Reminder: Never connect a generator to home wiring without transfer switch

Jay Hennigan jay at
Wed Aug 25 18:11:11 UTC 2021

On 8/25/21 10:25, Mel Beckman wrote:
> Jay,
> No, because transformers work in both directions :)

I think you mean, "Yes, because transformers work in both directions." 
First of all, I absolutely agree that no one should attempt to energize 
their home wiring with a standby generator unless there is a proper 
transfer switch in place. I very much understand the safety concerns.

The question that Ethan raised makes sense, however. If power to several 
blocks is out and I connect my little 2KW Honda to my house wiring 
without a transfer switch, because transformers work in both directions 
my generator will see the load of the whole neighborhood. This will 
immediately and severely overload the generator and at best cause it to 
stall out or trip its output breaker, at worst to fail catastrophically.

In the very rare case that the outage is at the fuse on the pole pig 
feeding just my house or that of me and one or two neighbors, then 
indeed the generator may continue to run and that transformer will have 
distribution voltage of 4KV or so on the utility side, a very dangerous 
condition. That's a pretty unusual situation, however. Typical power 
outages are substantially more widespread. My little generator would be 
looking at the load of the entire outage area reflected through the 
(bidirectional as you note) transformers. The load of half the town 
will, as Ethan speculated, completely overwhelm any practical 
residential standby generator to the point that it stops producing power 
either by failure or by tripping its breaker.

Even if the generator were massive and survived, its branch circuit 
breaker or the house main would trip long before sufficient power to 
feed a large area was able to flow back into the utility's wiring.

Yes, connecting a generator without a transfer switch is a horrible idea 
and likely to get someone killed, agreed.*  However, as the vast 
majority of power failures involve more than a single residence, the 
generator will fail to produce power immediately anyway due to looking 
at essentially a dead short.

* Every time I've seen utility workers working on lines that are assumed 
to be dead, the first thing they do is clamp them to ground to be 
certain. When the lines are assumed to be live, massive insulation 
sleeves, heavy gloves, insulated booms and the like are used.
Jay Hennigan - jay at
Network Engineering - CCIE #7880
503 897-8550 - WB6RDV

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