A standby generator permit set is sized by a load calculation, not by the customer’s wish list. NEC 2020 220.82, the optional feeder and service calculation for a dwelling unit, establishes the number the generator and the transfer switch have to answer. Get that sheet right and the rest of the package – single-line, site plan, fuel routing, anchoring – follows from it.
Key takeaways
- NEC 2020 220.82(B) takes the first 10 kVA of general load at 100 percent and the remainder at 40 percent. 220.82(C) adds the largest heating or air-conditioning load.
- A house generator is an optional standby system under NEC Article 702, not an Article 700 emergency system or an Article 701 legally required standby system.
- In NEC 2020, 702.4 is capacity and rating and 702.5 is transfer equipment. Under 702.4(B)(2), automatic transfer with an undersized generator requires load management.
- NEC 2020 445.18(D) requires a readily accessible emergency shutdown device outside the dwelling unit.
- NFPA 37 drives the 5-foot minimum clearance to structures and wall openings; propane pulls in NFPA 58 and the fire district.
- Sound limits and property-line setbacks are not NEC items. They come from zoning, noise ordinances and sometimes an air district permit.
Generators are the most natural adjacent line an installer can add: same service, same panel, same plan reviewer, different code basis. PV is a source under Article 705; a generator is a source under Articles 445 and 702, carrying a load set under Article 220. That is what we build into every residential generator plan set we draw.
When does a standby generator need a permit?
A permanently installed standby generator needs a building or electrical permit in every jurisdiction we submit to, and usually more than one. Gas-fired units pull a separate gas or plumbing permit. Propane and diesel commonly need fire district approval. Portable cord-and-plug-connected units are the exception, and the ones that cause backfeed problems.
What varies is the submittal. Contra Costa County, California wants a site plan, electrical line diagrams referencing NFPA 37, structural calculations where applicable, a plumbing plan for the gas piping, and equipment cut sheets. Permit Sonoma requires the generator make and model, the manufacturer’s installation instructions, and a single-line diagram showing wire size and transfer switch details per CEC Article 445.18.
The adopted cycle varies too. Nevada County writes to the 2022 California Electrical Code; the Leon County, Florida handout writes to the 2020 NEC. Jurisdictions are spread across several NEC editions, so the cover sheet has to name the one you drew to, and fire districts, flood-elevation rules and air quality districts each stack approvals on top – the same pattern as the permitting requirements that catch installers out.
How do you size a standby generator for a house?
Run the dwelling load calculation first. NEC 2020 220.82 is the optional calculation and the one used on residential generator work. Step one totals the general loads. Step two applies 100 percent to the first 10 kVA and 40 percent to the remainder. Step three adds the largest of the heating or air-conditioning loads. The result is what the generator and transfer switch have to answer.
The general-load list in 220.82(B) is specific, and leaving an item out is the fastest route to a correction. Per EC&M’s walkthrough of dwelling load calculations under the 2020 NEC, it takes general lighting at 3 volt-amperes per square foot, two small-appliance circuits and the laundry circuit at 1,500 volt-amperes each, fixed and permanently connected appliances at nameplate, and motors at 100 percent.
Step three is a selection, not a sum. NEC 2020 220.82(C) takes the largest of: air conditioning at 100 percent; a heat-pump compressor without supplemental heat at 100 percent; a heat pump with supplemental heat at 100 percent of the compressor plus 65 percent of the supplemental; central electric space heating at 65 percent for three or fewer separately controlled units, or 40 percent for four or more.
A worked 2,400 square foot example
| Step | Item | Volt-amperes |
|---|---|---|
| 220.82(B) | General lighting, 3 VA per sq ft x 2,400 | 7,200 |
| 220.82(B) | Two small-appliance circuits at 1,500 VA | 3,000 |
| 220.82(B) | Laundry circuit | 1,500 |
| 220.82(B) | Fixed appliances at nameplate | 25,100 |
| 220.82(B) | Connected total | 36,800 |
| 220.82(B) | First 10,000 VA at 100 percent | 10,000 |
| 220.82(B) | Remaining 26,800 VA at 40 percent | 10,720 |
| 220.82(C) | Air conditioning at 100 percent | 6,720 |
| Result | Calculated load | 27,440 |
27,440 volt-amperes at 240 volts is roughly 114 amperes. The 200-ampere service is comfortable; the generator is not, because a common 22 kW air-cooled unit does not carry 27.4 kVA on its own. That gap is the design conversation, and it is the same arithmetic that drives Level 2 EV charger permit plans on the load side.
What does NEC Article 702 require for an optional standby system?
Article 702 covers optional standby systems, which is what a residential backup generator is. Two sections carry the plan-review weight. In NEC 2020, 702.4 is capacity and rating and 702.5 is transfer equipment. Between them they decide whether the generator selection is legal and whether the transfer switch specified is the right one.
Capacity splits on how the system transfers. The Leon County handout, written to the 2020 NEC, sizes manual transfer equipment under 702.4(B)(1) and automatic transfer equipment under 702.4(B)(2), and states that where generator output capacity is less than the main breaker ampacity, a load management system is required. That is the code hook behind every load-shed module on the market. Transfer switches are required for all generators, rated for the connected load, with model number and manufacturer specifications in the submittal. The 2023 cycle says the same more cleanly: Schneider Electric’s explainer on NEC 2023 702.4 describes an automatic system as either supplying the entire connected load or, where an energy management or power control system is used, having capacity for the maximum load that system permits.
Signage is the item most often missed. NEC 2020 702.7 requires a sign at the service-entrance equipment identifying the location and type of the on-site optional standby source, with 230.2(E) covering the service plaque. Draw the placards with their actual text – missing labels are a recurring entry in why permits get rejected.
Do Articles 700 and 701 apply to a house?
Almost never. Article 700 covers emergency systems legally required for life safety; Article 701 covers legally required standby systems. A single-family backup generator is neither, so it lands in Article 702. AHJ handouts listing 700.7 and 701.7 beside 702.7 do so because the same handout covers commercial work. Where a reviewer applies Article 700 to a house, state the classification on the cover sheet.
Where does the transfer switch sit relative to the service?
The transfer switch decides everything downstream. It sits either ahead of the main panel as service-entrance rated equipment, or behind it feeding a separate backed-up panel. That one choice sets the scope of backup, the conductor runs, the grounding and bonding arrangement, and how much of the existing service you have to touch.
| Arrangement | Position | Backs up | Consequence |
|---|---|---|---|
| Service-entrance rated ATS | Ahead of or replacing the main disconnect | Whole service | Service conductors re-terminated; grounding and bonding re-established at the new disconnect |
| Downstream ATS with essential-loads panel | Fed from a breaker in the main panel | Circuits moved to the sub-panel | Main panel untouched; sub-panel bonded as a sub-panel |
| Managed whole-house ATS | Either, with a listed load management device | Whole service, non-essential loads shed | Satisfies NEC 2020 702.4(B)(2) |
| Manual transfer switch | Downstream, with an interlock | Selected circuits | Sized under NEC 2020 702.4(B)(1); no automatic start |
Whichever way it lands, the single-line has to show it. Nevada County enumerates that sheet: panel and sub-panel amperage and location, grounding and bonding conductor sizes, junction boxes, disconnect types, sizes and locations, conduit from generator to transfer switch, and the switch type, size and location. It also requires an auxiliary disconnect within three feet of the main service panel, and a readily accessible disconnect lockable in the open position within line of sight and 50 feet of the building served. Re-establish a service disconnect and the grounding and bonding changes with it.
Whole-house backup or an essential-loads panel?
Choose on the load calculation, not the sales conversation. If the calculated load under NEC 2020 220.82 exceeds what the generator can carry, there are three legal options: a bigger generator, a load management system under NEC 2020 702.4(B)(2), or an essential-loads panel that physically limits what is connected.
An essential-loads panel is the traditional answer. Relocate the circuits that matter – refrigeration, heating controls, well pump, lighting, network equipment – into a sub-panel fed from the transfer switch. The generator only ever sees those circuits, so sizing closes cleanly. The cost is labour: every relocated circuit is a re-terminated conductor, and both panel schedules go on the drawing.
Load management is the modern answer and the code supports it explicitly. A listed load-shed device drops the air conditioner, range or dryer as generator output approaches its limit, letting a whole-house transfer switch sit in front of a service the generator could not otherwise carry. Nevada County goes further, stating the standard load calculation is not required where load management modules control all 240-volt loads. Two things must be on the sheet: the listed device, and the loads it controls. A larger generator is often the wrong retrofit answer, because it moves the unit out of the air-cooled range.
How does a standby generator interact with PV and battery storage?
This is what separates a generator set drawn by a solar shop from one drawn by an electrician who has never seen an inverter. Adding a generator changes where the PV interconnection sits relative to the transfer switch, changes the busbar arithmetic the array was permitted under, and pulls Article 705 into a submittal that would otherwise be Articles 220, 445 and 702.
Contra Costa County’s generator checklist cites California Electrical Code Articles 220, 445, 702 and 705 together. Article 705 is the interconnection article, and its presence on a generator handout is the tell: on a house with an existing array, the reviewer reads both sources on one drawing. Our breakdown of which NEC articles govern a plan set maps how 690, 705 and 706 divide that sheet.
Position comes first. If the PV interconnection lands on the load side of the transfer switch, the array is part of the backed-up system and the generator has to tolerate it. On the utility side it is offline during an outage, and the customer needs that in writing before the sale. The busbar comes second: an array interconnected under the NEC 2023 705.12(B)(3)(2) 120 percent busbar allowance was calculated against a specific panel, main breaker and breaker position. Insert an ATS or move the array to a backed-up sub-panel and that calculation is redone. Keep the percentages straight – 120 percent belongs to the busbar, 125 percent to continuous current.
Listing comes third and is skipped most often. Whether a given inverter or battery may operate alongside a generator, and through what device, is answered by the equipment’s installation manual, not by the NEC – which is why Permit Sonoma requires those instructions in the submittal. Read the storage manual before the single-line is drawn: it decides whether the generator lands on the storage system’s generator input, on a microgrid interconnect device, or on a transfer switch that never lets the two sources meet. A generator permit and a solar plan set that disagree about the same panel get both rejected.
Fuel supply, clearances and anchoring
Three non-electrical items decide whether the generator can go where the customer wants it: the clearance envelope, the fuel supply, and the anchorage. All three belong on the site plan, and all three are checked before the electrical sheets are read.
Clearances come from NFPA 37 and from the manufacturer, whichever is stricter. Contra Costa County states that setback distances and clearances shall not be less than the 5-foot minimum, and requires the site plan to identify setbacks to property lines, clearances to building openings and vents, and the generator’s position relative to structures, switches, gas lines and fuel tanks.
Fuel is where sets fail review, because it is a plumbing submittal inside an electrical project. Nevada County requires gas line locations, materials and sizes, sizing calculations to the California Plumbing Code, and propane tank size. A generator starting under load is a large sudden BTU demand, and an undersized run produces a unit that starts then stalls. Propane adds NFPA 58, cited by Nevada County for the 5-foot clearance from the regulator vent to ignition sources.
Anchoring is structural. Nevada County requires equipment and pad anchoring to ASCE 7 – usually a manufacturer detail plus a seismic or wind check on an air-cooled unit, and a sealed calculation on anything larger. Same review path as structural load calculations on a rooftop array, and the same question as when a project needs a PE stamp.
Sound and setback ordinances: the part that is not in the NEC
The NEC says nothing about noise. Sound limits, property-line setbacks and screening come from local zoning and noise ordinances, and in some counties from an air quality district. They are enforced at the same counter as the electrical permit, and they are the most common reason a generator moves after the site survey.
The AHJ documents show how uneven this is. Permit Sonoma requires the site plan to show setbacks to property lines, combustibles and vegetation, publishes no distances on its back-up generator page, and notes additional permits may be required from the Northern Sonoma County Air Pollution Control District or the Bay Area Air Quality Management District. Nevada County publishes hard numbers for a different hazard: a minimum 100 feet to wells and water sources, plus setbacks to septic and leach lines. Neither publishes a decibel limit, because that lives in the zoning code.
So capture it at survey: property-line distance, distance to the nearest neighbouring dwelling and its windows, distance to the customer’s own bedroom windows, and any recorded easements. Confirm the noise ordinance with the planning counter, not the building counter, before the pad location is fixed.
What goes in an AHJ-ready generator plan set?
A generator plan set is smaller than a PV set but it is not a one-page sketch. A reviewer needs the code basis, the site, the fuel, the calculated load, the transfer arrangement, the disconnects and labels, and equipment listings that back every rating on the drawing.
- Cover and code sheet. Scope, the code edition adopted, and the Article 702 classification stated explicitly.
- Site plan. Property lines and setbacks, generator and pad, clearances to openings and vents, fuel tank and gas routing, conduit, easements, wells and septic.
- Load calculation sheet. The NEC 2020 220.82 calculation step by step, with the demand factors, the 220.82(C) selection, and the result against the service and generator ratings.
- Single-line diagram. Service and meter, main disconnect, panel and busbar ratings, transfer switch type, size and location, generator conductors and conduit, grounding and bonding, disconnects, and the PV or storage interconnection.
- Panel schedules. Existing, relocated and revised loads for both panels.
- Fuel plan. Pipe type, length, size and BTU demand, riser diagram, tank size and location, and the sizing calculation.
- Anchoring, placards and cut sheets. Pad anchorage to ASCE 7, the NEC 2020 702.7 service-entrance sign and the 230.2(E) plaque drawn with their actual text, and equipment listings.
The PV equivalent is set out in what belongs in a permit-ready plan set, and the pre-submittal checklist is the pass to run before anything reaches the counter.
How Avila Solar Drafting handles generator plan sets
Avila Solar Drafting produces the drawings; you submit them and you keep the customer. Standard plan set turnaround is 2-3 business days from the point we have complete and accurate project information, and revisions are free for six months. Every set is drawn to the code cycle the jurisdiction has adopted and designed to pass first-time review.
A standard package covers the site plan, electrical layout, pipe layout, pipe schematic, line diagram and anchoring details. Load calculations are ordered with the set, and electrical or structural Professional Engineering seals are available as adders through our engineering stamp service. Sealing is licence-bound: Avila Solar does not stamp in APS or SRP territory in Arizona, the City of Peoria or the City of Phoenix, Los Angeles County and LABD in California. Check the project address against that list first. Where a project reaches us directly from a homeowner rather than through an installer, a separate development and consultancy fee applies; every business is exempt — the fee applies only to homeowners running their own project.
If generator work is a new line for your crews, the bottleneck is rarely the install. It is a signed job waiting on a 220.82 calculation and a transfer switch decision nobody has had time to make. Same argument as outsourced solar drafting, and the reason we run EV charger permit designs off the same intake. Order a generator plan set at my.avilasolar.com/order/genset, or call 971-410-0655.
Frequently asked questions
Do you need a permit to install a standby generator?
Yes. A permanently installed standby generator needs a building or electrical permit in every jurisdiction we submit to, plus a gas or plumbing permit for natural gas and propane. Propane and diesel often need fire district approval.
What NEC code sizes a standby generator for a house?
NEC 2020 220.82, the optional feeder and service load calculation for a dwelling unit. It totals the general loads, applies 100 percent to the first 10 kVA and 40 percent to the remainder under 220.82(B), then adds the largest heating or air-conditioning load under 220.82(C).
Is a residential generator covered by NEC Article 700, 701 or 702?
Article 702, optional standby systems. Article 700 covers emergency systems legally required for life safety; Article 701 covers legally required standby systems. A single-family backup generator is neither, because nobody is compelled by law to install it.
Does a whole-house generator have to match the full service size?
No, provided the design accounts for the gap. Under NEC 2020 702.4(B)(2), where generator output capacity is less than the main breaker ampacity, a load management system is required. NEC 2023 702.4 says the same: an automatic system either supplies the entire connected load or has capacity for the maximum load an energy management system permits.
Where does the transfer switch go relative to the main service?
Either ahead of the main disconnect as service-entrance rated equipment, backing up the whole service, or downstream of it feeding an essential-loads panel. The first re-establishes the service disconnect and its grounding and bonding; the second leaves the main panel untouched but requires circuits to be relocated.
Can a generator work with an existing solar array and battery?
Yes, but the position of the PV interconnection relative to the transfer switch decides the design and the equipment listing decides what is permitted. On the load side the array is part of the backed-up system; on the utility side it is offline during an outage. Adding a transfer switch can also invalidate the busbar calculation the array was interconnected under.
