Dwelling Detached Structure Load Calculator

A detached garage, workshop, storage building, or similar outbuilding may have lighting, receptacles, fixed equipment, and HVAC supplied from a feeder originating at the dwelling. The first planning question is whether the known connected load groups fit within the feeder capacity already selected or proposed.

The Dwelling Detached Structure Load Calculator produces three planning values:

• Base detached-structure load — the sum of the entered lighting, receptacle, equipment, and HVAC loads.
• Planned load with spare — the base load increased by the selected Spare factor.
• Entered feeder capacity margin — the remaining capacity after the planned load is compared with the entered feeder capacity.

The resulting planned kW load can be carried into a feeder review that considers conductor ampacity, overcurrent protection, terminal ratings, voltage drop, raceway fill, grounding and bonding, and the actual electrical characteristics of the equipment being served.

Load Groups Entered

The worksheet separates the known load into four field groups. All values are entered in kW.

Input	Electrical use
Lighting load	General and task lighting connected to the detached structure supply.
Receptacle load	Receptacle-related load entered for the detached-structure screen.
Equipment load	Fixed or planned equipment load assigned to the structure.
HVAC load	Heating, cooling, ventilation, or similar HVAC load entered for the structure.
Spare factor	Additional planning allowance, expressed as a percentage of the base detached-structure load.
Entered feeder capacity	Feeder capacity value entered in kW for comparison against the planned load.

The calculator does not determine which individual loads must be included, whether any demand treatment is appropriate, or whether a motor, compressor, welder, EV charging load, or other equipment requires a separate calculation. Each entered kW value should therefore represent the load basis selected for the project.

A spare factor is commonly used during early feeder planning when a garage or outbuilding may later receive additional receptacle circuits, shop equipment, a mini-split, exterior lighting, or other anticipated utilization. It is a design allowance applied by the worksheet; it is not an ampacity adjustment factor or a conductor temperature correction factor.

Detached-Structure Load Formula

The calculator first totals the entered load groups:

\(\displaystyle \text{base structure load} = \text{lighting} + \text{receptacle} + \text{equipment} + \text{HVAC loads}\)

It then applies the entered spare percentage:

\(\displaystyle \text{planned structure load} = \text{base structure load} \times \left(1 + \frac{\text{spare percent}}{100}\right)\)

Finally, it compares the planned load to the entered feeder capacity:

\(\displaystyle \text{feeder capacity margin} = \text{entered feeder capacity} - \text{planned structure load}\)

A positive capacity margin indicates that the entered feeder capacity exceeds the planned load. A zero margin means the entered capacity equals the planned load. A negative margin indicates that the planned load exceeds the capacity value entered into the worksheet.

Calculation Example

For a detached structure with the following entered values:

Input	Value
Lighting load	0.8 kW
Receptacle load	1.2 kW
Equipment load	3.0 kW
HVAC load	2.5 kW
Spare factor	25%
Entered feeder capacity	12.0 kW

The base load is:

\(\displaystyle 0.8 + 1.2 + 3.0 + 2.5 = \text{7.5 kW}\)

The planned load with 25% spare is:

\(\displaystyle 7.5 \times (1 + 25/100) = 7.5 \times 1.25 = \text{9.375 kW}\)

The entered feeder capacity margin is:

\(\displaystyle 12.0 - 9.375 = \text{2.625 kW}\)

The calculator therefore returns:

Result	Value
Base detached-structure load	7.5 kW
Planned load with spare	9.375 kW
Entered feeder capacity margin	2.625 kW

In a practical feeder-design workflow, 9.375 kW is the planning load to carry forward when evaluating whether the feeder arrangement is adequate. The 2.625 kW margin is not a conductor size, breaker size, or available ampacity. It is simply the remaining difference between the entered capacity and the calculator’s planned kW load.

Feeder Design Follow-Up

A detached-structure load total does not by itself establish feeder conductor size. Conductor selection requires an electrical analysis using the actual system voltage, phase configuration, power characteristics, continuous-load treatment, equipment nameplates, and overcurrent protective device arrangement.

The later design review may include:

• Conversion of the applicable load from kW to current based on the actual supply system and load characteristics.
• Feeder conductor ampacity review using the correct AWG or kcmil conductors, insulation temperature rating, terminal rating, and permitted ampacity basis.
• Adjustment and correction review for current-carrying conductors, ambient temperature, and installation conditions where applicable.
• Raceway and conduit layout, including raceway fill, conductor pulling conditions, bends, and available space for equipment grounding conductors.
• Feeder voltage-drop review, especially where the detached structure is remote from the dwelling service or distribution equipment.
• Disconnecting means, panelboard capacity, grounding, bonding, and separation of neutral and equipment grounding conductor functions where required by the installation.
• Equipment-specific review for motors, HVAC equipment, compressors, welders, EV equipment, and other loads that may not be represented adequately by a simple connected kW total.

Field Verification

This worksheet performs the stated arithmetic only: it sums the entered load groups, applies the Spare factor, and compares the result with Entered feeder capacity. It does not perform feeder sizing, grounding, bonding, voltage-drop, raceway-fill, conductor ampacity, motor-load, or code-compliance calculations.

Verify the actual load basis, supply characteristics, equipment ratings, conductor installation conditions, and applicable requirements with the project documents and the authority having jurisdiction (AHJ) before using the result for construction, permit, or approval decisions.