NEC 90.15: calculation walkthrough

What NEC 90.15 Actually Says

NEC 90.15 is the rounding rule for ampere calculations. When a load calc produces a fractional ampere result, and that fraction is less than 0.5, you are permitted to drop it. Fractions of 0.5 or greater round up. Simple on paper, but it changes conductor sizing, OCPD selection, and sometimes panel scheduling.

The section was codified in the 2023 cycle to standardize what many engineers and inspectors had been doing informally for years. Before it landed in Article 90, rounding guidance was scattered through informational notes and local practice. Now it is in the front of the book, which means it applies broadly across load and ampacity calculations unless another article overrides it.

Why This Matters on the Job

A fractional amp is the difference between a #12 and a #10, or between a 20A breaker and a 25A. On a single circuit it is academic. On a service calc across dozens of circuits, those rounding decisions stack up and can push a service from 200A to 225A, or change your transformer selection.

Working the rule correctly also protects you during plan review. Inspectors verifying your load calc will round the same way you do, assuming you both follow 90.15. If you round per some other convention, expect red lines.

Field tip: round at the end of the calculation, not during intermediate steps. Premature rounding compounds and can shift your final number by a full ampere or more.

Calculation Walkthrough: Residential Service

Work a 2,400 sq ft single-family dwelling, standard method per NEC 220.42. General lighting load is 3 VA/sq ft, so 2,400 x 3 = 7,200 VA. Add two 1,500 VA small appliance branch circuits per NEC 220.52(A) and one 1,500 VA laundry circuit per NEC 220.52(B), giving 4,500 VA. Subtotal: 11,700 VA.

Apply the demand factors from NEC Table 220.42(A). First 3,000 VA at 100% is 3,000. Remainder is 8,700 VA at 35%, which is 3,045 VA. General load after demand: 6,045 VA. Add a 10 kW range at 8,000 VA per NEC Table 220.55, a 4.5 kW dryer at 5,000 VA per NEC 220.54, and a 5,000 VA water heater at 100%. Total: 24,045 VA.

Divide by 240V for a single-phase service: 24,045 / 240 = 100.1875 A. Here is where 90.15 applies. The fractional remainder is 0.1875, which is less than 0.5. Drop it. Your calculated load is 100 A, and a 100A service is compliant. Without 90.15, some would argue you need the next standard size up.

Where 90.15 Does Not Apply

The rule covers amperage rounding, but it does not override specific sizing rules elsewhere in the Code. A few places to watch:

• NEC 210.19 and 215.2 require conductors sized to the calculated load before any rounding. You still pick the next standard wire size per NEC Table 310.16.
• NEC 240.4(B) allows rounding up to the next standard OCPD, but only under specific conditions. 90.15 does not expand that allowance.
• Motor circuits governed by NEC Article 430 use their own multipliers (125% for continuous, 250% for some OCPDs). Apply those first, round per 90.15 after.
• Continuous load factors in NEC 210.20(A) and 215.3 (125%) must be applied before you round the final result.

In short, 90.15 is the last step, not a shortcut around sizing rules. Do the math the Code requires, then round the final ampere figure.

Common Mistakes

The most frequent error is rounding each load individually. If you round your lighting load to 30A, your receptacle load to 20A, and your HVAC load to 25A before summing, you have introduced noise that compounds in your favor or against it. Keep everything in VA or precise amperage until the final division.

Another trap: rounding down when the fraction is exactly 0.5. The Code says "less than 0.5" gets dropped. A result of 100.5A rounds to 101A, which matters if the next standard OCPD above 100A is 110A per NEC 240.6(A).

1. Calculate all loads in VA using the applicable article (220, 430, 440, etc.).
2. Apply demand factors and continuous-load multipliers.
3. Divide by system voltage to get amperes, carrying decimals.
4. Apply NEC 90.15 only at the final step.
5. Select conductor per NEC 310.16 and OCPD per NEC 240.6.

Documentation and Inspection

When you submit a load calculation, show the fractional result and the rounded value side by side. An inspector looking at "100.1875 A rounded to 100 A per NEC 90.15" accepts it without a question. A sheet that just says "100 A" invites them to run the numbers themselves and flag any discrepancy.

Field tip: keep a one-page load calc template on your tablet with the 90.15 citation in the footer. Inspectors and plan reviewers appreciate seeing the rule referenced, and it speeds up permits.

90.15 is a small rule with real consequences. Apply it at the right step, cite it in your paperwork, and know which articles keep their own sizing requirements ahead of it.