ECN Forum Forums Code Related Discussion NEC & other Code issues Fire Pump Transfer Sw.

Ernie- I think since both supplies are grounded wye's that you are required to have an EGC and the neutral brought to the transfer switch. The neutral will have to be switched by the transfer switch so you don't end up with redundant neutral bonding. As for the EGC - All of this equipment must be tied together via an EGC. I don't know what your Grounding Electrodes are or where they are in relation to your supplies but you will need a GEC running from the utility and the generator to the grounding electrode(s). I've got some engineer diagrams if I can find them I'll post them here. Drawn and published by IEEE.

Generator Feeds transfer Switch off a Breaker. (there are several breakers feeding
other transfer switchs besides Fire Pump). Thus Ist Disconnect Neurtal Must be grounded there (Generator), So from there To Fire Pump transfer Switch only a Ground is required as the Pumps 480 volt 3 phase.
Now the power co. side :
As theres NO disconect switch ahead of the Transfer swith Except the one on the Service rated Transfer switch The Neutral Must be brought in and grounded here. (The fire Pump transfer switch is a 3 pole only .

It appears that the load on the transfer switch needs a neutral - Where will it get a neutral when the utility is disconnected from the load?

If you don't need the neutral then just bring in the EGC and use a 3 pole sw.

[This message has been edited by George Little (edited 09-27-2006).]

I believe 250.30(A)(1) Exception 1 allows bonding neutrals from separately derived systems, and I can see no reason it wouldn't apply in this instance. I don't see a problem with bonding the neutrals.

[This message has been edited by SteveFehr (edited 09-28-2006).]

Steve- If you Bond the utility supply with the Main Bonding Jumper at the Service would you approve of installing a jumper wire between the neutral screw of a receptacle and the ground screw of the receptacle somplace else downstream on the system? That's essentially what you would have if you bond at the Utility Service and then remotely at a generator an don't switch the neutral with the transfer switch. The concern is having current flow on the grounding conductor. If a person is not using the Grounded conductor for a current carring conductor then I agree with you. Code reference: 250.28, 250.142 NEC '02 Ed.

Edited to add code reference.



[This message has been edited by George Little (edited 09-28-2006).]

George, I'm aware of the implications of bonding the neutral to ground at multiple points, but it IS legal by NEC in the specific case of multiple derived sources, and absolutely required for make-before-break connections like UPS maintanance bypass switches and paralleling/load-sharing generators where switching the neutral is impossible.

Thus, it is allowed by NEC 250.30(A)(1) in instances like this because there is no way around it. 250.30(A) allows exceptions to this in several other instances, too. Edit- my mistake, I said this was unchanged from 2002, but 250.30(A) is annotated as changed in NEC 2005.

[This message has been edited by SteveFehr (edited 09-28-2006).]

Steve.... I'm still not sure of the actual theory behind your statement (regarding make / break and UPS)...but I'm a believer!!

I got confused on this point, installed a UPS incorrectly, tha customer went to test the system..... and I got to spend $$$$ replacing all the stuff my incorrect neutral connection fried. OOPS!

renosteinke, I work in an industry where power interruption in not acceptable. We have redundant utility feeders, redundant generators and redundant UPS systems, most of which are designed for make-before-break switchover, with few exceptions. The equipment has to be designed to parallel and always includes circuitry to ensure phase-matching and open the breakers if paralleling is lost. It's especially important on UPS systems where the UPS, Static Switch and Maint Bypass switch are fed from 2 or 3 separate sources.

It certainly puts a burden on the contractor to test properly and do it right the first time, especially in situations when we're unable to schedule an outage or test offline and have to test on the operational building. But, that's why we're paid the big bucks!

As this has changed from 2002, I'll transcribe:
NEC 2005, 250.30(A)(1) Exception 1: For separately derived systems that are dual fed (double ended) in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single system bonding jumper connection to the tie point of the grounded circuit conductors from each power source shall be permitted.

That's a mouthful, but if you break it down, it pretty clearly authorizes bonding the neutrals together. It's stated elsewhere in 250 that the neutral bus bar is considered a bonding jumper.

Same clause, with extraneous verbage removed for clarity:
For separately derived systems that are dual fed in a common enclosure, a single system bonding jumper to the grounded circuit conductors from each power source shall be permitted.

[This message has been edited by SteveFehr (edited 09-28-2006).]

Steve thanks for your comments and insight. I agree with you entirely (Don't tell anyone I agreed with an engineer) and have seen the article in IEEE magizine March 2001 that explains it very well including the first make/last break concept. Good to banter with someone who has done his homework.

Upon closer inspection, I think I was reading it slightly wrong- it allows tying the neutrals of the two services together, but only if the two neutral systems are bonded to ground ONLY at that tie point. Thus preventing the ground wires carrying and neutral current.

So, Yoopers must bond the generator neutral and panel neutral to the utility neutral and ground at the transfer switch and not to ground anywhere else.