Panelboard load calculation
Panelboard load calculation determines the maximum active or apparent power (demand load) that a distribution panelboard must supply. It is obtained from the sum of connected loads, applying standardized demand factors that reflect the probability of simultaneous use. The goal is to select a rated capacity for the panelboard, its busbars, and its overcurrent protective devices that is sufficient and economical, while complying with the safety requirements of the applicable code.
Calculation formula
Section titled “Calculation formula”S_dem = ( Σ S_connected_i × F_dem_i ) × F_coinc
| Variable | Definition | Unit |
|---|---|---|
| S_dem | Total demand load of the panelboard | kVA |
| S_connected_i | Connected apparent power of load i | kVA |
| F_dem_i | Demand factor for load i | dimensionless (0 to 1) |
| F_coinc | Coincidence factor between loads (if applicable) | dimensionless (0 to 1) |
When not all loads operate simultaneously, a global coincidence factor is used; otherwise, F_coinc = 1 is assumed. The demand factors depend on the load type and local regulations, typically taken from NEC Article 220 or IEC 61439-2.
Demand factors by load type
Section titled “Demand factors by load type”The typical values come from US practice (NEC) and are widely adopted in Latin America as a reference for commercial and industrial installations.
General lighting and receptacles
Section titled “General lighting and receptacles”| Load type | Connected load range | Demand factor |
|---|---|---|
| Residential general lighting | First 3000 VA / 3 kVA | 1.00 (100 %) |
| Residential general lighting | From 3001 VA to 120 kVA | 0.35 (35 %) |
| Residential general lighting | Over 120 kVA | 0.25 (25 %) |
| Commercial lighting | Total lighting load | 1.00 (100 %) up to 15 kVA; then 0.50 (50 %) for the excess |
| General-use receptacles (offices) | Up to 10 kVA | 1.00 (100 %) |
| General-use receptacles (offices) | Excess over 10 kVA | 0.50 (50 %) |
Electric motors
Section titled “Electric motors”| Motor rated power (kW / HP) | Typical nameplate current (A) | Applicable demand factor (NEC 430.24) |
|---|---|---|
| 0.37 kW / 0.5 HP | 2.4 A (single-phase 230 V) | 125 % of largest motor full-load current + 100 % of the rest |
| 0.75 kW / 1 HP | 6.9 A (three-phase 208 V) | 125 % of largest motor full-load current + 100 % of the rest |
| 3.73 kW / 5 HP | 16.7 A (three-phase 208 V) | 125 % of largest motor full-load current + 100 % of the rest |
| 18.65 kW / 25 HP | 74.8 A (three-phase 480 V) | 125 % of largest motor full-load current + 100 % of the rest |
| 37.3 kW / 50 HP | 65 A (three-phase 480 V) | 125 % of largest motor full-load current + 100 % of the rest |
Kitchen and laundry equipment
Section titled “Kitchen and laundry equipment”| Appliance | Typical connected power (kVA) | Demand factor |
|---|---|---|
| Household electric range | 8 – 12 kVA | 0.80 (80 %) for one unit, 0.65 for two |
| Clothes dryer | 5 kVA | 1.00 (100 %) or per nameplate |
| Commercial dishwasher | 3 – 6 kVA | 0.90 (90 %) |
Calculation procedure
Section titled “Calculation procedure”- List all loads connected to the panelboard, classifying them by type (lighting, receptacles, motors, special equipment).
- Assign the connected apparent power (S_connected) for each load in kVA, using nameplate values or standardized tables (e.g., 180 VA per receptacle in offices).
- Apply the corresponding demand factor (F_dem_i) according to the load type and accumulated quantity, in accordance with the design code.
- Calculate the partial demand load by summing the products S_connected_i × F_dem_i.
- Apply the coincidence factor if several sub-panels or groups of loads that do not operate simultaneously are supplied (typically 0.7 to 0.9 for distribution panelboards).
- Verify that the rated capacity of the panelboard (in A or kVA) is greater than or equal to the calculated demand load, and that the main protective device does not exceed the panelboard rating (NEC 408.36).
- Record the demand load, design current, and available short-circuit current on the panelboard label in accordance with NEC 408.6.
Reference codes and standards
Section titled “Reference codes and standards”- NEC 408.30 – Panelboard rating not less than minimum feeder capacity based on load calculations from Article 220.
- NEC 408.36 – Overcurrent protection of the panelboard: the device shall have a rating not greater than that of the panelboard.
- NEC Article 220 – Branch-circuit and feeder load calculation methods, including demand factors by occupancy type.
- IEC 61439-1/-2 – Low-voltage switchgear and controlgear assemblies, heating criteria and verification of rated current capacity.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the demand factor for general lighting in a single-family dwelling?
Section titled “What is the demand factor for general lighting in a single-family dwelling?”The first 3000 VA are taken at 100 % and the remainder up to 120 kVA at 35 %, per NEC Table 220.42.
What minimum rating should a typical residential main panelboard have in 120/240 V systems?
Section titled “What minimum rating should a typical residential main panelboard have in 120/240 V systems?”The panelboard rating must be at least equal to the calculated demand load; in North American single-family homes, a 100 A or 200 A panelboard is typically installed.
How much load is assigned to each general-use receptacle in offices?
Section titled “How much load is assigned to each general-use receptacle in offices?”A connected load of 180 VA is assigned per single or multiple receptacle, and a demand factor of 100 % up to 10 kVA and 50 % for the excess may be applied.
How is motor load determined in an industrial panelboard?
Section titled “How is motor load determined in an industrial panelboard?”Take 125 % of the full-load current of the largest motor plus 100 % of the current of the remaining motors, per NEC 430.24.
What do NEC standards require regarding fault current marking on the panelboard?
Section titled “What do NEC standards require regarding fault current marking on the panelboard?”In installations other than one- or two-family dwellings, the available short-circuit current and the date of the calculation must be marked on the enclosure with a durable label (NEC 408.6).
Is it mandatory to apply a demand factor to heating and air conditioning loads?
Section titled “Is it mandatory to apply a demand factor to heating and air conditioning loads?”A demand factor is only applied if the local code allows it; in the NEC, fixed heating and cooling equipment is considered at 100 % of nameplate current, without demand reduction.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/fans-efficiency-power-consumption-d_197.html
- mikeholt.com: https://www.mikeholt.com/newsletters.php?action=display&letterID=2860