A generator is a generator. The wiring is not. The National Electrical Code puts backup power into three separate articles, and the article a given load falls under decides how it has to be wired, how fast it has to come on, and what hardware may be used. Two of those three classes are explicitly permitted to share raceways, cables, boxes and cabinets with ordinary building wiring. One of them is not.
That single difference is worth more than any other when a project is being priced, because the class is not a design choice. It is a property of the load, set by whichever code or authority requires the power to be there.
The class is set by the load, not by the generator
Each of the three articles opens by describing a different kind of load, and the descriptions are the whole classification.
Article | Class | Scope as the code states it |
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700 | Emergency systems | Circuits and equipment intended to supply, distribute and control electricity for illumination, power, or both, to required facilities when the normal supply is interrupted |
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701 | Legally required standby systems | Circuits and equipment intended to supply, distribute and control electricity to required facilities for illumination or power, or both, when the normal supply is interrupted |
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702 | Optional standby systems | Systems intended to supply power to public or private facilities or property where life safety does not depend on the performance of the system |
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The first two read almost identically, and the distinction is not in the wording but in which code requires the load. The third is defined by a negative: optional standby is what is left when life safety does not depend on it. The article’s note gives the typical cases — industrial and commercial buildings, farms and residences, serving heating and refrigeration, data processing and communications, and processes that would cause discomfort, interrupt a process or damage product if they stopped.
One consequence is easy to get wrong. A single engine-driven generator can serve loads in all three classes, and that does not merge them.
Three rules that change with the class
Three measurable requirements separate the classes, and each of them is stated as a number or as a permission.
Requirement | Emergency (700) | Legally required standby (701) | Optional standby (702) |
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Time to be available after normal supply fails | Within the time required for the application, not to exceed 10 seconds | Within the time required, not to exceed 60 seconds | No time stated |
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May the wiring share raceways, cables, boxes and cabinets with general wiring? | No. Wiring from the emergency source, or from emergency source distribution overcurrent protection to emergency loads, must be kept entirely independent of all other wiring and equipment, with five permitted exceptions | Yes. The wiring is permitted to occupy the same raceways, cables, boxes and cabinets with other general wiring | Yes. Same permission, in the same words |
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Must transfer equipment be automatic? | Yes. Transfer equipment, including automatic transfer switches, shall be automatic, identified for emergency use and approved by the authority having jurisdiction | Not stated as automatic | Not stated as automatic; transfer equipment must be suitable for the intended use |
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The three classes and what each one requires
One clause decides the wiring method
The sentence that carries the most weight in the whole subject is the second paragraph of 700.10(B), and it is a prohibition rather than a permission.
Wiring from an emergency source, or from emergency source distribution overcurrent protection to emergency loads, shall be kept entirely independent of all other wiring and equipment, unless otherwise permitted in five specific cases. All five are enclosures where two systems inevitably meet.
Permitted exception | Where two systems meet |
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Transfer equipment enclosures | Normal power wiring |
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Exit or emergency luminaires | Wiring supplied from two sources |
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A listed common junction box at those luminaires | Wiring from two sources |
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A common junction box at unit equipment | The branch circuit supplying the unit, and the emergency circuit it supplies |
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The combination arrangement | One source serving more than one class, as set out below |
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The combination arrangement is where the article is most precise. Where an emergency source supplies any combination of the three classes, it must do so from separate vertical switchboard sections, with or without a common bus, or from individual disconnects mounted in separate enclosures, supplied by single or multiple feeders without overcurrent protection at the source. And then the clause that decides the layout:
Legally required and optional standby circuits shall not originate from the same vertical switchboard section, panelboard enclosure, or individual disconnect enclosure as emergency circuits.
The unit of separation is the enclosure, not the conductor. Running the emergency feeder in its own conduit while landing it in a panelboard that also carries legally required standby circuits fails the clause, and so does a shared disconnect enclosure. Feeders are permitted to supply distribution equipment between the source and the point where the classes are separated, which is what makes a single switchboard with segregated sections workable.
Against that, 701.10 and 702.10 each say the same thing in the same words: the wiring of that class is permitted to occupy the same raceways, cables, boxes and cabinets with other general wiring. Two of the three classes may be run alongside ordinary power. One may not.
What the independence requirement actually separates
One source can carry all three classes, in a fixed order
The capacity clause settles a question that comes up on every project with more than one class of load. An alternate source is permitted to supply all three classes where it has adequate capacity, or where automatic selective load pickup and load shedding gives adequate power in this order: emergency circuits first, then legally required standby, then optional standby. The same source may be used for peak load shaving on the same conditions.
Two details are easy to miss. The priority order is written into the code, so the shedding sequence is not a design preference. And peak load shaving counts as satisfying the periodic test requirement, provided the other testing conditions are met — so a generator run regularly to shave peak demand carries part of its own maintenance obligation.
There is an apparent conflict nearby that is not one. Transfer equipment for the emergency system shall supply only emergency loads. The combination arrangement reaches the other two classes through the fifth exception to the independence rule and separate switchboard sections. The dedicated emergency transfer switch and the combination arrangement are two different mechanisms.
The fire-rated feeder rule has a size threshold
The provision most often quoted as a blanket requirement is 700.10(D), and it begins with a condition. The additional fire protection requirements apply in assembly occupancies for not less than 1000 persons, or in buildings above 23 m (75 ft) in height with any of these occupancy classes: assembly, educational, residential, detention and correctional, business, and mercantile. The code points to the Life Safety Code for the occupancy definitions.
Below those thresholds, and in occupancy classes not on the list, the additional requirements do not apply. That is not a license to be careless, because 700.10(C) separately requires emergency wiring circuits to be designed and located so as to minimize the hazards that might cause failure due to flooding, fire, icing, vandalism and other adverse conditions. But it does mean a two-story office building and a 40-story one are not held to the same feeder rule.
Where the rule does apply, the feeder-circuit wiring has to meet one of five conditions, and only two of the five involve a cable product at all.
Route | Condition |
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1 | Installed in spaces or areas fully protected by an approved automatic fire suppression system |
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2 | A listed electrical circuit protective system with a minimum 2-hour fire rating |
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3 | Protected by a listed thermal barrier system for electrical system components with a minimum 2-hour fire rating |
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4 | Protected by a listed fire-rated assembly with a minimum 2-hour fire rating that contains only emergency wiring circuits |
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5 | Encased in a minimum of 50 mm (2 in.) of concrete |
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Route 2 is the fire-rated cable route, and the article’s note points to the UL guide information for electrical circuit protective systems, which covers the installation conditions needed to keep the rating. That the listing depends on installation is the part that decides whether a specifier can claim the route at all. What a two-hour rating means for a cable itself, and why a jacket’s reaction to fire is a different property, is covered in LSZH versus PVC jacket selection.
Two companion requirements sit alongside. Equipment for feeder circuits, including transfer switches, transformers and panelboards, must be located either in spaces fully protected by approved automatic fire suppression systems or in spaces with a 2-hour fire resistance rating. And control conductors installed between the transfer equipment and the emergency generator must be kept entirely independent of all other wiring and must meet the same conditions as the feeder wiring — which puts the generator start circuit in the same category as the feeder it starts.
At the source end the threshold returns with a lower rating: source equipment in those same occupancies and heights must be installed in spaces protected by approved automatic fire suppression or in spaces with a 1-hour fire rating.
The five routes to a protected emergency feeder
What the emergency class adds beyond the wiring
Several requirements exist for emergency systems and appear nowhere in the other two articles. Each one is checkable on a walkthrough.
Provision | Requirement |
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Identification | All boxes and enclosures, including transfer switches, generators and power panels, for emergency circuits must be permanently marked so they are readily identified as part of an emergency circuit or system |
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Loads | No appliances and no lamps other than those specified as required for emergency use may be supplied by emergency lighting circuits |
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Single-lamp failure | The system must be designed so that failure of any individual lighting element, such as a lamp burning out, cannot leave a space requiring emergency illumination in total darkness |
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High-intensity discharge | Where HID lighting is the sole source of normal illumination, the emergency lighting system must operate until normal illumination is restored |
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Switch control | Only authorized persons may control emergency lighting; switches connected in series, and three-way and four-way switches, must not be used |
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Switch location | A control switch for emergency lighting must never be placed in a motion-picture projection booth or on a stage or platform |
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Overcurrent devices | Branch-circuit overcurrent devices in emergency circuits must be accessible to authorized persons only |
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Coordination | Emergency system overcurrent devices must be selectively coordinated with all supply-side overcurrent protective devices |
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Ground-fault protection | The alternate source is not required to have ground-fault protection of equipment with automatic disconnecting means; ground-fault indication is required instead |
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The signal requirement is quantified. Where the emergency system is a solidly grounded wye of more than 150 volts to ground and the circuit protective devices are rated 1000 amperes or more, a ground-fault signal is required, its sensor located at or ahead of the main system disconnecting means for the emergency source and its maximum signal setting at a ground-fault current of 1200 amperes. Instructions for what to do when it operates must be posted at or near the sensor.
Selective coordination is required for emergency and for legally required standby. Optional standby is not covered by that requirement.
The three classes share the same hardware rules
The source and storage requirements barely differ between the three articles, which is easy to miss when the wiring rules differ so sharply.
Requirement | Value |
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Storage battery capacity, emergency and legally required standby | Total load for at least 1½ hours, without voltage falling below 87½ percent of normal |
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Automatic battery charging | Required |
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Automotive-type batteries | Must not be used |
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Generator automatic starting and transfer | Required on failure of the normal service |
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Retransfer time delay | A setting of 15 minutes must be provided, to avoid retransfer on a short-time reestablishment of the normal source |
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On-site fuel for an internal combustion prime mover | Not less than 2 hours of full-demand operation |
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Fuel and cooling independence | Prime movers must not be solely dependent on a public utility gas system for fuel or a municipal water supply for cooling |
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Generator sets needing more than 10 seconds | Permitted only where an auxiliary power supply energizes the emergency system until the generator can pick up the load |
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Fuel cell system | Not less than 2 hours of full-demand operation; where a single fuel cell is the normal supply, it must not be the sole source for the standby system |
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Unit equipment battery | 87½ percent of nominal for at least 1½ hours, or 60 percent of initial illumination for at least 1½ hours |
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Unit equipment installation | Permanently fixed in place, not portable; flexible cord-and-plug connection permitted if the cord does not exceed 900 mm (3 ft) |
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Unit equipment supply circuit | The same branch circuit as the normal lighting in the area, connected ahead of any local switches |
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Portable generator, optional standby | Temporary connection without transfer equipment is permitted only where maintenance and supervision ensure that only qualified persons service the installation and the normal supply is physically isolated by a lockable disconnecting means or by disconnection of the normal supply conductors |
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Service-entrance sign, all three classes | A sign is required at the service-entrance equipment identifying the type and location of the on-site power sources |
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Grounding warning sign, all three classes | Required where removing a grounding or bonding connection in the normal power source equipment would interrupt the grounding electrode conductor connection to the alternate source. The sign text reads: SHOCK HAZARD EXISTS IF GROUNDING ELECTRODE CONDUCTOR OR BONDING JUMPER CONNECTION IN THIS EQUIPMENT IS REMOVED WHILE ALTERNATE SOURCE(S) IS ENERGIZED |
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FAQ
Can the same generator feed emergency and optional standby loads?
Yes, on two conditions: the source must have adequate capacity, or automatic selective load pickup and load shedding must be provided, and where an emergency source supplies a combination of classes the wiring must come from separate vertical switchboard sections or individual disconnects in separate enclosures. Legally required and optional standby circuits must not originate from the same switchboard section, panelboard enclosure or individual disconnect enclosure as the emergency circuits.
Does every emergency feeder need a two-hour fire-rated wiring system?
No. The additional fire protection requirements apply in assembly occupancies for not less than 1000 persons, or in buildings above 23 m (75 ft) in height with an assembly, educational, residential, detention and correctional, business or mercantile occupancy. Where they apply, the feeder must meet one of five conditions, and a listed circuit protective system is one of them rather than the only one. The other four are a fire-suppression-protected space, a listed thermal barrier system, a listed fire-rated assembly containing only emergency wiring, and 50 mm of concrete.
Conclusion
Three articles, three classes, and one decisive difference. Emergency system wiring must be kept entirely independent of all other wiring and equipment, and where classes are combined the separation is enforced at the switchboard section, panelboard enclosure or disconnect enclosure. Legally required standby and optional standby wiring may share raceways, cables, boxes and cabinets with general wiring — the code says so in those words twice.
The rest follows from that. The availability time is 10 seconds, or 60, or unstated. The fire-rated feeder rule carries a height and occupancy threshold, and where it applies it offers five routes of which a fire-rated cable is one. Selective coordination is required for two of the three classes. The loads keep their class all the way to the terminal.
Cable for the circuits the class governs
ZONG Cable builds the constructions that emergency and standby feeders are specified with, and quotes what each one is listed to rather than what it looks like. If the project needs route 2 under 700.10(D)(1), tell us the listing and the installation conditions it has to hold under, and we will confirm what the construction can support. The range sits under cable, sealing where a feeder crosses a rated wall is covered in cable penetration firestopping, feeder sizing against the load is worked through in voltage drop and conductor sizing, what a laboratory report does and does not prove is covered in test report validity, and private-label construction for a specified project is handled under OEM support.
Sources
- NFPA 70, National Electrical Code, 2011 edition. Read directly for this article: Article 700, Emergency Systems — 700.1 Scope and its four informational notes citing Article 517, NFPA 99-2005, NFPA 101-2009 and NFPA 110-2010; 700.3 Tests and Maintenance paragraphs (A) through (E); 700.4 Capacity, including (A) on capacity and rating for loads operated simultaneously and (B) on selective load pickup, load shedding and peak load shaving with its stated priority order; 700.5 Transfer Equipment paragraphs (A) through (D); 700.6 Signals including (D) on the ground-fault signal, its sensor location and its 1200 ampere maximum setting; 700.7 Signs; 700.10 Wiring, Emergency System, including (A) identification, (B) wiring with the five permitted exceptions and subparagraph (5) with its items on separate vertical switchboard sections, feeders without overcurrent protection at the source, the prohibition on legally required and optional standby circuits originating from the same switchboard section, panelboard enclosure or individual disconnect enclosure as emergency circuits, and feeders to the point of separation, (C) wiring design and location, and (D) fire protection with its occupancy and height threshold, the five conditions in (D)(1) including the informational note on UL guide information for electrical circuit protective systems, (D)(2) on feeder-circuit equipment and (D)(3) on generator control wiring; 700.12 General Requirements including the 10 second limit, the 1-hour fire rating for source equipment in the listed occupancies and heights, (A) storage battery, (B) generator set with its 15 minute retransfer delay, 2 hour fuel supply, fuel and cooling independence, 10 second provision and outdoor generator provisions, (C) uninterruptible power supplies, (D) separate service, (E) fuel cell system and (F) unit equipment; 700.15; 700.16 including the high-intensity discharge provision; 700.17; 700.18; 700.20 including the prohibition on series and three- and four-way switches; 700.21; 700.22; 700.23; 700.25; 700.26; 700.27. Article 701, Legally Required Standby Systems — 701.1 Scope; 701.4; 701.5; 701.6; 701.7; 701.10 stating that the wiring is permitted to occupy the same raceways, cables, boxes and cabinets with other general wiring; 701.12 General Requirements including the 60 second limit and (A) through (G) including (E) on connection ahead of the service disconnecting means; 701.25; 701.26; 701.27. Article 702, Optional Standby Systems — 702.1 Scope; 702.2 definition of optional standby systems and its informational note; 702.4; 702.5 Transfer Equipment including the requirement for transfer equipment and the exception for temporary connection of a portable generator; 702.6 Signals; 702.7 Signs including (B) and the full text of the warning sign; 702.10 Wiring Optional Standby Systems; 702.11 Portable Generator Grounding; 702.12 Outdoor Generator Sets. Source text read from a full-text copy of the 2011 edition.
- ZONG Cable range. https://zh-zong.com/cable/
