Number of Atomatic Transfer Switches Required?

[cconoly]

Why does the code require separate automatic transfer switches (ATS) for emergency loads and non-emergency loads for general applications, but for healthcare you are allowed a single ATS for systems under 150 kVa? If I am remodeling a school that has a small generator (less than 150kVa) used only for life safety items such as emergency lighting and emergency egress, and they want to add a few receptacle circuits for their computer servers, then why is it required that I put them on separate ATS when healthcare allows it? It would seem to me that it should be completely opposite and healthcare should be separate no matter what? For reference (from the National Electrical Code Handbook 12th ed - M. Earley, et al): General Emergency Systems:

Article 700.5 (D):"Use. Transfer equipment shall supply only emergency loads."

Explanation: "Although the alternate power source for the emergency system is permitted to supply emergency loads as well as other loads, 700.5(D) limits the transfer switch to emergency loads, that is, loads classified as emergency in accordance with 700.1. Other loads, such as legally required standby loads or optional standby loads (covered by Articles 701 and 702), are not permitted to be supplied from the emergency system transfer switch. Where a single generator is used to supply both emergency and nonemergency loads, multiple transfer switches are required."

Healthcare:

Article 517.30(B)(4): "Transfer Switches. The number of transfer switches to be used shall be based on reliability, design, and load considerations. Each branch of the emergency system and each equipment system shall have one or more transfer switches. One transfer switch shall be permitted to serve one or more branches or systems in a facility with a maximum demand on the essential electrical system of 150 kVA."

Explanation: "In larger health care facilities, the requirements of 700.5(D) and 517.30(B)(4) correlate from the perspective that there is o ne or more transfer switches that supply only emergency loads. However, 517.30(B)(4) also differs from 700.5 in smaller facilities with a maximum demand on the essential electrical system of 150 kVa or less. In those cases, a single transfer switch is permitted to supply the entire essential electrical system including loads that are not part of the emergency system. This is an example of where a requirement in Article 517 differs from a requirement in Article 700, and Article 517 takes precedence as is specified in 517.26."

 

[charlie b]

Good question. I initially thought I knew the answer. But after looking in the book again, I realized that I do not. The handbook is not helping, and that is not a surprise. If you didn't already know what the rule was, the handbook is telling you what the rule is, but using different words. It is not providing any "why" information. I can say that this 150KVA rule dates back at least as far as 1999. My guess that the rule is the result of pressure from smaller medical facilities that could not afford a complex electrical system, and that likely did not deal with surgical procedures that pose the greatest risk to the patients. 150 KVA is not much of a power source, and it would not support a great deal of medical equipment.

Welcome to the forum.

 

[copper chopper]

because in a health care facility the computer network is considered an essentian part of the place and can go on that ats. in a school its not. Hospitals, nursing homes, even out patient facilities have there nurse call, security,paging all runs through there main network hub... if you have ever worked at a facility like these with a careless worker who does not check to see if there is a filter in the shop vac before vacuming up dust from a coreing job and sets off the fire alarm. and 12 differnt poeple get alerted about what just happened. So this is why...some times we have to find things out the hard way...

 

[eesac]

It is my interpretation that the 150kvA rule under Art 517 is for "essential electrical systems", which is broken out into 2 separate systems of "emergency"(EM) and "equipment"(EQ). The EQ branch is still essential for hospital operation, it is just not "Life Safety"(LS) or "Critical"(CR). In other "regular" bldgs which falls under Art 700, the only "essential" systems are typicall emergency egress lighting, nothing else deals with any life safety issues (at least none I can think at this time). "Emergency" under Art 700 and "Essential" under Art 517 are basically the same thing. In a hospital when I put a duplex receptacle located in a regular office (which is non-essential) on back-up generator power, it goes into another separate ATS just for non-essential loads, this should hold true whether the essential system is less than, equal to, or larger than 150 kVA. This is the similar scenario in Art 700.