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NEC 215.2 / Article 220 — minimum ampacity, 125% continuous, conductor lookup
Build a load schedule, mark continuous loads, and compare calculated amps to standard breakers and 75°C conductor tables used in the tool.
Calculate feeder and service entrance conductor sizing per NEC 215.2 and Article 220. Enter your connected loads, apply demand factors, and get the minimum feeder ampacity, conductor size, and required overcurrent protection — with full step-by-step math so you can show your work at inspection.
Related tools: Circuit Breaker Sizing · Wire Ampacity Calculator · Residential load calculator (NEC 220.82)
NEC Table 220.42 — General Lighting Demand Factors
| Portion of Load | Demand Factor | Application |
|---|---|---|
| First 3,000 VA | 100% | All occupancies |
| Next 3,001 to 120,000 VA | 35% | Dwelling units |
| Remainder over 120,000 VA | 25% | Dwelling units |
NEC 215.2 — Feeder Sizing Rules Summary
| Rule | NEC Section | Requirement |
|---|---|---|
| Minimum ampacity | 215.2(A)(1) | Not less than non-continuous load + 125% of continuous load |
| Minimum overcurrent protection | 215.2(A)(1) | Not less than non-continuous + 125% continuous |
| Demand factors permitted | 220.42, 220.53, 220.54 | Apply per applicable NEC section |
| Minimum feeder — dwelling | 215.2(A)(1) Ex. | 100A for a single dwelling |
Aluminum Service Conductor Sizes — Common Sizing
| Load (Amps) | Aluminum Conductor | Copper Equivalent | Typical Application |
|---|---|---|---|
| 60A | 4 AWG Al | 6 AWG Cu | Sub-panel, small loads |
| 100A | 1/0 AWG Al | 4 AWG Cu | Small home service |
| 125A | 2/0 AWG Al | 2 AWG Cu | Average home service |
| 150A | 3/0 AWG Al | 1 AWG Cu | Medium home service |
| 200A | 4/0 AWG Al | 2/0 AWG Cu | Standard 200A service |
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Tools for feeder & service work
Clamp meters, distribution gear, and conductors electricians use when sizing feeders.
Ideal 61-757 600A Clamp Meter
- True RMS AC/DC to 600A
- Large jaw for service and feeder conductors
- Backlit display for panel work
Square D QO140L200PG 200A Load Center
- 40-space layout for subfeeders and branch breakers
- QO series common in service upgrades
- Matches 200A feeder and service planning
Fluke 117 Electricians True RMS Multimeter
- Dead-front checks before energizing panels
- Non-contact voltage detection
- CAT III 600V for distribution work
Klein Tools ET310 Circuit Breaker Finder
- Map existing circuits before feeder changes
- Built-in GFCI tester
- Saves time during panel surveys
Klein 1000V Insulated Screwdriver Set
- IEC 60900 insulation for energized-adjacent work
- Useful for lugs, breakers, and panel terminations
- Multiple profiles in one service kit
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NEC 215.2 — Feeder conductor sizing rules
NEC 215.2(A)(1) sets the minimum ampacity for feeder conductors: they must carry the non-continuous load at 100%, plus the continuous load at 125%. A feeder serving 60A of continuous load and 40A of non-continuous load needs a minimum ampacity of (60 × 1.25) + 40 = 115A — not simply 100A. That is the value you compare to conductor ampacity tables.
The overcurrent device protecting the feeder follows the same sizing basis. The OCPD must be rated at not less than the non-continuous load plus 125% of continuous load, then you select the next standard device per NEC 240.6(A).
Demand factors — when and how to apply them
NEC Article 220 allows demand factors on specific load types because not everything runs at full load simultaneously. Common references include:
- NEC Table 220.42 — general lighting: First 3,000 VA at 100%, next 3,001–120,000 VA at 35%, remainder at 25% (dwelling units).
- NEC 220.53 — fixed appliances: With four or more fixed appliances (excluding dryer, range, A/C, and heat), 75% may apply to the total.
- NEC 220.54 — electric dryers: 5,000W or nameplate, whichever is larger, with additional rules for multiple dryers.
- NEC 220.55 — cooking equipment: Demand tables for ranges and cooktops by quantity and rating.
Residential vs. commercial feeder calculations
Residential (NEC 220.82 optional method): For dwelling units, the optional method in NEC 220.82 groups general lighting (3 VA per sq ft), small-appliance and laundry circuits, and fixed loads, then applies set demand factors. Use our residential load calculator (NEC 220.82) for that path.
Commercial (standard method): Commercial feeders typically follow the standard method — sum loads, apply Article 220 demand factors where permitted, and separate continuous vs. non-continuous classification for the 125% rule.
Pro tip Aluminum is common for large feeders and services. Size from the aluminum column of NEC Table 310.16 (or engineering supervision where required), use connectors listed for aluminum, and follow manufacturer torque and antioxidant requirements.
Frequently asked questions
What is the minimum feeder size for a residential service?
For a single dwelling unit, the feeder must be sized for the calculated load but not less than 100A per NEC 230.79(C). Many new homes are built with 200A service to allow for EV and electrification.
What is the difference between a feeder and a branch circuit?
A feeder runs between service equipment (or another supply point) and the final branch-circuit overcurrent device. A branch circuit is downstream of the last OCPD before the load. Feeders fall under NEC Article 215; branch circuits under Article 210.
Do I need to apply the 125% rule to feeder conductors?
Yes. NEC 215.2(A)(1) requires feeder conductor ampacity not less than the non-continuous load plus 125% of the continuous load for loads operated as continuous (typically 3 hours or more).
Can I use aluminum conductors for feeders?
Yes, when permitted by the design. Size per NEC Table 310.16, use listed lugs and connectors, and follow termination preparation practices for aluminum conductors.