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120/240 V Generator Leg Balance
A portable generator's wattage is split across two 120 V legs, and each leg has its own limit.
Example
You enter
- Generator rated running watts (W) 10800
- Per-leg rating from the data sheet (A, 0 = watts / 240) 0
- 240 V loads, total running current (A) 10
- L1: 120 V load 1 (A) 6
- L1: 120 V load 2 (A) 8
- L1: 120 V load 3 (A) 12
- L1: 120 V load 4 (A) 0
- L2: 120 V load 1 (A) 5
- L2: 120 V load 2 (A) 3
- L2: 120 V load 3 (A) 7
- L2: 120 V load 4 (A) 0
You get
- Leg rating a 45
- L1 36.0 A (80% of 45.0 A)
- L2 25.0 A (56% of 45.0 A)
- L1 (%) 80
- L2 (%) 55.5556
- Neutral current 11.0 A
- Total running load 7,320 W (68% of rated watts)
- Total (%) 67.7778
- Balanced current per leg 30.5 A (heavier leg now: L1)
Checked against published worked examples from: WINCO.
Details, formula, and sources
ScopeA 240 V load -- a well pump, a water heater -- draws the same current on both legs; a 120 V load draws only on the leg it is plugged into (WINCO, Portable Generator Load Balancing Guide). So a generator at two-thirds of its rated watts can still trip a leg breaker because the refrigerator, the sump pump, and the microwave all ended up on one side. The calculator adds each leg separately against the per-leg rating (the rated watts over 240 V, or the data sheet figure), and reports the neutral current, the total watts, and the balanced current both legs would carry. A 10,800 W generator (45 A per leg) with a 10 A well pump, 6 + 8 + 12 A on L1, and 5 + 3 + 7 A on L2 carries 36 A (80%) on L1 and 25 A (56%) on L2 with 11 A on the neutral -- only 68% of its wattage, but L1 is at the level most makers treat as the continuous limit. The balanced split is 30.5 A per leg, and one plug moved gets close to it. The same arithmetic serves a spider box fed from a 120/240 V source. It does not address three-phase sources (neutral-imbalance), motor starting (the Generator Sizing for Motor Starting calculator), conductor sizing, GFCI and grounding, or building connection, which needs a transfer switch or interlock. The manufacturer's ratings, NEC Articles 445 and 590 as adopted, and the AHJ govern.
Formula or tableper-leg rating = rated watts / 240 V (or the data sheet figure); L1 A = 240 V loads + 120 V loads on L1; L2 A = 240 V loads + 120 V loads on L2; neutral A = |120 V on L1 - 120 V on L2|; total W = 240 x 240 V amperes + 120 x all 120 V amperes; balanced leg A = 240 V loads + all 120 V loads / 2.
Source and editionWINCO, Portable Generator Load Balancing Guide (L-1008), named as the method (a 240 V ampere loads both legs, a 120 V ampere loads one); NEC Articles 445 and 590 cited by number for generators and temporary installations; the generator manufacturer's per-leg rating named as governing.
Free accessAddition on the user's own load currents against the generator's rating; no code table is reproduced.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesrated_w, per_leg_rating_a, load_240_a, l1_load1_a, l1_load2_a, l1_load3_a, l1_load4_a, l2_load1_a, l2_load2_a, l2_load3_a, l2_load4_a, leg_rating_a, l1_a, l2_a, l1_pct, l2_pct, neutral_a, total_w, total_pct, balanced_leg_a
- 240 V loads on both legs a 240 V ampere loads each leg equally and carries no neutral currentWINCO L-1008
- Per-leg rating rated watts / 240 V unless the data sheet states a per-leg figuregenerator manufacturer data
- Running current only motor starting and surge are checked separatelyproject method