Feeder Load Calculator

Related tools: Commercial Load Calculator, Continuous Load Calculator, and Electrical Panel Load Calculator.

A feeder load calculator converts a known connected load into an adjusted feeder load in VA and the corresponding feeder current in amperes. The result supports the next stages of feeder design: selecting conductor ampacity, checking overcurrent protection, evaluating voltage drop, planning raceway fill, and reviewing the load supplied by a panelboard or distribution equipment.

The calculator applies the entered Demand factor to the full Connected load, then adds the additional VA required by the entered Continuous load portion and Continuous multiplier. It converts the resulting adjusted VA load to current using the selected system voltage and phase arrangement.

This workflow is intended for situations where the connected load basis, applicable demand factor, continuous-load portion, voltage, and phase have already been established.

Adjusted Feeder Load

The calculator produces two primary design values:

• Adjusted feeder load — the calculated VA load after demand is applied and the continuous-load adder is included.
• Feeder current — the calculated line current at the entered System voltage for either single-phase or balanced three-phase current math.

The adjusted feeder load is not simply the connected load. A connected load can include equipment that is not expected to operate simultaneously, which is why a demand factor may reduce the initial VA basis. A continuous load portion, however, requires an additional multiplier in this worksheet’s arithmetic.

The output current can then be compared with the allowable ampacity of the proposed feeder conductors. Final conductor selection still requires the applicable conductor insulation temperature rating, terminal rating, ambient-temperature correction factor, adjustment factor for current-carrying conductors, and the installation’s overcurrent-protection requirements.

Calculation Inputs

Input	Electrical purpose
Connected load	Total known load basis in VA before demand is applied
Demand factor	Percentage applied to the connected load to calculate demand load
Continuous load portion	Portion of the load in VA that requires the entered continuous multiplier
Continuous multiplier	Multiplier used to calculate the added VA above the demand load for the continuous portion
System voltage	Nominal voltage used to convert adjusted feeder VA into current
Phase	Selects single-phase or balanced three-phase feeder-current math

The Continuous load portion must be identified correctly before using the calculation. It represents the portion for which the selected continuous multiplier applies; it is not automatically derived from Connected load or Demand load.

Demand and Continuous-Load Formula

The calculator performs the following VA arithmetic:

\(\displaystyle \text{Demand load} = \text{Connected load} \times \left(\frac{\text{Demand factor}}{100}\right)\)

\(\displaystyle \text{Continuous adder} = \text{Continuous load portion} \times \left(\frac{\text{Continuous multiplier}}{100} - 1\right)\)

\(\displaystyle \text{Adjusted feeder load} = \text{Demand load} + \text{Continuous adder}\)

For the current conversion:

\(\displaystyle \text{Single-phase feeder current} = \frac{\text{Adjusted feeder load}}{\text{System voltage}}\)

\(\displaystyle \text{Balanced three-phase feeder current} = \frac{\text{Adjusted feeder load}} {\sqrt{3} \times \text{System voltage}}\)

The balanced three-phase formula uses line-to-line voltage. It assumes the entered feeder load is balanced across the three phases.

Calculation Example

Use the entered values:

Input	Value
Connected load	60,000 VA
Demand factor	80%
Continuous load portion	12,000 VA
Continuous multiplier	125%
System voltage	480 V
Phase	Balanced three-phase

First, apply the 80% demand factor:

\(\displaystyle 60{,}000\ \text{VA} \times 0.80 = 48{,}000\ \text{VA}\)

Demand load = 48,000 VA

Next, calculate the added VA associated with the 125% multiplier on the 12,000 VA continuous-load portion:

\(\displaystyle 12{,}000\ \text{VA} \times (1.25 - 1.00) = 3{,}000\ \text{VA}\)

Continuous adder = 3,000 VA

Add that adder to the demand load:

\(\displaystyle 48{,}000\ \text{VA} + 3{,}000\ \text{VA} = 51{,}000\ \text{VA}\)

Adjusted feeder load = 51,000 VA

For a 480 V balanced three-phase feeder:

\(\displaystyle I = \frac{51{,}000} {\sqrt{3} \times 480} = 61.3435\ \text{A}\)

Feeder current = 61.3435 A

Feeder Design Application

A calculated feeder current of 61.3435 A is the load current used to begin ampacity selection. It does not, by itself, establish the final conductor size. For example, the next design review may include:

• Conductor material and size in AWG or kcmil
• Allowable ampacity at the applicable terminal rating
• Insulation temperature rating and permitted use of higher-temperature ampacity values for correction or adjustment
• Ambient-temperature correction factor
• Adjustment factor for the number of current-carrying conductors in a raceway or cable
• Feeder overcurrent protective-device rating
• Raceway fill and conductor pulling practicality
• Feeder voltage drop over the actual route length
• Available fault current, equipment rating, and grounding or bonding requirements

For a long 480 V feeder, the calculated current is also an input to voltage-drop review. A conductor that satisfies ampacity may still need to be increased in AWG or kcmil size when route length, load characteristics, or performance requirements produce unacceptable voltage drop.

Field Verification

The calculator performs transparent VA and current arithmetic; it does not determine whether the selected Demand factor is permitted for a particular occupancy, equipment type, or load category. Confirm the load classification, demand treatment, continuous-load determination, conductor ampacity, terminal limitations, adjustment and correction factors, and overcurrent protection under the applicable code requirements and the local AHJ.

Use balanced three-phase selection only where the feeder load can reasonably be treated as balanced. Significant phase imbalance requires phase-by-phase load review, including neutral loading where applicable. Motor feeder calculations, nonlinear loads, harmonic effects, separately derived systems, and equipment-specific nameplate requirements may require additional design calculations beyond the adjusted feeder load shown here.