Load Calculations for EV Circuits: Demand, Continuous Loads, and Service Capacity Checks

What you are trying to prove

Before adding an EV circuit, you need to verify that the existing electrical service and the panelboard can support the added load without exceeding allowable demand. In practice, you are answering three questions:

• Service capacity: Will the calculated demand (existing + EVSE) stay within the service rating (e.g., 100 A, 150 A, 200 A)?
• Panel capacity: Is there physical space (breaker positions) and bus rating capacity for the new breaker?
• Feeder/circuit sizing: Can the EV branch circuit and any upstream feeders be sized correctly under continuous-load rules?

This chapter focuses on load calculations and the decision-making that follows.

Terminology you must use consistently

Demand load

Demand load is the portion of the connected load expected to be on at the same time, after applying code-permitted demand factors (diversity). Demand is what you compare to service/feeder ratings.

Continuous load

A continuous load is expected to run at maximum for 3 hours or more. EV charging commonly qualifies as continuous because charging sessions can last many hours. Continuous loads trigger special sizing rules for conductors and overcurrent protection.

Diversified vs. non-diversified loads

• Diversified (coincident) loads: Loads that are unlikely to all run at full output simultaneously (general lighting, receptacles, typical household appliances). These often receive demand factors in dwelling calculations.
• Non-diversified (100% counted) loads: Loads that must be assumed to run when needed or are treated at full value in the calculation (many fixed appliances, HVAC selections, EVSE depending on method). When in doubt, treat the EVSE as a full continuous load unless an approved load management system is used and documented.

Continuous-load sizing rules for EVSE (branch circuit and feeder impact)

For EV charging equipment that is treated as a continuous load, apply the 125% rule (or equivalently, size the circuit at continuous load ÷ 0.8).

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Two equivalent ways to think about it

• Given EVSE output current: Minimum circuit rating = EVSE continuous current × 125%.
• Given breaker size: Maximum continuous EVSE current = breaker rating × 80%.

Common pairings (quick reference)

Important: This sizing is about the branch circuit and upstream conductors/feeder. Separately, you still must include the EVSE load in the service load calculation (often as a continuous load at its rated output unless a documented control strategy changes the effective demand).

Step-by-step residential load calculation workflow (practical field method)

The exact worksheet format varies by jurisdiction and code edition, but the workflow below matches how electricians typically build a defensible dwelling-unit load calculation and then “stress test” it by adding EVSE at different levels.

Step 1: Gather inputs and document them

• Service rating (e.g., 120/240 V, 1φ, 3-wire, 100/150/200 A).
• Dwelling size (square footage) for general load calculation.
• Small-appliance and laundry circuits (typically counted as required loads).
• Fixed appliances (nameplate kW/VA): range/oven, dryer, dishwasher, disposal, microwave, water heater, well pump, etc.
• HVAC: identify whether it’s electric heat, heat pump with strips, furnace blower, A/C condenser, etc. Determine the largest applicable heating vs cooling load per rules used in your jurisdiction (often not both at full).
• Existing EVSE or other large continuous loads (pool heater, spa, sauna, server rack, etc.).
• Proposed EVSE output current (or desired breaker size) and whether it is adjustable.

In your notes, record whether each value is from a nameplate, manual, or a reasonable assumption (and why).

Step 2: Calculate general load (lighting + general receptacles)

Use the dwelling general load method required in your area (often based on square footage plus required small-appliance and laundry loads). Apply the permitted demand factor to the general load portion.

Field tip: If you don’t have a formal worksheet in front of you, still separate “general load” from “fixed appliances” and “HVAC,” because the demand factors and selection rules differ.

Step 3: Add fixed appliances

List fixed appliances and apply the demand factor rules that apply when you have multiple appliances. Be consistent: either use nameplate VA/kW or a permitted standard value, and note which you used.

Step 4: Add cooking equipment and dryer per applicable rules

Ranges/ovens and dryers often have special calculation rules and demand factors. Use the method your jurisdiction expects (worksheet/table-based). Document the method used (e.g., “standard dwelling calc table” or “nameplate with demand factor”).

Step 5: Add HVAC (largest of heat vs A/C, as applicable)

In many dwelling calculations, you include the larger of heating load or cooling load (plus associated blower/fan where required). Electric resistance heat and heat pump auxiliary strips can dominate the entire calculation—treat these carefully and verify nameplates.

Step 6: Add the EVSE load (and apply continuous-load thinking)

For service load calculations, include the EVSE at its intended output. If the EVSE is set to 40 A output, include 40 A × 240 V = 9.6 kVA (unless your local method specifies otherwise). If you are evaluating multiple possible EVSE settings, run the calculation multiple times (32 A, 40 A, 48 A, etc.).

If a listed, approved load management system will limit EV charging based on service loading, you may be able to justify a reduced effective demand—but only if you can document the control method and it is acceptable to the AHJ.

Step 7: Convert total VA to amperes and compare to service rating

For a 120/240 V single-phase dwelling, service current is typically:

I_service = Total_VA / 240

Compare the calculated amperes to the service rating. Many electricians also apply a practical margin (for example, avoiding designs that land at 99–100% of service rating) because real-world usage and future loads can push it over.

Guided example: typical dwelling load calc + EVSE at different amperage levels

Scenario: 2,000 ft² single-family dwelling, 120/240 V, 1φ, existing 200 A service. Gas space heat (so no large electric heat), electric water heater, electric dryer, electric range, central A/C. We will calculate an approximate dwelling demand and then add EVSE at 32 A, 40 A, and 48 A output.

1) General load

• General lighting/receptacles: 2,000 ft² × 3 VA/ft² = 6,000 VA
• Small-appliance circuits: 2 × 1,500 VA = 3,000 VA
• Laundry circuit: 1 × 1,500 VA = 1,500 VA

Subtotal general = 6,000 + 3,000 + 1,500 = 10,500 VA

Apply a common dwelling demand approach for general load (first 10,000 VA at 100%, remainder at 40%):

• 10,000 VA × 100% = 10,000 VA
• 500 VA × 40% = 200 VA

Demanded general = 10,200 VA

2) Fixed appliances (example nameplates)

• Electric water heater: 4,500 VA
• Dishwasher: 1,200 VA
• Disposal: 800 VA

Appliance subtotal = 6,500 VA

If your method allows a demand factor for multiple appliances, apply it as required; for this guided example we will conservatively count them at 100% (you can re-run with the permitted factor if applicable in your jurisdiction).

3) Range and dryer (example nameplates)

• Range: 12,000 VA (12 kW)
• Dryer: 5,000 VA (5 kW)

Depending on the code method used, these may be reduced by demand factors. For a conservative, easy-to-follow example, we will include them at nameplate here and note that a formal worksheet may reduce the range demand.

4) HVAC (largest of heat vs A/C)

• Central A/C condenser: 4,000 VA (example)
• Gas heat (blower only): already embedded in general load or small motor load depending on method; we will not add a separate large heating kW.

HVAC added = 4,000 VA

5) Total existing calculated load (before EVSE)

• Demanded general: 10,200 VA
• Fixed appliances: 6,500 VA
• Range: 12,000 VA
• Dryer: 5,000 VA
• A/C: 4,000 VA

Total (no EVSE) = 37,700 VA

Convert to service current:

I = 37,700 VA / 240 V ≈ 157 A

On a 200 A service, this example has headroom (about 43 A) before adding EV charging.

6) Add EVSE at different output levels

Compute EVSE VA as output current × 240 V.

7) Tie the service calc back to branch-circuit sizing

Even if the service can support it, the EV branch circuit must still follow continuous-load sizing:

• 32 A output → minimum 40 A circuit (32/0.8 = 40)
• 40 A output → minimum 50 A circuit (40/0.8 = 50)
• 48 A output → minimum 60 A circuit (48/0.8 = 60)

Make sure the panel has the correct breaker space and that any feeder or subpanel supplying the EV circuit is also evaluated for added continuous load.

Recognizing red flags that often force mitigation

Red flag 1: Already-high calculated demand before EVSE

If the dwelling is already calculating near the service rating (common on 100 A services), adding even a modest EVSE can push it over. Treat “near” as a trigger to re-check every assumption and consider managed charging early.

Red flag 2: Electric space heat or large auxiliary heat strips

Electric resistance heat, heat pump auxiliary strips, and electric boilers can dominate the load calculation. If present, expect EVSE output to be limited or require load management/service upgrade.

Red flag 3: Multiple large appliances and lifestyle loads

Second ovens, large spas/hot tubs, pool equipment, multiple refrigerators/freezers, workshops, and server/AV racks can make the “typical dwelling” assumptions invalid. Nameplate verification matters more in these homes.

Red flag 4: Subpanels fed from marginal feeders

Even when the service is adequate, a garage subpanel fed by a small feeder may not have capacity for a continuous EV load. Check feeder ampacity and existing loads on that feeder.

Red flag 5: Customer expects maximum charging but has limited service

If the customer requests 48 A charging on a 100–150 A service with electric heat, plan on either a reduced EVSE setting, load management, or an upgrade. Set expectations early and document the options.

How to document assumptions so your calculation is defensible

• Source every value: “Nameplate photo,” “manual page,” “existing breaker rating,” “measured square footage,” etc.
• State the EVSE setting: Record the configured continuous output current (e.g., “EVSE set to 32 A max”). If adjustable, note who is authorized to change it.
• Identify the calculation method: “Standard dwelling unit load calc worksheet” or “existing demand calc updated with EVSE.”
• Note demand factors used: If you applied a demand factor for general load or appliances, write it down explicitly.
• List exclusions: If you did not include a load because it is gas (range, water heater, heat), state that and confirm fuel type.
• Capture panel/service details: Main breaker rating, panel bus rating, available spaces, and any derating conditions you relied on.

Decision matrix: what to do with the result