Subpanel Load Calculator

A subpanel serving a shop, ADU, or addition draws its supply from a connected load figure expressed in volt-amperes, but the conductor and breaker feeding that panel are never sized to the connected load directly. The demand load — connected load reduced by an applicable demand factor — is the number that determines feeder current, and feeder current is the number that gets compared against conductor ampacity tables and the subpanel’s own bus rating. This calculator converts a known connected load and demand factor into demand load, feeder current, panel utilization, and remaining spare capacity in a single pass, so a contractor can screen whether an existing or proposed subpanel has room for additional circuits before pulling out ampacity tables or running a full NEC Article 220 load calculation.

Inputs and What They Represent

• Connected load (VA) — the sum of nameplate or calculated loads feeding the subpanel, entered as the basis for this screen. In this example, 48000 VA.
• Demand factor (%) — the percentage applied to connected load to reflect the fact that not all connected loads operate simultaneously at full draw. Entered here as 80%.
• System voltage (V) — the line voltage used to convert demand load (VA) into current (A). Entered as 240 V.
• Phase — Single-phase or balanced three-phase, since the current formula differs between them.
• Subpanel rating (A) — the ampere rating of the subpanel bus or main breaker, used only as a comparison ceiling for utilization and spare capacity. Not a design input to the demand or current math.
• Target spare capacity (%) — the spare capacity the contractor wants reserved against the subpanel rating, expressed as a percent of that rating.

Calculation Sequence

Demand load is connected load multiplied by demand factor:

\(\text{Demand load (VA)} = \text{Connected load (VA)} \times \text{Demand factor (\%)}\)

For single-phase systems, feeder current converts demand load to amperes at the entered voltage:

\(I = \frac{\text{Demand load (VA)}}{\text{System voltage (V)}}\)

Three-phase balanced systems use the standard \(\sqrt{3}\) multiplier in the denominator instead. Once feeder current is known, subpanel utilization compares it against the subpanel rating:

\(\text{Utilization (\%)} = \frac{\text{Feeder current (A)}}{\text{Subpanel rating (A)}} \times 100\)

Spare capacity is the ampere difference between the subpanel rating and the feeder current actually drawn, and target spare capacity converts the entered spare-capacity percentage into the same ampere basis for direct comparison.

Worked Result

With connected load at 48000 VA, demand factor at 80%, single-phase, 240 V, and a 200 A subpanel:

Field	Value
Connected load	48000 VA
Demand load	38400 VA
Feeder current	160 A
Subpanel utilization	80%
Spare capacity	40 A
Target spare capacity	40 A

Demand load of 38400 VA comes directly from 48000 × 0.80. Feeder current of 160 A comes from 38400 ÷ 240. Utilization of 80% is 160 ÷ 200. Spare capacity of 40 A is the 200 A rating minus the 160 A feeder current, and it happens to land exactly on the 20% target spare capacity requested (40 A on a 200 A base), meaning this subpanel is fully loaded to the stated planning margin with no room to spare for additional circuits without derating something else first or upsizing the panel.

Where Feeder Current Goes Next

The 160 A feeder current is the figure carried into conductor sizing against an ampacity table, adjusted for the conductor’s insulation temperature rating, terminal rating at the panel and breaker, and any applicable adjustment or correction factors for ambient temperature or the number of current-carrying conductors in the raceway. It is also the current used for a separate voltage-drop review on the feeder run length, and it is the amperage that determines whether a 200 A main breaker feeding this subpanel, or the feeder conductors themselves, have any headroom for a future circuit addition.

Field and Code Limitations

This calculator performs arithmetic only on the demand factor entered by the user. It does not apply, verify, or substitute for the demand factor tables in NEC Article 220, and it does not calculate connected load from individual appliance or circuit nameplates. The subpanel rating entered is used strictly as a comparison ceiling for utilization and spare capacity — it is not validated against the actual bus rating, main breaker, or listing of the physical panel installed in the field. Conductor sizing, terminal ratings, raceway fill, and any additional demand factors for continuous loads, motors, or existing versus new load combinations must be confirmed separately against the applicable NEC tables and verified with the authority having jurisdiction before a feeder is installed or upsized based on this screen.