Generator Paralleling

[dema]

A number of brands of generators will now parallel with on board controls, but they have to parallel to something. I looked up bus cabinets and that brings me to a second question - why does Husky have a 3000A bus cabinet that only handles four conductors per phase up to 750MCM? I mean that isn't even one direction (source or load) - what's the point? There must be a point, right? Please explain it to me.

But my main question has to do with code requirements for multiple generators. Standard paralleling gear has a breaker per generator. The NEC says that for one generator, if the generator is within site of the building a second breaker (beyond the one that comes with the generator) is not required. Okay - so now I have three generators and the paralleling is being done with their own breakers. What does the code say? Do I need a fuse upstream of the transfer switch? Seems like I do unless I have a main breaker in my gear. I'm thinking that this configuration is correct. But do I require the fuse? What if I do have a main breaker in my MDP? Do I require a fuse then? The generator paralleling would prevent three generators from feeding the bus at once.

I am thinking that if a separate building houses the equipment, that I must have some sort of a main in the main building anyway. In that case, I particularly wouldn't want to pay for the fuse in the attached diagram.

 

[dema]

Conductors

Conductors

I think the fuse is necessary either way due to protecting the conductors heading towards the main. I haven't fully investigated tap rules (I don't usually use them), but since there is a common bus, I don't think there is a way to get around the OCP on the output.

The issue here is that I have had two generator dealers tell me that they can parallel without gear. So, I'm trying to get my head around the issues - and sometimes when I think hard on one issue another flies right by my ear.

I was told that if you put all the lugs on the transfer switch itself that that was okay - but I don't think that is any different at all. There still is the issue of protecting the load conductor from the transfer switch and three separate breakers (two really, because one generator would be on standby at all times) can't protect the outgoing unless somehow one of the tap rules applies.

Would appreciate discussion on this.

 

[Smart $]

Have a look at Article 705 Interconnected Electric Power Production Sources. While you will not be connected to the utility, each generator is a production source.

 

[dema]

That does not compute

That does not compute

Okay, these guys say they have been putting these in and at least one installation was actually a military installation. But when I read article 705 of the NEC 705.12.D.1 says that each source interconnection must be made at a dedicated breaker.

This doesn’t say made WITH an interconnection, it says AT an interconnection.

I'm going to ask them about this too. I would love, love to have a cost effective way to have a redundant generator - but the gear often costs as much as the generator - sometimes more. And I don't see that they can get around it.

 

[dema]

Befuddled

Befuddled

I am obviously befuddled and reading things incorrectly. I apologize. C applies and not D. I am flustered about this and therefore my intelligence appears to be on vacation. If you have input, please reply.

 

[Smart $]

If connection is your main problem, then you have the wrong equipment... or you will have to revise the equipment you have. Is your bus cabinet custom made? If manufactured en masse, is it listed? ...do the instructions prohibit modification?