School Generator


Electrical Designer
I believe this topic may have been addressed so I apologize in advance if I haven't found all the references on the forum yet.

On a typical school project and other commercial buildings I have worked on , myself and a other engineers have struggled with the transfer switch design when emergency lighting is intended to be served by the generator.

After much discussion, once we decide to put the lighting on the generator we have somewhat agreed it is considered a Level 1 emergency power system and isolate the lighting with a separate transfer switch/distribution equipment and also provide a separate 'Emergency electrical room" to house the life safety distribution equipment, per NFPA 110.

I don't do much healthcare but recently came across the following in regards to healthcare facilities.

NEC 517.31(B) states the following:
One transfer switch and downstream distribution system shall
be permitted to serve one or more branches in a facility with a
maximum demand on the essential electrical system of 150 kVA.

On the other transfer switch we typically have the tele/data equipment including any HVAC equipment serving main data rooms etc.
Especially with LED lighting having minimal loads, this separate life safety system for lighting looks like a real waste of money and square footage.
I now this will start some design approach topics, for example I do use emergency battery packs or centralized inverters from time to time to avoid this mess but unfortunately sometimes the emergency lighting is desired to go on the generator no matter what.

I'm surprised the above exception hasn't been expanded to cover more facility types, and seems like a reasonable request/proposal. I'm not sure where it would go as the current exception I refenced is in 517, but would be interested on any comments and feedback.

Thanks , long time reader , but don't post much


Senior Member
Field coordinator/ technical support
Why not do both? As long as the inverter can power the lights for 90 minutes, the feed to the inverter can be on general use generator power. It would be an optional standby system instead of emergency. Done that many times on retrofit services. The existing emergency lighting system just has to be maintained.