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A neighbor in my (US) building just found out that his laundry room (and others in the building including mine) are wired incorrectly. When the original washer and dryer were replaced, the dryer stayed on the existing 240 V circuit while the washer (needs only 120 V) was wired...elsewhere. In his case it was "bridged to some kitchen outlets" (mine is on the same circuit as outlets in the living room, I had to flip the breakers to figure it out).

He has since been told that this is not up to code and is a dangerous fire hazard. I believe it but I don't understand why. I'm an asic designer, with no experience in residential or power electronics. If my washer draws too much current while my TV, cable box, and video games are all drawing from the same path, wouldn't the breaker flip long before a fire? Isn't that the point of a breaker?

The breaker for the laundry room is a double-wide 30 A breaker. The washer is now on a standard width 15 A breaker. What's the problem?

  • It’s not just a matter of overload, if you get a fault and there’s too much resistance between the panel and the fault location, you may not have enough current to trip the breaker. The intent of the code is to avoid situations like that and many others... – relayman357 Nov 18 at 22:22
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    A circuit breaker is not guaranteed to stop fires. Imagine what would happen if you stuffed your toaster with paper napkins and switched it on. Would the napkins catch fire? Probably so. Would the circuit breaker pop? Probably not. Just as a toaster can generate enough heat to start a fire without popping a breaker, so too can some wiring faults. – Solomon Slow Nov 18 at 22:28
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    I would see this as less of a fire hazard and more of a general code violation. Modern codes require a lot of division of circuits. With a lot of attention given to anything that could be in contact with water, or large equipment often is now required to be on dedicated circuits. – MadHatter Nov 18 at 22:31
  • Having a lot of stuff running off of one circuit is certainly a temptation for a resident to play games with the breaker box. When I was a kid in the early 1970's, my dad was often called on to fight fires, the cause of which was a penny in the fuse box (old screw-in style fuses could be defeated by putting a penny in the hole and screwing the fuse on top of it). The cause of the penny in the fuse box was houses wired for typical 1920's to 1940's use, that had 1970's expectations by the residents. – TimWescott Nov 18 at 22:35
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It's a lot like aviation. "Little" code violations can stack up and interact to become big ones. The electrical code is written in blood and ash, which is to say it's less about theory and more about actuarial analysis of accident patterns - pro and con. For instance blown insulation around knob-and-tube wiring was banned, and is now being un-banned because better data showed it wasn't actually causing fires.

The dryer circuit sounds like the bog-standard typical 120/240V 30A dedicated circuit: both poles of hot, neutral and (we hope) safety ground. Safety ground can be retrofitted using any viable path back to a grounded location (not a water pipe). All 3 (non-ground) conductors appropriate to 30A, that means 10 AWG (rules don't allow you to run the wire at 90C thermal rating, so #12 is right out).

The washing machine circuit must necessarily be a different circuit from the dryer (since the dryer uses almost 80% of circuit capacity, and also, it's a different voltage). A few washers are designed to plug into partner dryers; these are popular in large complexes because it means one less circuit to the washer/dryer area.

You're correct that a 15A breaker "should" protect you, because it's small enough to protect wire as small as #14, the smallest permitted in US mains wiring. However, that assumes everyone behaves properly, and that frequent trips don't bait someone into doing something stupid. Sort of like the way you can cause accidents by getting on a section with 20 miles of no-passing zone and driving 5 under the speed limit -- it incites stupid people to do stupid things to get around you. There's no accounting for stupidity in accident statistics.

For instance, plain overloads cause breakers to trip in thermal mode. The breaker heats up in theory about as fast as the wall wiring does. In practice, they cool off a lot faster than in-wall wires - especially if you hit them with canned air. So if a person resets the breaker over and over; the breaker cools off during off times, but the wires just keep getting hotter and hotter.

Both washing machine and kitchen countertop circuits are required to be 20A and dedicated. You have a 15A circuit serving both loads, so you're bound to have overloads -- most kitchen appliances having anything to do with heat are 1500W (12.5 amps) because UL imposes a 1500W limit on such appliances. That means so much as making toast with the washing machine running makes a trip likely. Two kitchen appliances also overloads a 20A circuit, but with only a 25% overload (25A vs 20A) you may have enough time to make toast.

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    So, sort of in summary are you saying that the statement that it "IS" a fire hazard is slightly hyperbolic? Meaning it would be more accurate to say "it IS a code violation, and MAY BE a fire hazard?" – DaveInCaz Nov 19 at 14:10
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    @DaveInCaz the root causes of why it is a fire hazard are (1) the Byzantine domestic electricity supply standards in the USA, requiring two voltages and two-phase supplies (including the fact that I^2R heating caused by poor workmanship is four times higher in a 120V supply than at 240V) and (2) the fact that the USA builds houses from wood. The codes are just band-aids to mitigate the root causes. – alephzero Nov 19 at 15:03
  • Thank you, this also explains the comment from my neighbor about how the original washer shared the dryer outlet, obviously using a setup like you describe with the "partner dryer". – Matt Nov 19 at 16:25
  • @DaveInCaz if you prefer to think of it that way, have a field day! But I feel like what you're really trying to do is castigate NEC as frivolous and unnecessary. I would not say that about NEC. – Harper - Reinstate Monica Nov 19 at 16:46
  • @Harper-ReinstateMonica I see why you might think that but I was asking a genuine question. I have nothing against electric codes. But I do like to try and understand their basis to better inform my own projects. – DaveInCaz Nov 20 at 12:32
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Circuit breakers and fuses are for overcurrent protection. They are not heat detectors. Therefore they only trip if the current exceed the breaker rating.

The most common cause of a burn or fire comes from a poor or loose connection in a circuit. When a load is applied to a circuit with a poor connection and, the conductor begins to heat up. This causes resistance to rise in a circuit not decrease. Meaning the circuit does not exceed the rating of the overcurrent device, but just the opposite and the conductor continues to increase in temperature eventually creating enough heat to melt insulation,destroy equipment or start a fire. Many refer to this as a high resistance fault.

This is why you will find many expert comment not to use the "stab-in" hole in a cheap receptacle but to attach the conductor to the screw on the side and create a sound mechanical and electrical manner.

Arc fault detection devices were created and one of its applications is to help prevent this situation. That is why you may get an arc fault trip and not the circuit breaker. The arc fault detects heat in an arc but can also detect a heat rise that may not necessarily a classic arc by definition.

Hope this helps.

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Having 240V and 120V outlets at the same room gives a theoretical possibility to connect a wire between two outlets. That wire can make a short circuit through lower spec 120V wire but the only breaker in the current route is the one for 240V outlet. The low spec wire can have so big resistance that the breaker trips far too late => overheatened wire inside a wall.

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