Know specific, unique applications of generators in mission critical facilities

Overview

Rener's 2024 article explains how NEC load classification (emergency, legally required standby, optional standby, COPS and healthcare essential systems) dictates generator response time, wiring separation, fire protection and coordination requirements. Emergency (NEC 700) power must be available within 10 seconds and legally required standby (NEC 701) within 60 seconds. Emergency systems require selective coordination (NEC 700.30).

It gives derating rules for ambient temperature and altitude: 0.4% per 10 F above 77 F and 1.5% per 1,000 ft above 3,300 ft, with generators rated at 0.8 PF lagging. Leading power factor from IT loads is discussed. Medium-voltage (15 kV class) generators are typically considered at 2.5 MW and above. Emergency fuel needs at least 2 hours of runtime and mission-critical sites 8-72 hours; NFPA 110 7.9.5 limits diesel stored inside or on the roof to 660 gal.

Fuel options cover natural gas, HVO and fuel cells. HVO can cut carbon emissions by up to about 90% in existing diesel engines at roughly a 10% cost premium, and generator lead times are 18-24 months. The article also covers acceptance and periodic operation testing. As a trade article set in the US NEC context, it should be read alongside local Turkish regulations when applied here.

Key points

1. Emergency (NEC 700) power within 10 s; legally required standby (NEC 701) within 60 s.
2. Derate 0.4% per 10 F above 77 F and 1.5% per 1,000 ft above 3,300 ft; generators rated at 0.8 PF lagging.
3. Emergency distribution needs 2-hour rated enclosure or sprinkler protection in buildings with occupancy of 1,000 or more or certain buildings over 75 ft.
4. Emergency systems require selective coordination (NEC 700.30).
5. Medium-voltage (15 kV class) generators typically considered at 2.5 MW and above.
6. Emergency fuel at least 2 h; mission-critical sites 8-72 h; NFPA 110 7.9.5 limits diesel stored inside or on the roof to 660 gal.
7. HVO can cut carbon by up to about 90% in existing diesel engines at about 10% cost premium; lead times 18-24 months.

Using it in design

• Generator catalogue entries need kW/kVA at 0.8 PF plus derating inputs; the tool should auto-derate for site altitude and design ambient.
• Keep NEC 700 loads off the IT generator or on a separately classified path.
• Require selective coordination on emergency branches and warn on leading-PF IT loads.
• Flag medium-voltage generators as an option above roughly 2.5 MW.

Who should read it

Electrical engineers designing standby generator systems for mission-critical facilities.

Related standards

• NFPA 70 Art. 517/700/701/702/708
• NFPA 110
• NFPA 99
• IEEE 446
• UL 1008
• ANSI/NETA ATS-2021

Publication details

Authors

Brian Rener

Publisher

Consulting-Specifying Engineer

Year

2024

Document language

English

Source type

Expert / community

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation