Does anyone know if the NEC has verbiage for a warning sign for a switchboard that is backfed similarly to the verbiage it has for a disconnect switch? This is for a project where power is generated onsite and fed directly into the switchboard. The main breaker won't de-energize the bus, it just de-energizes the downstream panel. I believe the switchboard will still be service entrance rated because the G-N are bonded at the switchboard.
backfeed warning sign
- Thread starter mjmike
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kwired
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I call your description a "Main lugs" unit instead of a main breaker/main switch unit.
You could have up to six such disconnects in this unit and still call it one service.
Not following you on this? It isn't a service, no utility, so the 6-disconnect rule is not applicable.
kwired
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Maybe I didn't quite follow what you have either, but I understand you do have something already designated as a main switch or breaker, yet it isn't exactly serving as the main. You have to either be supplying this via main lugs or a switch/breaker that is not the one marked as the main.
Six disconnect rule applies to buildings supplied by a feeder as well as a service.
The main breaker in the switchboard will serve the feeder leaving the switchboard. The switchboard branch breakers (6 of them) will put power onto the switchboard bus from the onsite power source.
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So basically both the main breaker and the branch breakers in the switchboard are carrying power in the opposite to normal direction?
The same setup that you would see in a PV system AC combiner panel?
Are all of the breakers involved suitable for backfeed (no line/load markings), and are the branch breakers additionally retained if appropriate?
That is correct.
iceworm
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mj -
A couple of questions:
Any chance you could post a one-line?
How many and what size gens are we discussing?
The Switchboard contains CBs for 6 incomming gens and one output CB to another switchboard (or maybe an MCC) that powers all loads?
edit to make q's a bit clearer (hopefully)
ice
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correct.
K8MHZ
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Just curious as to how one keeps 6 generators in phase.
Maybe I should know this, but I don't. Except in a power plant, I have never seen generators in parallel.
Maybe I should know this, but I don't. Except in a power plant, I have never seen generators in parallel.
ggunn
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Don't some gennies make DC and use an inverter to interconnect?
kwired
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That was going to be my next question.
iceworm
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Now how about a one-line giving some gen sizes, bus capacities, CB sizes? - Just so we know the magnitude.
A pencil sketch will be fine. No need to be accurate. Just state approximates sizes - not design.
ice
iceworm
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kwired
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I guess that depends on your definition of small. Something tells me anything smaller then maybe 100 KVA (maybe even more then that) is not worth the cost of what is necessary to keep them all in phase vs purchasing a larger generator that can handle the load.
iceworm
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Yes, for this kind of stuff, 500kva is about as small as I have seen in more than 3 parallel. I've only seen one 6 - parallel (750kva asI recall). I never had to work on it.
I suspect (that means I have not ever had to figure it out - nor seen any paper) that the most economical is to size the generation at n+1 to account for maintenance issues. If one had 2000kva load (4 - 500s) then the ideal might well be 3 each 1000kva sets.
They are easy to keep in phase. Once the gens are synced and on-line, it is hard to get them out of phase. The hard part is keeping the load-share, var-share, auto-start operating reliability. There gets to be enough parts, that MTBF starts creeping toward zero.
I've told you more than I know - time to stop.
ice
iceworm
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(edit to add) Six generators with cooresponding paralleling gear, load share, var share, auto start, ...., is really complex. It definitely requires qualified personnel to operate.
With the information we have, and no knowledge of what is driving the question: The best I can tell you is, I've never seen any multiple gen system having backfeed warning labels on the common bus switchgear. (edit to add) Having such a label won't make it any safer, nor lessen the requirements for qualified operators.
That answer could change if we ever hear the rest of the story.
ice
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K8MHZ
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I was working on a new gas turbine plant that had 5 generators. 4 gas turbines and one HRSG. I was pretty much in idle mode one day and I asked one of the engineers how it was done.
The turbines are starter / generators. The generators start turning using power from the grid, which gets them spinning in phase. Once up to a certain speed, the turbines are lit and start taking over as the grid power is throttled back. This goes on until the turbine is up to speed. All this is monitored in a control room on multiple large screens. It took at least two people 24/7 to watch what was going on and to push the right buttons when wrong things happened.
That's why I asked.
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