In my office, I have a server (powered through a UPS) and a laser printer on the same circuit. When the laser printer turns on, the lights dim and the UPS will briefly switch to battery. This prematurely wears out the battery, and, if there were no UPS, would cause extra wear and tear on the server's power supply.

Can I electrically isolate these two from each other by putting them on separate legs of my house's split-phase power supply? In other words, do loads on one leg of a split-phase power supply affect loads on the other leg?

Possibly related answer here.

  • This is a very common problem. I have had it myself with my desktop computer for many years and I was at a customer earlier today and noticed the same problem. May 16 '19 at 4:34
  • Right now, are they on the same circuit, i.e. if you shut off or trip the breaker powering one, does the other lose power? If on 2 breakers, what is the physical relationship of the breakers to each other in your panel? Put a stickynote to mark each breaker and shoot us a photo of that area of your panel. I'm also fishing for panel make/type (in very coarse terms; we need to know QO vs Pushmatic, but not QO2448 vs QO3042). May 16 '19 at 4:45
  • I'm pretty sure that they're on the same circuit, but I will check tomorrow when there's daylight.
    – matmat
    May 16 '19 at 5:33

If the outlets are on completely separate wire runs from the breaker panel OR sharing a neutral but on opposite legs (sharing on the same leg is illegal), then the wire resistance from the panel to the outlets will be isolated between them and the voltage drop will be minimized. (The current draw is low enough that there should be no noticeable drop at the panel).

So yes, even if they were on the same breaker, let alone phase, as long as the wire runs are separate, the server should not see a drop from the laser printer. Also good if a neutral is legally shared.

(The reason that the shared neutral is OK is that each circuit's return current will be 180 degrees out of phase. Therefore, the worst case current through the neutral will be one circuit's. In fact, when the laser turns on, the voltage may even rise slightly to the server as some of the server return current will actually flow through the laser, bypassing the neutral.)


The problem is they're on the same circuit, and they may actually be overloading that circuit. The classic example is a gaming PC (10A) or server (13A) and a laser printer (7-12A in operation, 50A when cycling on the fuser assembly). In case you're counting, circuits are typically 15 or 20 amps. So you overload them right quick. The circuit breaker is a slow-trip, and might take 15 minutes to trip at a modest overload.

However, the overload is happening on the same 2 wires -- and the resistance of those wires (against the high current) is causing an excessive voltage drop.

Voltage Drop = Current * Wire Resistance. 

The resistance of a wire is proportional to its length, so if it's a long wire run back to the panel, it will make the voltage drop worse.

Another thing that will make voltage drop worse is poor connections in receptacles or switches. This is common with "back-stab" connections and can happen with screws also. Feel for any outlets on the circuit getting warm. Outlets shouldn't get warm.

Running another circuit from the panel will solve the problem.

It will not matter if the circuit is on the same pole or the opposite pole - the problem is the long wire run. So the laser printer will suffer its own voltage drop in its cable run, and the computer will feel only its own voltage drop in its cable run. If the computer does feel it, that indicates a supply problem - too small a subpanel, or a developing problem in the service drop from the street.

You could consider a multi-wire branch circuit - however that could cause interaction because now you have 3 resistors in play instead of just 2. MWBCs are generally more trouble than they're worth, especially now that AFCI is required and GFCI is widely used.

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