Branch Circuit Calculator
Free branch circuit calculation tool for load analysis, wire sizing, overcurrent protection, and NEC compliance. Calculate outlet loads, voltage drop, and circuit requirements for residential and commercial applications.
Circuit Configuration
Load Information
Wire & Protection
Results
Circuit Requirements
Wire Protection
- 14 AWG: 15A maximum OCPD
- 12 AWG: 20A maximum OCPD
- 10 AWG: 30A maximum OCPD
- Continuous loads: 125% factor
- Voltage drop: ≤3% recommended
Derating Factors
Apply correction factors per NEC Table 310.15(B)(1)
4-6: 80%, 7-9: 70%, 10-20: 50%
Branch circuit calculations ensure safe and code-compliant electrical installations. Proper load calculations, wire sizing, and overcurrent protection are essential for electrical safety.
Load Calculations
I = P / V
I = Current (A), P = Power (W), V = Voltage (V)
Adjusted Load = Load × 1.25
For loads operating 3+ hours continuously
Demand Load = Connected Load × DF
DF varies by load type per NEC Table 220.42
Wire Sizing & Protection
Adj. Ampacity = Base × TCF × CFF
TCF = Temperature Correction, CFF = Conduit Fill Factor
OCPD ≤ Wire Ampacity
With exceptions per NEC 240.4(D)
Ampacity ≥ Adjusted Load Current
Consider voltage drop and mechanical strength
Voltage Drop Analysis
VD = 2 × I × R × L
2-wire circuit (hot + neutral)
VD = 1.732 × I × R × L
3-wire circuit calculation
VD% = (VD / V_nominal) × 100
NEC recommends ≤3% for branch circuits
P_loss = I² × R × 2 × L
Energy wasted as heat in conductors
NEC Requirements Summary
General Requirements
- NEC Article 210: Branch Circuits
- NEC Article 220: Load Calculations
- NEC Article 240: Overcurrent Protection
- NEC Article 310: Conductors
Specific Circuits
- 210.11(C)(1): Kitchen small appliance
- 210.11(C)(2): Laundry circuits
- 210.11(C)(3): Bathroom circuits
- 210.19(A): Conductor sizing
Example Calculation
Kitchen 20A small appliance circuit, 12 AWG THWN, 75 ft run:
• Load: 20A × 120V = 2400W
• Voltage drop: 2 × 20A × 1.93Ω/1000ft × 75ft = 5.79V (4.8%)
• Recommendation: Use 10 AWG to reduce voltage drop to 2.9%
• OCPD: 20A circuit breaker
• NEC compliance: Meets all requirements with 10 AWG
How to Size a Branch Circuit: Step-by-Step
Proper branch circuit sizing ensures safe operation and code compliance. Each circuit must have the right breaker, wire gauge, and device ratings to handle its load.
Step 1: Determine the Load Current
Calculate the circuit's load current from the connected equipment. For resistive loads, divide watts by voltage. For motor loads, use the nameplate FLA or NEC motor tables.
Step 2: Apply 125% for Continuous Loads
If the load operates for 3 hours or more (lighting, commercial equipment), multiply the load current by 1.25 per NEC 210.20. For example, a 16A continuous lighting load becomes 16 x 1.25 = 20A for circuit sizing.
Step 3: Select the Breaker Size
Choose the next standard breaker size that equals or exceeds the adjusted load. Standard sizes are 15, 20, 25, 30, 40, 50, and 60 amps. The breaker rating must not be less than the non-continuous load plus 125% of the continuous load.
Step 4: Match Wire Gauge per NEC 310.16
Select wire with ampacity equal to or greater than the breaker rating. Common pairings: 15A breaker = #14 AWG, 20A = #12, 30A = #10, 40A = #8, 50A = #6. Use the 75 degrees C column for standard terminations.
Step 5: Check GFCI and AFCI Requirements
NEC requires GFCI protection for bathrooms, kitchens, outdoors, garages, and other wet locations. AFCI protection is required for most living spaces including bedrooms and living rooms. Some circuits require both.
Formula
Breaker Size ≥ Non-continuous Load + (Continuous Load x 1.25)
Where: Non-continuous loads are used at full value, Continuous loads (3+ hours) are multiplied by 125% per NEC 210.20
Worked Example
Scenario: Size a branch circuit for a 16A continuous commercial lighting load at 120V.
- Step 1: Load current = 16A (continuous)
- Step 2: Adjusted load = 16 x 1.25 = 20A
- Step 3: Select 20A breaker (matches adjusted load exactly)
- Step 4: 20A breaker requires #12 AWG copper minimum
- Step 5: Commercial lighting does not require AFCI. Check local codes for additional requirements
Result: Use a 20A breaker with #12 AWG copper wire for the 16A continuous lighting circuit.
Branch Circuit Questions & Answers
What's the maximum number of outlets on a 20-amp branch circuit?
The NEC doesn't specify a maximum number of outlets, but limits the load. For general-use receptacles, use 180VA per outlet (NEC 220.14). A 20-amp circuit at 120V = 2400VA, so theoretically 13 outlets max. However, practical limits are 10-12 outlets considering actual loads and the 80% continuous load rule. Kitchen small appliance circuits are limited to kitchen, pantry, breakfast room, and dining room areas only.
When do I need AFCI protection on branch circuits?
NEC 210.12 requires AFCI protection for most 15A and 20A branch circuits in dwelling units. This includes bedrooms, living rooms, hallways, closets, bathrooms, and most other areas. Exceptions include bathrooms, garages, and unfinished basements in some cases. Commercial buildings have different requirements - check local codes. AFCI breakers cost about $45 vs $8 for standard breakers, but they prevent arc-fault fires.
What's the 80% rule for branch circuits?
Continuous loads (operating 3+ hours) can't exceed 80% of the branch circuit rating. A 20A circuit can handle 16A continuous load maximum. This applies to lighting circuits, some appliances, and commercial equipment. Non-continuous loads can use 100% of circuit capacity. Size overcurrent protection and conductors for 125% of continuous loads (same as 80% rule, different perspective).
Can I use 14 AWG wire on a 20-amp breaker?
No, 14 AWG wire is only rated for 15A maximum per NEC Table 310.15(B)(16). Use 12 AWG minimum for 20A circuits. The wire must be sized for the overcurrent protection device, not the actual load. Even if your load is only 10A, you need 12 AWG wire if you're using a 20A breaker. This is a fundamental safety rule - never undersize conductors.
How do I calculate voltage drop for branch circuits?
Use the formula: VD = (2 × K × I × L) / CM, where K is the conductor constant (12.9 for copper), I is current in amps, L is one-way distance in feet, and CM is circular mils. Keep voltage drop under 3% for branch circuits, 5% total (feeder + branch). For a 20A load at 200ft with 12 AWG (6530 CM): VD = (2 × 12.9 × 20 × 200) / 6530 = 15.8V drop - way too high!
What's the difference between MWBC and individual branch circuits?
Multi-wire branch circuits (MWBC) use a shared neutral between two hot conductors on different phases. They save wire but require handle-tied breakers and careful load balancing. Individual circuits are simpler and safer for troubleshooting. MWBC circuits can be dangerous if the neutral opens - you'll get 240V across 120V loads. Use MWBC for fixed loads like lighting, individual circuits for receptacles.
Why can't I put lights and receptacles on the same circuit?
You can in most cases, but it's not recommended practice. NEC allows mixing lighting and receptacles except in specific areas like kitchens (small appliance circuits), bathrooms (dedicated 20A required), and laundry rooms. Separate circuits give better reliability - if one trips, you don't lose both lights and power. Commercial work often requires separate lighting circuits for code compliance.
How many 15A circuits can I put on a 100A panel?
Theoretically 100A ÷ 15A = 6.7 circuits if all fully loaded, but that's not how it works. Panels are sized for diversity - not all circuits operate at full load simultaneously. A typical 100A residential panel has 20-40 circuit spaces. Load calculations determine actual capacity, not simple math. You might have 30 circuits totaling 450A capacity on a 100A service because of diversity factors.
What happens if I overload a branch circuit?
The breaker should trip to protect the wire from overheating. If the breaker doesn't trip (defective or wrong size), the wire overheats, insulation melts, and you get fires or electrocution hazards. This is why proper sizing is critical. Nuisance tripping usually means too much load on the circuit, not a bad breaker. Add circuits rather than upsizing breakers without changing wire size.
Do I need a neutral wire for 240V branch circuits?
Depends on the load. Straight 240V loads (water heaters, baseboard heat, some A/C units) only need two hots and a ground. 240V loads with 120V components (electric ranges, dryers) need a neutral for the 120V parts. NEC 250.140 allows existing 3-wire dryer/range circuits but new installations need 4-wire (2 hots, neutral, ground). When in doubt, run the neutral.
Can I tap off a branch circuit to feed another circuit?
No, branch circuits must originate from the panelboard per NEC 210.25. You can't tap one branch circuit to create another - that's called a "sub-circuit" and it's not allowed. Each branch circuit needs its own overcurrent protection at the panel. Junction boxes are fine for splicing within the same circuit, but not for creating new circuits. Install a subpanel if you need more circuits in a remote location.
What's the most common branch circuit mistake?
Mixing up wire size and breaker size. People see 20A receptacles and think they need 20A breakers, but 15A breakers with 14 AWG wire are fine for most receptacle circuits. Or they upsize breakers without changing wire - that's dangerous. Also, not understanding AFCI/GFCI requirements leads to failed inspections. Always match wire ampacity to overcurrent protection, not the load or receptacle rating.
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