9.2 Feeders, Load Calculations & Demand Factors
Key Takeaways
- A feeder carries power from the main service (or a distribution point) to a subpanel or another distribution point; a branch circuit is the final run from a panel to outlets or fixed equipment.
- Total connected load is calculated by summing every individual load (in VA or watts) served by a feeder or service.
- Not every connected load operates simultaneously at full nameplate capacity, so codes allow a demand factor — a percentage less than 100% — to be applied to certain load categories when sizing a feeder.
- Applying a demand factor lets a feeder be sized smaller than the full sum of all connected nameplate loads, reflecting realistic simultaneous usage rather than a worst-case sum.
- Demand-factor style calculations are illustrative of the method tested on the RME exam; specific percentages must always be verified against the applicable PEC table rather than assumed.
Sizing a feeder correctly requires two things working together: knowing exactly what a feeder is versus a branch circuit, and understanding how demand factors let a designer size that feeder based on realistic usage rather than a worst-case sum of every connected load running at once. Both concepts are core to PEC Part 1 load-calculation questions on the RME exam.
Feeders vs. Branch Circuits
A feeder is the set of conductors that carries power from the main service (or an upstream distribution point) to a subpanel, distribution board, or another downstream distribution point. A feeder does not directly supply outlets, fixtures, or equipment — it supplies a panel, which then splits the load out further.
A branch circuit, by contrast, is the final run of conductors between the last overcurrent protective device (a breaker in a panel) and the outlets, lighting fixtures, or fixed equipment actually using the power.
| Characteristic | Feeder | Branch Circuit |
|---|---|---|
| Where it starts | Main service or upstream distribution point | The last OCPD (panel breaker) |
| Where it ends | A subpanel or distribution point | Outlets, fixtures, or fixed equipment |
| What it supplies | An entire panel's worth of downstream branch circuits | A single circuit's actual loads |
| Typical sizing basis | Total connected load of everything downstream, adjusted by demand factors | The individual load(s) on that one circuit, adjusted for continuous-load rules (see Section 9.1) |
A simple way to remember the distinction: power flows from service, through feeders, into panels, out through branch circuits, to the load. A large commercial building might have a main service feeding a main distribution feeder, which feeds several subpanel feeders, each of which feeds dozens of branch circuits.
Total Connected Load
Before any demand factor is applied, the designer must first establish the total connected load — the sum of every individual load that a feeder or service is expected to serve, expressed in volt-amperes (VA) or watts. This means literally adding up: general lighting loads (often calculated per unit area using a VA-per-square-meter or VA-per-square-foot allowance), receptacle outlet loads, fixed appliance and equipment nameplate loads, and motor loads, among others.
This total connected load is the starting point — the largest possible number the feeder could theoretically ever need to carry if absolutely everything ran at full nameplate rating simultaneously. In real buildings, that scenario almost never happens, which is exactly the problem demand factors are designed to solve.
Demand Factors: Sizing for Reality, Not Worst Case
A demand factor is a percentage, less than 100%, that codes permit designers to apply to certain categories of connected load when calculating the load a feeder must actually be sized to carry. The underlying logic is straightforward: not every light fixture, receptacle, and piece of equipment in a building draws its full rated current at the exact same moment. Occupants turn lights on and off at different times, not every receptacle has something plugged in and running simultaneously, and multiple motors in an installation rarely all start and run at full load together.
If a feeder had to be sized for 100% of every connected load's nameplate rating, added straight across the board, the resulting conductors, panels, and OCPDs would be dramatically oversized — more copper, larger conduit, bigger breakers, and higher cost, all to cover a combined-peak scenario that essentially never occurs in practice. Demand factors correct for this by letting certain load categories (commonly, general lighting loads, receptacle loads above a certain connected total, and multiple-motor installations, among others defined in the applicable PEC demand-factor tables) be counted at less than their full connected value once the total exceeds specified thresholds.
It is important to be precise here: the exact demand-factor percentages and thresholds are defined in PEC demand-factor tables for specific load categories, and those tables must be consulted directly rather than assumed or memorized incorrectly — this section teaches the method and reasoning, not a specific quoted percentage as an authoritative figure.
Worked Example: Applying a Demand Factor Conceptually
To see the mechanism at work, walk through a simplified, illustrative scenario — a typical demand-factor style calculation, not a quotation of an exact PEC figure:
| Step | Description | Illustrative Value |
|---|---|---|
| 1. Total connected general lighting load | Sum of all lighting loads calculated for the building or area | 40,000 VA (illustrative) |
| 2. Applicable demand factor for this load category and quantity | A percentage less than 100%, taken from the applicable demand-factor table for the load type and quantity involved | Illustrative reduced percentage (verify actual value against the applicable PEC table) |
| 3. Feeder-sizing demand load | Total connected load x applicable demand factor | A value smaller than the 40,000 VA connected total |
| 4. Feeder conductor/OCPD sizing | Based on the reduced demand load (Step 3), not the full 40,000 VA connected total | Smaller conductors and OCPD than a full-sum approach would require |
The critical exam-relevant takeaway is the direction and mechanism of the calculation: connected load is calculated first by simple addition, the applicable demand factor is then applied to reduce that figure for feeder-sizing purposes (never to increase it), and the feeder is sized to the resulting, smaller demand load rather than the raw connected-load sum. A demand factor can never be used to size a feeder for less than what the applicable code table specifies as the minimum demand load — it is a codified reduction, not an arbitrary designer discretion.
Why This Distinction Matters on the Exam
RME exam questions on this topic typically test whether a candidate understands that: (1) branch circuits are generally sized on their own connected/continuous load per Section 9.1's 125% rule, without demand factors applied at that level, while (2) feeders serving multiple branch circuits or multiple loads are where demand factors come into play, because the feeder is aggregating many loads that statistically will not all peak simultaneously. Confusing these two levels — for example, trying to apply a demand factor to a single dedicated branch circuit serving one continuous load — is a common wrong-answer trap on load-calculation questions.
What is the key structural difference between a feeder and a branch circuit?
Why are demand factors permitted when sizing a feeder, rather than requiring the feeder to carry the full sum of every connected load's nameplate rating?
In a typical demand-factor style calculation for feeder sizing, what is the correct order of operations?
An RME exam question describes a single dedicated branch circuit serving one continuous 20A lighting load, and asks whether a demand factor should be applied when sizing that circuit's conductors. What is the correct reasoning?