Trades Math Basics: Circuit Load and Panel Sizing (Watts, Amps, Demand Factors per NEC)

Trades Math – Circuit Load and Panel Sizing (Watts, Amps, Demand Factors per NEC)

What This Is

This is how you figure out how much power a building or machine will actually pull so you can size the wires, breakers, and electrical panel correctly. If you get it wrong, breakers trip constantly, wires overheat, or the inspector red-tags your work. Real-world example: You’re wiring a small machine shop with three 240V motors (5 HP, 3 HP, 1 HP), a 120V welder, and twenty 120V outlets. The NEC says you can’t just add up all the watts—you have to apply demand factors (discounts) to some loads. This guide shows you how to calculate the real load so you can pick the right panel and breakers without wasting money on oversized gear.

Key Terms & Formulas

• Watt (W): Unit of power. Example: A 100W light bulb uses 100 watts when on.
• Ampere (A or “amps”): Unit of current (flow of electricity). Example: A 15A breaker trips if more than 15 amps flow through it.
• Volt (V): Unit of electrical pressure. Example: Most U.S. homes use 120V for outlets and 240V for big appliances.
• Ohm’s Law: P = I × E (Power = Current × Voltage). Example: A 120V circuit pulling 10A uses 1,200W (120 × 10).
• Continuous Load: A load that runs for 3+ hours straight (e.g., HVAC, shop lights). NEC rule: Size the breaker at 125% of the continuous load.
• Non-Continuous Load: Runs for less than 3 hours (e.g., a drill press, table saw). NEC rule: Size the breaker at 100% of the load.
• Demand Factor: A percentage the NEC lets you apply to reduce the total load for sizing. Example: For a house with 10 small-appliance circuits, you only count 3,000W (not 10 × 1,500W = 15,000W).
• General Lighting Load (NEC Table 220.12): 3 VA per sq. ft. for houses. Example: A 2,000 sq. ft. house needs 6,000 VA (2,000 × 3) for lighting.
• Small-Appliance Circuits (NEC 220.52): 1,500 VA each for kitchen, laundry, and bathroom outlets. Example: A house with 2 kitchen circuits = 3,000 VA (2 × 1,500).
• Motor Loads (NEC 430.22): Size wires at 125% of the motor’s full-load current (FLC). Example: A 5 HP, 240V motor pulls 15A (from NEC Table 430.248). Wire size = 15 × 1.25 = 18.75A-Use #12 AWG (20A rated).
• Feeder/Service Calculation (NEC 220.82): For a house, add up all loads, apply demand factors, then size the main panel and service drop.

Step-by-Step / Process Flow

Step 1: List All Loads

• Write down every device that will pull power (lights, outlets, motors, appliances).
• Group them by type (lighting, small-appliance, motor, etc.).
• Example:
• General lighting: 2,000 sq. ft. house-6,000 VA (2,000 × 3)
• Small-appliance circuits: 2 kitchen, 1 laundry-4,500 VA (3 × 1,500)
• 5 HP motor (240V): 15A (from NEC Table 430.248)
• 3 HP motor (240V): 9.6A
• 1 HP motor (120V): 16A (from NEC Table 430.248)
• Welder (120V, 20A breaker): 2,400W (120 × 20)

Step 2: Apply Demand Factors (NEC 220.42 & 220.52)

• General lighting: First 3,000 VA at 100%, the rest at 35%.
• 6,000 VA-3,000 + (3,000 × 0.35) = 4,050 VA
• Small-appliance circuits: First 3,000 VA at 100%, the rest at 35%.
• 4,500 VA-3,000 + (1,500 × 0.35) = 3,525 VA
• Motors: No demand factor—use 125% of FLC for continuous loads.
• 5 HP motor: 15A × 1.25 = 18.75A-20A breaker (next standard size)
• 3 HP motor: 9.6A × 1.25 = 12A-15A breaker
• 1 HP motor: 16A × 1.25 = 20A-20A breaker
• Welder: 2,400W (no demand factor, but check if it’s continuous).

Step 3: Convert Everything to VA or Amps

• VA = Watts (for resistive loads like heaters, lights).
• For motors: Use FLC (Full-Load Current) from NEC tables.
• Example conversions:
• 5 HP motor: 15A (from NEC Table 430.248)
• Welder: 2,400W ÷ 120V = 20A

Step 4: Add Up All Loads

• Lighting + small-appliance: 4,050 + 3,525 = 7,575 VA
• Motors:
• 5 HP: 18.75A (240V)-4,500 VA (18.75 × 240)
• 3 HP: 12A (240V)-2,880 VA
• 1 HP: 20A (120V)-2,400 VA
• Welder: 2,400 VA
• Total VA: 7,575 + 4,500 + 2,880 + 2,400 + 2,400 = 19,755 VA

Step 5: Size the Panel & Service

• Convert VA to amps:
• 120V loads: 2,400 (1 HP) + 2,400 (welder) = 4,800 VA ÷ 120V = 40A
• 240V loads: 4,500 (5 HP) + 2,880 (3 HP) = 7,380 VA ÷ 240V = 30.75A
• Lighting/small-appliance: 7,575 VA ÷ 240V = 31.56A
• Total amps: 40 + 30.75 + 31.56 = 102.31A
• Next standard panel size: 125A (common for small shops).
• Main breaker: 125A (must be-total load).

Step 6: Check Wire & Breaker Sizes

• Feeder wires (from meter to panel): Must handle 125A.
• #2 AWG copper (115A @ 75°C, NEC Table 310.16).
• Branch circuits:
• 5 HP motor: #12 AWG (20A breaker, 125% of 15A).
• 3 HP motor: #14 AWG (15A breaker, 125% of 9.6A).
• 1 HP motor: #12 AWG (20A breaker, 125% of 16A).
• Welder: #12 AWG (20A breaker, 100% of 20A).

Common Mistakes

• Mistake: Adding up all motor loads at 100% and sizing the panel based on that. Correction: Motors have high startup currents—NEC requires 125% of FLC for continuous loads. Why? Motors pull 6x their FLC for a split second when starting, so breakers must handle the extra surge.

• Mistake: Forgetting to apply demand factors to lighting and small-appliance loads. Correction: Use 35% for loads over 3,000 VA (NEC 220.42). Why? Not all lights and outlets run at full power at the same time—this prevents oversizing.

• Mistake: Using the motor’s nameplate amps instead of NEC Table 430.248. Correction: Always use the FLC from the NEC table, not the motor’s label. Why? Nameplate amps can vary by manufacturer, but the NEC table standardizes calculations.

• Mistake: Sizing a breaker for a continuous load at 100% instead of 125%. Correction: For loads running 3+ hours, multiply by 1.25. Why? Breakers heat up over time—125% ensures they don’t trip under sustained load.

• Mistake: Mixing 120V and 240V loads without converting to VA first. Correction: Convert everything to VA before adding. Why? You can’t add amps directly if they’re on different voltages.

Trade-Specific Insights

Field Trick: For small shops, 125A panels are the sweet spot—big enough for most tools but not overkill. If you’re unsure, go 200A for future-proofing.

Code Shortcut: For residential panels, the NEC lets you use 100A service if the total load is-10,000 VA (after demand factors). Most houses fall under this.

Motor Wiring Hack: If a motor’s FLC is ? 16A, you can use #14 AWG wire (but still size the breaker at 125%). Example: A 1 HP, 120V motor (16A FLC)-20A breaker but #14 wire is allowed.

Inspector Red Flags: - No demand factors applied-Instant fail. - Breakers sized at 100% for continuous loads-Must be 125%. - Using nameplate amps instead of NEC tables-Always use the book.

Quick Check Questions

1. A 5 HP, 240V motor has an FLC of 15A. What size breaker do you need?
2. Answer: 20A (15A × 1.25 = 18.75A-next standard size).

3. Why? NEC 430.22 requires 125% of FLC for continuous loads.

4. A house has 2,500 sq. ft. of living space. What’s the general lighting load in VA?

5. Answer: 7,500 VA (2,500 × 3 VA/sq. ft.).

6. Why? NEC Table 220.12 says 3 VA per sq. ft. for lighting.

7. You have three 120V small-appliance circuits (kitchen, laundry, bathroom). What’s the total VA after demand factors?

8. Answer: 3,525 VA (3 × 1,500 = 4,500 VA-3,000 + (1,500 × 0.35) = 3,525 VA).
9. Why? NEC 220.52 says first 3,000 VA at 100%, the rest at 35%.

Last-Minute Cram Sheet

1. Ohm’s Law: P = I × E (Power = Amps × Volts).
2. Continuous load breaker size: 125% of FLC (NEC 430.22).
3. Non-continuous load breaker size: 100% of load.
4. General lighting load: 3 VA per sq. ft. (NEC 220.12).
5. Small-appliance circuits: 1,500 VA each (NEC 220.52).
6. Demand factors: First 3,000 VA at 100%, rest at 35% (NEC 220.42).
7. Motor FLC: Use NEC Table 430.248, not nameplate.
8. 120V vs. 240V: Convert all loads to VA before adding.
9. Panel sizing: Must be-total load after demand factors.
10. Wire size: Must handle 125% of continuous loads (NEC 210.19(A)).