You're referring to different methods for calculating heating loads in buildings, likely within the context of electrical panel sizing or HVAC design. Let's break down why the percentages differ

Standard Method (or "Demand Factor" Method with 100% nameplate)
Concept

This is generally a simpler, more conservative approach. It assumes that all connected equipment will operate at its full nameplate rating simultaneously.

Why 100% Nameplate

The idea is to cover the worst-case scenario. If everything connected to a circuit or panel runs at full power at the same time, the system needs to be able to handle it. This method prioritizes safety and avoids potential overloads.

When Used

Often used for residential applications or situations where there's less data available about actual usage patterns. It's also common when dealing with critical equipment where you absolutely cannot risk undersizing.

Optional Method (typically with a 65% demand factor) Concept

The optional method is more sophisticated. It recognizes that in many buildings, not all connected equipment will operate at 100% capacity simultaneously. It uses a demand factor, which is a percentage, to reduce the calculated load based on real-world usage patterns.

Why 65% (or another percentage)

This percentage (often 65% for general-use receptacles in certain applications) is based on statistical analysis and empirical data of how electrical loads are actually used in buildings. It assumes that, on average, general-use receptacles are not constantly drawing their maximum rated power. The specific percentage varies based on the type of load, occupancy, and applicable codes or standards. For example, heating appliances like stoves or clothes dryers might have different demand factors.

When Used

More common in commercial or larger residential buildings where detailed load calculations can be justified. It allows for potentially smaller panel sizes and lower installation costs without compromising safety, as long as the assumptions about usage are accurate.

More nuanced Demand Factors

The optional method doesn't just use 65% across the board. It may use different demand factors for different types of loads (lighting, HVAC, appliances, etc.), based on their expected usage patterns. This allows for a more accurate representation of the overall electrical demand.

In summary



Standard (100%)

Simple, conservative, assumes everything runs at full power simultaneously.

Optional (with demand factors)

More complex, uses historical data or load profiles to estimate actual usage patterns, allowing for reduced calculated loads.

Important Considerations



Applicable Codes and Standards

Always refer to the relevant electrical codes (like the NEC in the US) for specific requirements and accepted demand factors. These codes dictate which methods are permitted and what demand factors should be used for different applications.

Accuracy of Assumptions

The optional method relies on accurate assumptions about load usage. If your building has unusual usage patterns (e.g., a lab with many constantly running high-power instruments), using a low demand factor could be dangerous. Therefore, careful analysis and potentially load monitoring are important when using the optional method.

Safety Margin

Even with the optional method, a safety margin is typically included to account for unexpected increases in load or future expansion.

Example to illustrate the difference

Imagine a small office with 10 general-use receptacles, each rated for 15 amps at 120 volts (1800 watts).

Standard Method

10 receptacles 1800 watts = 18,000 watts. You'd size the circuit based on this worst-case scenario.

Optional Method (65%): 18,000 watts 0.65 = 11,700 watts. You'd size the circuit based on this lower, more realistic load estimate.The optional method recognizes that it's highly unlikely that all 10 receptacles will be drawing their full 1800 watts simultaneously.Therefore, the difference in percentages is about the design philosophy and the level of detailed analysis used in load calculations. The standard method is a safe, straightforward approach, while the optional method aims for a more optimized and cost-effective design. Always follow the requirements of your local electrical code. Flag for review