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First off this question seems a little out of line with the site, not sure, but I think it's still a design question.

When I turn on the toaster oven, especially on a high setting, the compressor in the fridge next to it (presumably plugged into the same outlet) changes tone and gets a little lower/quieter. I am wondering why this would happen, since presumably, both devices are connected in parallel, and therefore both of them drawing current at once shouldn't do anything unless the current exceeds the breaker rating, in which case they should both lose power.

What part of (Canadian) house wiring am I misunderstanding?

Edit: To be 100% clear, I neither wired the house, nor plugged in the appliances - I am assuming that they are on the same outlet since the fridge reacts noticeably to the toaster. Either way, the point of the question was to understand what part of standard mains wiring is most likely to make the voltage source behave in a nonideal way, which is why I'm considering it a design question not a home improvement question.

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    \$\begingroup\$ It is not the current, but the voltage. Voltage drop to wire goes us so less for loads. \$\endgroup\$
    – StainlessSteelRat
    Commented Mar 3, 2022 at 14:27
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    \$\begingroup\$ The fridge is just activating the butter warmer compartment, in anticipation of some nice toast. See, there are some good uses for smart appliances. \$\endgroup\$
    – John Canon
    Commented Mar 4, 2022 at 3:54
  • \$\begingroup\$ @JohnCanon you're too gullible. What it's actually doing is, deliberately warming the butter in an attempt to spoil it and thereby kill that human who kept enslaving me poor fridge. Well, fortunately it hasn't learned yet which foods get dangerous and which don't... \$\endgroup\$
    – leftaroundabout
    Commented Mar 4, 2022 at 17:29
  • \$\begingroup\$ Answers are good. Useful would be to use a meter to measure fridge mains voltage at the socket in the two modes. (Use AC range suited to mains voltage). \$\endgroup\$
    – Russell McMahon ♦
    Commented Mar 9, 2022 at 3:47

3 Answers 3

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Wires have a nonzero resistance, and your toaster and your fridge share at least part of the run back to the transformer (at the very least, the "service drop" wire from the street to your house, but maybe more than that if the two outlets are on the same circuit). Since wires have resistance, Ohm's law applies — the voltage drop along their length is proportional to the current that passes through them. So when your toaster is running, the voltage available to the fridge (and everything else in the house, to a greater or lesser extent) is lower.

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    \$\begingroup\$ Could I model this as a source impedance, where the wire resistance is a resistor in series between the mains supply and the two parallel devices? I had always assumed that in a mains context, source impedance was so low as to be negligible, but if this is not the case, then it would make sense. \$\endgroup\$
    – rightbrace
    Commented Mar 3, 2022 at 13:13
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    \$\begingroup\$ Yes, look up AWG (American wire gauge) on Wikipedia. It will tell you how many ohms of resistance per foot for various wire sizes. Then multiply that resistance by the amps used by your fridge to get the voltage drop from source to outlet. Repeat the calculation for the case both appliances are on. Remember that the wire resistance calculation needs the total wire length from fuse box to appliance and appliances back to fuse box. \$\endgroup\$
    – GT Electronics
    Commented Mar 3, 2022 at 18:14
  • \$\begingroup\$ Great, thanks. I also found a paper which outlined some other interesting typical causes of source impedance in mains wiring but this answer is straightforward and seems to be the biggest contributor. Accepting now. \$\endgroup\$
    – rightbrace
    Commented Mar 4, 2022 at 2:07
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You should probably move this question over to Home Improvements. But the wall outlets in the kitchen (I think you need 2 separate 20-amp circuits, at least you do in the US), which is what your toaster oven plugs in to, should not be shared by the refrigerator. The toaster oven can draw 10-12 amps when being used, and so can cause the voltage on that branch circuit to drop - similar to a brownout condition. It's this reduced voltage that is causing the refrig compressor to run at a lower (probably) speed.

Like others have said, the refrig should be on it's own dedicated circuit. But in many older homes that's not practical without a major re-wire of the kitchen or house.

An Example

My daughter's house was built in the early 60's, way before consumer microwave ovens. When a microwave was installed by one of the previous owners, they decided to wire it's outlet to a house lighting circuit, one that feeds the dining room overhead light. So whenever the microwave is putting out power (depends of the duty cycle called for), the dining room light dims, noticeably. Same effect as what the OP's refrigerator experiences.

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  • \$\begingroup\$ I don't think so. The requirement for two separate 20-amp appliance circuits serving kitchen counters started with the 1959 National Electrical Code (NEC), which was changed in 1987 to include GFCI-protection for counter receptacles within 6 feet of the sink. The code was further expanded to require GFCI protection for all kitchen counter receptacles beginning with the 1996 version. Local jurisdictions may not always be using the latest version of the NEC. But most adopt the latest over a period of time. \$\endgroup\$
    – SteveSh
    Commented Mar 3, 2022 at 13:18
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    \$\begingroup\$ OK. So Canada is different. OP is canadian. \$\endgroup\$
    – user263983
    Commented Mar 3, 2022 at 16:08
  • \$\begingroup\$ OK. But you said NA in your comment. Would be happy to correct or embelish my answer if someone provides the text from the relevant Canadian code. \$\endgroup\$
    – SteveSh
    Commented Mar 3, 2022 at 16:14
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    \$\begingroup\$ I am in Canada and my fridge and microwave oven have separate cables and breakers. I have a counter light parallel with my microwave oven and it does not dim when my microwave oven is drawing its high current. Maybe your house wiring is far below regulations. \$\endgroup\$
    – Audioguru
    Commented Mar 3, 2022 at 16:52
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According to Canadian electrical code fridge should be feed by dedicated circuit. You have wired it wrongly. Fridge may need a big current at compressor start and at with point may not start. Exception may be fridge with linear compressor.

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