MD84
Senior Member
- Location
- Stow, Ohio, USA
I need to provide optional standby power for a couple small loads. Does the transfer switch need to be specifically listed for this purpose? Would a double throw power relay be sufficient for this purpose?
Optional Standby Transfer Switch
- Thread starter MD84
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MD84
Senior Member
- Location
- Stow, Ohio, USA
I'm not following. I used the term optional standby as referenced by article 702. The generator would be a portable unit. It would be started manually. This is a residential application. Main panel feeds a standby panel via DT relay. NC contact of DT relay connected to feeder. NO contact of DT relay connected to generator power inlet. DT common connected to standby panel main lugs. DT relay coil connected to generator power inlet.
Power goes out. Plug in generator. Start generator. DT relay disconnects utility feed to standby panel and then connects generator. I realize there are other possibilities. For the simplicity of the question let's assume the above configuration.
Does NEC code prohibit use of a double throw relay being used as a transfer switch for a optional standby applications?
Power goes out. Plug in generator. Start generator. DT relay disconnects utility feed to standby panel and then connects generator. I realize there are other possibilities. For the simplicity of the question let's assume the above configuration.
Does NEC code prohibit use of a double throw relay being used as a transfer switch for a optional standby applications?
qcroanoke
Sometimes I don't know if I'm the boxer or the bag
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- Roanoke, VA.
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- Sorta retired........
Not to my knowledge.
As long as relay contacts are good for the connected load.
What are you doing with the generator neutral?
MD84
Senior Member
- Location
- Stow, Ohio, USA
Generator nuetral is floating. Cord and plug will carry nuetral to power inlet. Power inlet nuetral terminal will be wired to standby panel nuetral bar. Standby panel nuetral wired to main panel nuetral.
This is a non SDS and power inlet will be labeled as such. Generator case to nuetral is not to be bonded.
Looking at a 50A DPDT relay. Looking at about 3500VA load.
This is a non SDS and power inlet will be labeled as such. Generator case to nuetral is not to be bonded.
Looking at a 50A DPDT relay. Looking at about 3500VA load.
- Location
- Illinois
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- retired electrician
You need to check with your AHJ. Many read the requirement in 702.5 to required the use of listed transfer equipment. While the code wording does not specify the use of listed transfer equipment, it does require that the transfer equipment be suitable for the purpose. The easiest way for the AHJ to make sure that the transfer equipment is suitable for the purpose is to require the use of listed transfer equipment.
My mistake. When you said optional transfer switch, I assumed it was a commercial install, and there was also a required transfer switch.
Personally, I think using a DT relay has too much chance of backfeeding power back to the utility during if the relay failed to operate properly.
My local power company wouldn't allow it. See page 12 of this document for an example:'
https://ameren.com/-/media/corporate-site/Files/Reliability/SafeConnectDocumentC0Form.pdf
I would suggest sticking with a listed manual transfer switch. They aren't that expensive.
MD84
Senior Member
- Location
- Stow, Ohio, USA
IMO double throw inherently has a high level of safety in that it would be almost impossible for the relay to backfeed. How would a DT relay fail in which it would backfeed power from the generator to the utility?
Page 12 of the document from your local power company actually references double throw configuration for manual and automatic transfer.
I appreciate the feedback. The 50A DPDT relay I was considering is about 35$. Any recommendations on transfer equipment that is not too expensive relative to the relay?
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