Legally Required Standby Generator Transfer Time

[pnunley]

I have read several commentaries including the NEC handbook in 701.1 stating 60 seconds or less for the maximum transfer time to standby power. We are a hospital and also have Emergency generators which have to transfer in 10 seconds. I couldn't find anything in NFPA 99, 101 or 110 but could have overlooked something. Any help in locating the actual code(s) specifying these transfer times would be appreciated.

Thank you,

Paul Nunely

 

[ron]

For regular installations:
700.12 Emergency Systems
701.11 Legally Required Standby Systems

For hospitals
517.31 Emergency System.
517.42 Automatic Connection to Life Safety Branch

 

[templdl]

I have read several commentaries including the NEC handbook in 701.1 stating 60 seconds or less for the maximum transfer time to standby power. We are a hospital and also have Emergency generators which have to transfer in 10 seconds. I couldn't find anything in NFPA 99, 101 or 110 but could have overlooked something. Any help in locating the actual code(s) specifying these transfer times would be appreciated.

Thank you,

Paul Nunely

Interesting. The first step is that it be established that there is a power los. What defines that there has been a power loss not just brief power loss. Then there must be a start signal for the genset start. The genset then must excellerate and establish the voltage and frequency before being allowed to transfer.
Knowing this there would be as one sort of very minimum time for the loss of power to be determined and the genset started to the transfer.
There are also time delay options such as a time delay remergancy to normal which may be used to assure that the normal source has in fact been lost. There is also a time delay emergency back to normal option which is used to assure that the normal source has been established before transfer back to normal is allowed.

 

[Onsite_energy]

NFPA 110 type 10 requires it to start and assume load in 10 seconds or less


Sent from my iPhone using Tapatalk

 

[iwire]

Interesting. The first step is that it be established that there is a power los. What defines that there has been a power loss not just brief power loss.

My comments will be based on my experiance with emergency generators.

An emergency generator requires an Automatic Transfer Switch, most will have many adjustments for high and low voltage limits, frequency limits and time delays. In most applications the ATS controls the generator although Generac has moved the brains into the genertor and the genertor controls their proprietary transfer switch.

Typically the engine start delay is set to 0 seconds so even a small blip will cause the engine to start.

If the utility power returns while the genertor is starting the ATS will not transfer and the engine will run unloaded for until the utility supply is stable for the length of the engine stop delay. Typically the stop delay is 10 to 20 minutes.

If the utility does not return, once the engine has started and the supply from the genertor is stable the transfer switch will transfer.

Once the utility power returns and is stable the retransfer timer starts, typically this is set for about 15 minutes.

When that timer runs down the load transfers back to utility and the engine shut down timer starts.

The genertor will have block heaters so it will fire up quickly, often it can be up and carrying the load in 5 seconds.

 

[LIM]

My comments will be based on my experiance with emergency generators.

An emergency generator requires an Automatic Transfer Switch, most will have many adjustments for high and low voltage limits, frequency limits and time delays. In most applications the ATS controls the generator although Generac has moved the brains into the genertor and the genertor controls their proprietary transfer switch.

Typically the engine start delay is set to 0 seconds so even a small blip will cause the engine to start.

If the utility power returns while the genertor is starting the ATS will not transfer and the engine will run unloaded for until the utility supply is stable for the length of the engine stop delay. Typically the stop delay is 10 to 20 minutes.

If the utility does not return, once the engine has started and the supply from the genertor is stable the transfer switch will transfer.

Once the utility power returns and is stable the retransfer timer starts, typically this is set for about 15 minutes.

When that timer runs down the load transfers back to utility and the engine shut down timer starts.

The genertor will have block heaters so it will fire up quickly, often it can be up and carrying the load in 5 seconds.

All of what you said is true and is covered by section 6.2 of NFPA 110.

 

[dema]

NEC 700.12

NEC 700.12

It is under Sources of Power. Not to exceed 10 seconds.


Also See table 4.1b in the 2016 NFPA 110.

There are different classes of loads and they aren't defined in this document.