kVA (104A at 240V). Utility companies will use a single 25kVA transformer to supply as many as four homes (26A each) based on calculated load diversity even though the connected load (services) may be 800A (4x200A). If the individual circuits (connected loads) were added up, the total could easily exceed several thousand Amps.
They certainly are not! As I’ve said many times in this forum and elsewhere, I often ask AI questions that I already know the answers to. My questions most often are related to the electrical fields. First iteration answers are wrong far in excess of 50% of the time. It is only after multiple successive iterations that answers are correct or nearly correct. A good example is the recent discussion on this forum of the misuse of the term “Isolated”. A member used AI as his defense of the misuse of “Isolated”. I went deeper using the same AI and the AI acknowledged that it had provided inaccurate information. My conversation with the AI is in a link I provided elsewhere in this forum.
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Once again, you are wrong. Derating has nothing to do with reducing the calculated electrical load. Derating is an expression of diminished current carrying capacity. With respect to conductor sizes, as an example, derating because of diminished current carrying capacity (aka Ampacity), could require a larger conductor, whereas demand factor is an expression a smaller presumed load, therefore requiring a smaller conductor. Derating and Permissible Reduction (aka Demand Factor) are in effect the opposite of each other.
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Where this all started from was someone asking if you total all of the amperage rating of all breakers to determine sevice main sizing. below is direct from the NEC!
Yes, both derating and demand factors are commonly used when calculating the size of a residential electrical service, but they serve slightly different purposes and are applied at different stages. I’ll break it down step by step based on standard practices from the National Electrical Code (NEC) in the U.S. (specifically Article 220 for load calculations and Article 310 for conductor sizing). Note that local codes may vary, so always consult a licensed electrician or your local authority for specific projects.
1. Demand Factors
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What they are: Demand factors are multipliers (typically less than 100%) applied to various loads in a home to account for the fact that not all appliances or circuits will be running at full capacity simultaneously. This prevents over-sizing the service while ensuring it’s adequate for realistic usage.
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How they’re used in residential service sizing:
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You start by calculating the total connected load (e.g., lighting, appliances, HVAC, EV chargers, etc.).
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Apply demand factors from NEC Table 220.42 (for lighting and receptacles) or other sections (e.g., 220.50 for motors, 220.56 for kitchen equipment).
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For example:
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General lighting and receptacles: First 10 kVA at 100%, then the remainder at 35–50% depending on the size.
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Fixed appliances: Often 75% for four or more.
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The result is the “calculated load,” which determines the minimum ampacity needed for the service (e.g., 100A, 200A, or 400A service).
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Why they’re important: They make the service size practical and cost-effective without compromising safety.
2. Derating Factors
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What they are: Derating (or adjustment factors) reduce the ampacity rating of conductors or equipment based on environmental or installation conditions to prevent overheating.
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How they’re used in residential service sizing:
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After calculating the demand-adjusted load, you select service entrance conductors (e.g., SER cable or individual wires).
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Apply derating if conditions warrant it, per NEC Tables 310.15(B)(2) for ambient temperature or 310.15(B)(3) for more than three current-carrying conductors in a raceway or cable.
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For example:
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If conductors are in a hot attic (ambient >86°F/30°C), derate by a factor like 0.87.
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If bundled with many wires, derate to 80% or less.
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This ensures the conductors can safely carry the calculated load under real-world conditions.
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Note: Derating is more situational and might not always apply in a standard residential setup (e.g., if wires are in a cool, unbundled run). It’s separate from demand factors but builds on them.
Overall Process for Sizing a Residential Service
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Calculate total connected loads (e.g., square footage for lighting at 3VA/ft², plus appliances).
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Apply demand factors to get the adjusted service load.
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Select service size (e.g., panel rating) based on that load.
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Choose conductors and apply derating if needed to ensure ampacity meets or exceeds the load.
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Factor in overcurrent protection (e.g., main breaker) and grounding.
Example
For a 2,000 sq ft home with standard appliances:
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Lighting/receptacles: ~6,000VA (after demand: ~3,600VA).
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Add appliances, HVAC, etc., with demand factors.
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Total calculated load: Say 150A.
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If derating applies (e.g., hot environment), you might need larger conductors than the minimum for 150A.
If you’re doing this for a specific project, I recommend using an NEC-compliant load calculation worksheet or software. If you provide more details about the home (e.g., size, appliances), I can help walk through a rough estimate! Always prioritize safety and professional advice.