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

Mel Beckman mel at
Tue Aug 31 16:13:50 UTC 2021

It’s germane to NANOG. Just last week I visited a “data center” that uses a roll up generator and a cheater cord to power the racks. “Oh, this is safe”, they told me. “We have a policy that you must throw the main breaker before plugging in the generator. Since you have to open the garage door to wheel in the generator (!), you walk right by the switchgear and a sign that reminds you to throw the breaker.”

Code enforcement is on the way. :)


On Aug 31, 2021, at 7:50 AM, Josh Luthman <josh at> wrote:

Is this conversation really taking place on NANOG?

Don't backfeed power.  Got it.  Stupid people are going to be stupid, we won't solve it here.

Josh Luthman
24/7 Help Desk: 937-552-2340
Direct: 937-552-2343
1100 Wayne St
Suite 1337
Troy, OH 45373

On Tue, Aug 31, 2021 at 10:41 AM Mel Beckman <mel at<mailto:mel at>> wrote:

But you said “Gas-fired furnaces or heaters should not have an impact because the only electrical requirement is to fire up the pilot light.” There is no gas-fired furnace I know of that doesn’t require a blower fan. How else does the heat get out of the furnace?

To answer your question, you need to understand that this safety system has two components. The first component, the furnace interlock relay, is designed to interlock the blower with the forced-air system, which also includes an outside air supply valve. When the blower is energized, a circuit inside the furnace gets power. The blower and furnace operate continuously when this circuit is energized, and the supply valve opens and closes as needed to ensure the air doesn’t get stale.

The safety second component is the limit switch, which primarily turns the blower fan on and off, but also has a safety role. When the temperature in the air supply plenum gets too hot, the limit switch turns off the furnace burner (or boiler, in a water-based system) to prevent damage, and possibly a fire, from overheating.

The actual state mechanics are thus not as simple as “if the blower fails the furnace won’t light”. And it’s because of these complex state mechanics that furnace electricity is hard wired.

Without AC power, no furnace can operate in a power outage. So that’s certainly not “no impact” from a utility failure. But the many thousands of deaths that occurred in homes and offices before these safety systems were put into the code is why you need a generator transfer switch if you want heat (or A/C) in your home during an outage.


> On Aug 31, 2021, at 7:15 AM, Mark Tinka <mark at> wrote:
>> On 8/31/21 16:06, Mel Beckman wrote:
>> I think you’re forgetting about the all-important blower fan in a gas-fired furnace.
> Well, I was referring to a pure electric furnace, not one that uses a blower over a gas-fired one :-).
> In that case, the blower is not a major draw on power.
> But again, we don't have those things here, so :-).
>> That said, the reason the code requires furnaces to be hardwired is to ensure that the blower interlock system can’t be bypassed. An electrical interlock ties a heat recover ventilator to circulation air blower operation of a forced-air furnace system. This ensure that the blower circulates supply and return air within the structure. A plug-in power source leads to the possibility that this interlock could be accidentally defeated, resulting in an overheat within the flame box.
> Makes sense.
> Does this, then, mean that if the blower itself were to fail, the gas burner would not light?
> Mark.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the NANOG mailing list