Solar AC Size Calculator
Size AC output circuits to NEC 690 standards and calculate how many solar panels you need to run air conditioning.
Solar AC Size Calculator
Size AC output circuits to NEC 690 standards, or calculate how many solar panels you need to power an air conditioning system.
| AWG / kcmil | Cu 60°C | Cu 75°C | Cu 90°C | Al 75°C |
|---|
Fill in inverter details above to see NEC-compliant wire and breaker sizing results.
Select your state and enter building details to see how many solar panels you need to power your AC.
⚡ AC Circuit Sizing (Mode 1)
Following NEC 690.8(A)(1), the continuous load is calculated as:
I_design = (W_total / V / PF) × 1.25
Wire ampacity is then derated for ambient temperature (Table 310.15(B)(1)) and conduit fill (Table 310.15(C)(1)). NEC 690.9(B) requires the breaker to be rated ≥ 125% of rated AC current.
120% Rule (NEC 705.12(B)(3)): Solar backfeed breaker + main breaker ≤ 1.20 × busbar rating.
❄️ Solar for AC (Mode 2)
Cooling load is estimated using IECC climate zone BTU/sqft benchmarks adjusted for ceiling height, insulation quality, occupant loads, and sun exposure - a rule-of-thumb approximation within ±25% of Manual J.
Panels needed: kWh_daily / (PSH × panel_W / 1000 × efficiency)
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What This Tool Covers
Two calculators in one. Switch between AC circuit sizing for NEC compliance and panel-count sizing for powering air conditioning loads.
Mode 1 - AC Circuit Sizing
Size AC output circuits for solar inverters per NEC Article 690 and 705. Enter your inverter configuration and conductor parameters to get wire size, breaker size, voltage drop, and 120% busbar rule compliance.
- Wire size (AWG) with temperature and conduit fill derating
- AC disconnect breaker size per NEC 690.9(B)
- Voltage drop percentage with pass/fail status
- 120% busbar rule compliance check (NEC 705.12)
- Copyable permit summary block
Mode 2 - Solar Panels for AC
Calculate how many solar panels you need to power an air conditioner. Enter your state, building details, and AC system specs to get panel count, system size, and estimated annual energy cost.
- Number of panels needed based on location and load
- DC system size in kilowatts
- Cooling load in BTU/hr and tons
- Daily and annual AC energy consumption (kWh)
- Estimated annual electricity cost and roof area needed
Why Solar Installers Use This Tool
NEC-compliant sizing in seconds. No spreadsheets, no manual table lookups.
NEC 690 & 705 Compliant
Every calculation follows the current NEC code - 125% design current, temperature-corrected ampacity tables, conduit fill factors, and the 120% busbar interconnection rule.
Dual-Mode Design
One tool covers both design tasks - electrical circuit sizing for the AHJ and solar panel sizing for the customer. Switch between modes without leaving the page.
Permit-Ready Summary
Mode 1 generates a formatted permit summary block you can copy directly into your AHJ submission package - wire size, breaker size, ampacity, and compliance status included.
How It Works
Mode 1: AC Circuit Sizing
Set System Voltage & Inverter Config
Choose system voltage (120V, 240V, 208V, or 480V), number of inverters, and rated power per unit in watts or amps.
Enter Conductor Parameters
Select conductor material (copper or aluminum), insulation rating, ambient temperature, number of conductors in conduit, and one-way wire run length.
Enter Panel Details
Input main breaker rating, busbar rating, and any existing solar backfeed breaker size for the 120% busbar compliance check.
Get Your Results
Recommended wire size, breaker size, voltage drop status, ampacity values, derating factors, and 120% busbar compliance - all calculated instantly.
Mode 2: Solar Panels for AC
Select Your State
Choose your state and the tool auto-fills your IECC climate zone and location-specific peak sun hours - no manual lookup needed.
Enter Building & AC Details
Input conditioned area, ceiling height, insulation quality, occupant count, sun exposure, AC system type, SEER rating, and daily runtime hours.
Set Panel Specs
Enter panel wattage (STC) and system efficiency percentage to reflect your actual equipment and installation conditions.
Get Panel Count & System Size
See panels required, DC system size, cooling load, daily and annual kWh consumption, estimated annual electricity cost, and roof area needed.
Built for Every Solar Professional
Solar Installers & Electricians
Use Mode 1 during system design to confirm wire size, breaker size, and NEC compliance before submitting to the AHJ. Generate the permit summary block in one click.
Solar System Designers
Use both modes together - Mode 2 to size the solar array for AC loads, then Mode 1 to confirm the AC output circuit meets NEC requirements for the inverter configuration you've chosen.
Solar Sales Representatives
Use Mode 2 during site visits to show homeowners how many panels they need to offset their air conditioning load - a clear, tangible reason to go solar.
Calculation Methodology
Transparent formulas based on NEC Article 690/705 and standard IECC cooling load benchmarks.
Design Current (NEC 690.8)
(W_total ÷ V ÷ PF) × 1.25 Total inverter watts divided by voltage and power factor, multiplied by the NEC 125% continuous load factor to determine the minimum circuit ampacity.
Wire Ampacity (NEC 310.15)
Base Ampacity × Temp Factor × Conduit Fill Factor Base ampacity from NEC Table 310.16 derated using temperature correction (Table 310.15(B)(1)) and conduit fill adjustment (Table 310.15(C)(1)) for real-world conditions.
Voltage Drop
(2 × L × R × I) ÷ (V × 1000) Single-phase formula using one-way run length, conductor resistance per 1000 ft, and design current. Three-phase uses the √3 factor. Results flagged: under 2% excellent, 2–3% acceptable, over 3% high.
120% Busbar Rule (NEC 705.12)
Main Breaker + Solar Breaker ≤ 1.20 × Busbar Rating Sum of the main breaker and all solar backfeed breakers must not exceed 120% of the panel busbar rating per NEC 705.12(B)(3). Failures require a main breaker reduction or bus-side tap.
Cooling Load (BTU)
sqft × BTU/sqft benchmark × Adj. Factors IECC climate zone BTU/sqft benchmarks adjusted for ceiling height, insulation quality (1.0–2.0x factor), occupancy (400 BTU/person), and sun exposure. ±25% approximation - not a Manual J calculation.
Panels Required
kWh/day ÷ (PSH × Panel W ÷ 1000 × Efficiency) Daily AC energy divided by what each panel produces per day (peak sun hours × panel wattage × system efficiency). Uses state-specific PSH data ranging from 2.0 to 7.0 hours/day.
Pro Tips for Accurate Sizing
Never skip temperature derating
Wire in conduit on a hot roof can see ambient temps of 40–45°C, which reduces allowable ampacity by 15–20% compared to the 30°C base. Skipping derating is one of the most common NEC violations on solar permits.
Check the busbar rule before designing
Run the 120% busbar check at the start of design, not after. If the panel fails, you'll need to reduce the main breaker or use a bus-side tap - changes that affect the whole system design and cost.
Target voltage drop under 2% for long runs
For wire runs over 100 feet, voltage drop often drives wire sizing more than ampacity. Upsize the wire even if ampacity technically passes - it protects against production losses and heat buildup over the system's 25-year life.
Treat Mode 2 results as a ballpark
The Solar for AC mode uses rule-of-thumb BTU benchmarks, not a full Manual J load calculation. It's accurate enough for proposals and initial sizing, but always confirm with a licensed HVAC contractor before final equipment selection.
Frequently Asked Questions
How do you size an AC circuit for a solar inverter?
Per NEC 690.8, the design current equals the inverter's rated output current multiplied by 125%. You then select a wire with derated ampacity (accounting for ambient temperature and conduit fill) that meets or exceeds the design current. Breaker size is the next standard size at or above 125% of rated AC current per NEC 690.9(B). This calculator automates all of these steps.
How many solar panels do I need to run an air conditioner?
It depends on your AC system size, location, and how many hours per day it runs. A 3-ton central AC system running 8 hours per day in a moderate climate typically requires 10–18 panels (400W each). A 1-ton mini-split running 6 hours per day may need only 4–7 panels. Use Mode 2 of this calculator with your specific inputs to get an accurate estimate for your project.
What is the NEC 120% busbar rule?
Per NEC 705.12(B)(3), the sum of the main breaker rating and all solar backfeed breaker ratings must not exceed 120% of the panel busbar rating. For example, a 200A busbar panel can accept a main breaker up to 200A plus a solar breaker up to 40A (200 × 1.20 = 240A total). Exceeding this limit requires reducing the main breaker size or using a bus-side tap instead of a backfeed breaker.
What wire size do I need for my solar inverter's AC output?
Wire size depends on four variables: design current (125% of rated output), conductor material (copper or aluminum), ambient temperature and conduit fill (which derate the base ampacity), and wire run length (which affects voltage drop). For a 7,600W inverter on a 240V system with copper wire in conduit at 40°C, the design current is about 40A - typically requiring 8 AWG copper before derating. Enter your specific values into Mode 1 for an exact result.
Does this calculator follow NEC 2023?
The calculator follows NEC Article 690 (solar PV systems) and Article 705 (interconnected power production equipment) logic - specifically NEC 690.8 for design current, NEC 690.9(B) for breaker sizing, NEC 310.15 for conductor ampacity with derating, and NEC 705.12(B)(3) for the 120% busbar rule. Always verify your final design against the specific NEC edition adopted by your local AHJ.
How accurate is the Solar for AC mode?
Mode 2 uses IECC climate zone BTU/sqft benchmarks - a standard rule-of-thumb approach that gives results within roughly ±25% of a full Manual J load calculation. This is accurate enough for proposals, initial system sizing, and customer conversations. For final HVAC equipment selection or permit submissions, always have a licensed HVAC contractor perform a proper Manual J calculation.
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