Utility feeds

Utility feeds represent the external power source from the electric utility company. This is where grid power enters your facility's electrical system.

What utility feeds represent

The utility feed models the infinite bus - an idealized power source with:

• Constant voltage magnitude
• Unlimited short-circuit capacity
• Zero impedance (ideally)

In reality, utilities have finite short-circuit capacity and source impedance. Utility feed parameters model these real-world characteristics for accurate power flow and fault analysis.

Creating a utility feed

1. Drag Utility Feed from component toolbar.
2. Drop on canvas (typically at top of diagram).
3. Connect bottom handle to service entrance equipment or main bus.
4. Configure source voltage and short-circuit capacity.

Utility feeds typically appear at the top of the diagram following industry SLD conventions (power flows from top to bottom).

Source voltage configuration

Specify the utility delivery voltage:

Distribution voltages (common in North America):

Transmission and subtransmission:

Voltage must match utility service agreement and incoming transformer configuration.

Short-circuit MVA input

Short-circuit capacity (also called fault level) determines maximum available fault current at the utility connection point:

Typical utility short-circuit MVA:

Get actual short-circuit MVA from utility company. This information may be on your utility service agreement or available by request.

Calculating fault current from MVA:

Fault Current (kA) = SC MVA / (√3 × Voltage kV)

Example: 500 MVA at 13.8kV

I_fault = 500 / (1.732 × 13.8) = 20.9 kA

X/R ratio

The X/R ratio (reactance-to-resistance ratio) of utility source impedance:

Typical X/R ratios:

X/R ratio affects:

• Power factor of fault current
• DC offset during asymmetric fault
• Circuit breaker duties

Higher X/R (more inductive) = longer time to zero-crossing = harder to interrupt.

Use 15 as default if unknown. Get actual value from utility for critical applications.

Input methods

Two methods to specify utility source impedance:

Method 1: Short-circuit MVA and X/R ratio (recommended):

• More intuitive
• Matches utility-provided data
• Directly interpretable

Method 2: Impedance components (R and X):

• Source resistance in ohms
• Source reactance in ohms
• Less common
• For advanced users with detailed utility data

Most users should use Method 1 (MVA and X/R).

Slack bus relationship

The utility feed connects to the slack bus in power flow analysis:

• Slack bus balances system real and reactive power
• Provides voltage angle reference (0 degrees)
• Typically the bus immediately downstream of utility feed

Designate the bus connected to your utility feed as the slack bus. See Buses for slack bus configuration.)

Grid contribution to faults

During short-circuit analysis, the utility feed contributes fault current:

Symmetrical fault current:

I_fault = SC MVA / (√3 × V_kV)

Asymmetrical (peak) fault current:

I_peak = I_fault × √2 × √(1 + 2e^(-2π/X/R))

Higher short-circuit MVA = higher fault current = higher interrupting capacity required for downstream breakers.

Service entrance configuration

Typical utility service entrance components:

1. Utility feed (modeled component)
2. Service entrance cable - From utility to main disconnect
3. Main service disconnect - First overcurrent device
4. Service entrance equipment - Main switchboard or panelboard
5. Main bus - Distribution point for building feeders

Connect utility feed to service entrance components in this order.

Multiple utility feeds

Some facilities have redundant utility services:

Dual utility feeds:

• Two separate utility connections
• Automatic or manual transfer switch
• High reliability for critical facilities
• Model as two separate utility feed components

Single utility feed with generator backup:

• Utility feed as normal source
• Generator as emergency source
• Automatic transfer switch between sources
• Model utility feed and generator separately

Metering point

Utility revenue metering typically occurs at or near the utility feed connection:

• Service entrance location for <600V
• Primary side of service transformer for medium voltage
• Billing demand calculated from metering data

The utility feed component represents the point after the meter (on the customer side).

Related topics

• Buses - Connecting utility feed to main bus (slack bus
• Short-circuit analysis - Utility contribution to fault currents
• Power flow analysis - Slack bus and power balance