Multiple 3Pole ATSs and gensets

5 replies [Last post]
ECMLuna's picture
Joined: 2014-03-15

In a 3000amp bus system (neutral and gnd bonded here), for two 2000amp branch loads (office and HVAC), each load is fed with one 2000amp, three phase breaker (with GFP), 3 pole ATS, and a standby by generator (neutral and gnd not bonded).  Two gen sets are not paralleled.  This is my first encounter, two 3pole with two gen sets in one system.  So the questions are:
1.  Is this circuit/system consider paralleled circuit/ssytem? 
2.  Is this a stable system with shared neutral through the main bus? 
3.  Is there any code issue with two 3pole ATSs in GFP system?

electrogasman's picture
Joined: 2014-03-16

This system is not parallel, as each gen set is a separately derived system. In my experience, installations like this were made with 4-pole ATSs, to keep the separately derived neutral "separate". If the ATS is wired properly, the ground-fault breakers (2,000 amp) will be removed from the system all-together when the load transfers.

ECMLuna's picture
Joined: 2014-03-15

I do agree that 4-pole ATSs are needed, but the installation is operating with two 3-pole ATSs. The installation did experience one generator shutting down during transfers due to low voltage/frequency, according to the monitor read out. During last test, the two generators ran at the same time. The major difference is that one generator was loaded lightly when compared to the other generator -- load (less than 10%). Both loads were heavily loaded when the installation experienced an outage during transfer. As to parallel or not, if one draws out the circuits, it does look like parallel.

APR15's picture
Joined: 2013-11-21

I see in your original post that the generator neutral points are not bonded to ground; if this is truly the case then the generators would not be considered as a separately derived systems. For a non-separately derived system, a 3-pole (solid neutral) ATS is the correct equipment, as the generator system would rely on the bonding jumper in the main normal equipment for voltage reference and ground-fault current return. If a 4-pole ATS were used, the generator neutral would not be solidly connected to the grounding electrode system at any point, and there would not be an intentional path on which ground fault current could return to the generator neutral point.

Ground fault protection on the normal main equipment and the distribution equipment downstream of the ATS would function properly, as any fault current would return on the grounding conductor (outside of the sensor window / CTs) and cause the breaker to open. Ground fault protection applied at the generator would be ineffective, as any ground fault current would be returning to the generator neutral point via the bonding jumper in the normal system equipment.

mdshunk's picture
Joined: 2014-03-14

It only looks like parallel until you realize that the ATSs open the "normal" feed and they're just two independent systems. It's functionally no different than you and your neighbor (at home) being served from the same utility transformer, and you each have a backup generator for your home. You're not in parallel. You're both disconnected from the utility supply when operating on generator power.

filltee8's picture
Joined: 2014-04-10

Does a 120VAC GFCI outlet, under a counter in a kitchen, need to be installed in a weatherproof box?

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