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I am looking to confirm my understanding of how my circuit would fail on an over current event.

I have a single phase 40a 240v circuit run with four 8awg solid copper wire (two hots, neutral, and ground). The total length being 40'.

On the circuit I run two devices infrequently and often not at the same time.

  1. 120v/240v Plug in car charger (20amp max charging rate) connected to the two hots and the neutral
  2. 240v 15amp rated air compressor
  3. 120v exhaust fan (peak 15amp, 10amp running) connected to one hot and the neutral

Expected use cases:

  1. pump turns on a 1-3 times a day at random to fill an air tank, running for no more than 5m
  2. exhaust fan runs randomly for 10m at a time over a period of time when work is being done in the garage.
  3. car charger runs once a day for 1-2hrs, typically after I just got home

Risk: The load on a either leg can approach and exceed the 40amp circuit and breaker rating

Failure mode triggers considered:

  1. If all three things start at the same time, the max attempted current draw would be 50amps on one leg of the breaker and 35amps on the other. The breaker would trip and I would be a tad annoyed at the need to reset it.

    • Secondary Risk: breaker fails to trip. Its a new breaker, so I am viewing this as a low risk, with bounded period of 5m. See next entry.
  2. The car charger is running, the exhaust fan starts and reaches steady state (10amp), the air compressor turns on. One leg is running close to 40amps (slightly high or low) for about 5m and not causing the breaker trip. This is with the bounds of the safety margins. It should not occur frequently (maybe once/twice a year?)

Mitigation: Exhaust fan has a "smart" switch, which can be used to prevent it and the "smart" car charger from running at the same time. Making the likelihood of 1) and 2) very low.

Question: Is there anything I am overlooking here? I know enough to be dangerous and to articulate what I am doing, but I have fairly large gaps in my electrical knowledge. I know enough

Other things considered:

  • Adding a sub panel or a second circuit - rejected in that there is not feasible room to do one and the location of the things will change.
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    How are these things all connected to a 40A circuit? For example is the fan connected via an ordinary 120V 20A plug and socket, that is wired in to this 40A circuit? Also, how does adding a subpanel make it harder to relocate these devices? Do you currently have a long 40A cable dangling loose in the garage? Sorry that's a bit sarcastic but I don't see how subpanel or not makes these things more or less mobile.
    – jay613
    Commented Apr 11, 2023 at 15:58
  • "low risk" isn't really good enough when the consequence of failure could be that your house burns down. That's why the rules say to go for defense in depth: use a breaker and don't knowingly overload the circuit to begin with. You can probably install some kind of outlet or interlock switch to prevent using the car charger and the other devices at the same time. Commented Apr 11, 2023 at 18:25
  • So, you have no effective overcurrent protection for 2 15A and one 20A rated devices directly attached to a 40A circuit. Your insurance company will pat themselves on the back when they don't have to pay a cent after your house burns down. Also strange to have neutral connected on a car charger - usually those are hot-hot-ground, not hot-hot-neutral. You're definitely right about knowing enough to be dangerous...
    – Ecnerwal
    Commented Apr 12, 2023 at 1:40
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    The circuit has two NEMA 14-50 outlets on opposite sides of the space. The pulsar wallbox car charger is plugged into one and configured for a max draw of 20amps. The air compressor and exhaust fan are near the other, and there is not a reasonable way to add a dedicated circuit for the exhaust fan. To connect both of them to the outlet, I was considering making a breakout box with a 120v 20AMP outlet and a 240v 20AMP outlet. The wall is not roomy enough to replace the outlet with a subpanel.
    – unexpected
    Commented Apr 12, 2023 at 17:53
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    The difference is devices with 15A and 20A plugs are UL certified to operate safely or trip a (at most) 20A circuit breaker (or fuse). They are not certified to operate safely or trip a 40A breaker. They might start a fire on a 40A breaker.
    – nobody
    Commented Apr 13, 2023 at 0:24

2 Answers 2

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You are overlooking that there is nothing protecting the 120V 15 amp circuit from drawing as much current as possible before the 40 amp breaker trips. Same is true with the air compressor. I assume the electric charger has it's own protection.

I suggest you fix the safety issues first. You need a subpanel. Maybe you can ask a follow up question on how to design that setup.

If you have an '8amp shop vac and a 15amp circular saw on the same standard 15amp garage outlet' this outlet is protected by a 15 amp breaker not a 40 amp.

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  • This is the insight I was struggling to find but could not see. I was failing to consider that my 15a devices have a failure causing them to draw more than 15amps, overloading them... I was too focused on the down stream 40amp rated pieces... Thank you
    – unexpected
    Commented Apr 13, 2023 at 18:43
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You are bordering on max circuit load, but it is not a tragic oversubscription.

In the worst case (all 3 devices starting at the same time) you will be drawing 50A for short time and at most 45A continuously for a couple of minutes. This is "fine" for regular breakers and wires.

Breakers do not magically trip at 40A. In fact, they will not trip at twice that. Not immediately. Breakers trip either Magnetically, when you exceed their ratings several times (quick googling says usa breakers trip at ~4 times the rated current), or Thermally, after they conduct above the rated current for a long time (quick google says 1.3 nominal amperes for 1 hour).
Since the loads, as you know them, are not continuous in the magnitude of hours, but rather minutes, this is fine.

There may be a problem with how the main wire splits to the devices. Is it coming into a plug-box with several plugs and connected to the box leads? Or is there some kind of junction box, and thinner wires go to separate receptacles?

The breaker needs to be sized for the smallest wire or smallest rated socket between the device and panel, so if you have a 15A rated socket fed by a 15A rated wire, you need a 15A breaker there somewhere.

Subpanel and splitting the line with breaker per device would be best.
If you can't put a subpanel anywhere, but really can't, you could put a small 1 breaker surface mount junction box, and run the 15A socket for the fan from it. Just put it next to the wall socket, should be enough space. The breaker can be at either end of a wire, in this case, it will be the far end of this thin wire.

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  • Breaker can be where? Surely you're not suggesting a 14ga splice with a 40A circuit breaker? Commented Apr 13, 2023 at 12:31
  • An 15A socket with appropriate 15A wire and a new breaker on that 15A branch. Right now the situation is 15A socket on 40A breaker, connected by hopefully 40A wire all the way through.
    – Thomas
    Commented Apr 13, 2023 at 12:57
  • Thank you very much for this well written technical detail! I will indeed be adding a breaker, given that from the reply below I can now see in my head a concrete example of how it can fail
    – unexpected
    Commented Apr 13, 2023 at 18:49
  • @StevenTegreeny -- can you provide photos of the situation at the outlet box where the air compressor and fan are located? Commented Apr 14, 2023 at 2:52

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