Wiring a sub-panel with an equal sized breaker on each end of the feed, which will trip first?

Question

I'm wiring up a small backyard tiny house for use as a rental.  Main power will be supplied by a 30A breaker and 10/2 wire coming from the main house, to an RV box, then to an RV plug, then into the sub panel in the tiny house where it feeds a couple of 20A circuits.

So heres the issue, with this amount of power, popping the breaker will happen from time to time, and I want the person in the house to be able to reset it there. (I can't be the first person to have this issue with sub panels)

If I didn't care about electrical code, I'd just go ahead and put an oversized breaker at the main panel and backfeed the sub panel through a 30A single pole breaker.   I like to keep things as compliant as I can though, so I'm trying to figure out if there is a different way to do this.  It's too bad they don't make 29 amp breakers or I could just backfeed the subpanel with one of those and the problem would be solved!  Maybe it's not an issue?  in this scenario if I had a 30 amp breaker on BOTH ends of the 100' 10/2 line, which breaker would trip first, the one in the main panel or the one in the sub panel?  Any ideas for how to do what I'm trying to do here?

thanks for any replies!   

Answer 1

If you're anticipating overload, this is a sign you've underdesigned your electrical system

The fact you are anticipating breaker trips from overloads is a sign you are about to underprovision the circuit in question. A better alternative would be to simply provision more power to the location; I would simply trench in a 2" PVC conduit, as that will let you run as much power as you'll ever need out to your tiny-house, anything from the RV-sized 30A@120V you're considering to a full 200A residential service, with a simple change of wire and breakers required if you want to upgrade later. (Using direct buried cables means you have to dig up the cable in order to upgrade, and who wants to rent a trencher again six months down the road?)

Selectively Discoordinated Breakers (or, don't try to make any guarantees here, because you can't)

In most electrical distribution systems, there will be at least two breakers in series between the service point and any given outlet (light fixture, receptacle, etc). This means that your situation isn't all that odd. However, you cannot make any guarantees or predictions about which breaker will trip first here, as the properties of breakers connected in series are complex to predict. Unpredictable dynamic arc impedances during the trip process, variations in magnetic trip coil and contact timing, and tolerances during thermal trip calibration all will cause variations in breaker trip timing.

This is undesirable in some situations, such as those involving emergency power systems or continuous industrial processes. Hence, the major makers of commercial/industrial (i.e. molded case) circuit breakers publish tables that can be used by an engineer to select compatible sets of circuit breakers from the same model and series so that the furthest-downstream breaker trips first on a short circuit. This process is called selective coordination.

However, it's considered impractical to selectively coordinate the loadcenter-type miniature circuit breakers used in residential and light commercial applications; their trip characteristics are not as well documented, and selective coordination requires significant engineering effort vs. the mere nuisance an upstream trip poses in a residential environment. Compare this to the stakes of an inadvertent feeder trip in a hospital emergency room, where critical life-support equipment could be taken offline, or a continuous industrial process, which could cause a serious safety hazard due to a disorderly shutdown.

There is also the factor that selective coordination only works for short circuits; in fact, the upstream overcurrent protection device needs to be somewhere around twice as large, or larger, to have a hope for selective coordination to work at all. Two equal-sized breakers simply cannot be coordinated on overloads due to the limitations of trip calibration.

Answer 2

Getting moved around? Use bolt down breakers!

Most breakers panels allow breakers to unclip and tip out. Easy dor servicing under load. Not awesome for popping out due to vibration. They also make breaker panel models where the breakers bolt down instead of snap in. And they make bolt-down kits for snap-in breakers; these are intended for backfeeds, but if you use them for normal breakers, I won't tell :)

If this tiny house gets moved more than once or twice, consider bolt-down breakers.

A "main breaker" at the subpanel is mandatory

Well not a breaker, just a shutoff switch, but everyone uses a breaker because it's cheaper. Your backfeed plan is fine and legal if you bolt it down, but you must do something to that effect.

Selective coordination is hopeless

And that is where you are going with that 29A breaker. See ThreePhaseEel's poetry for why.

It's not impossible, but it'll only work with slow trips (38A on a 30A breaker) and only on much bigger differences in trip ampacity.

Upsize the main... and the wire

10/2 makes me think you are really trying to slumlord this thing. Stop it. Even a 10/3 would be a step in the right direction, and not much more coin. At this point you have two 30A/120V feeds; inside the tinyhouse you breaker each at 15A for a better chance of local branch circuit trip. Of course this trips at the branch circuit level, so 20A+5A will trip it.

If the people in the tiny house community are telling you 10/2 and a TT30 socket, that is not wrong, but it is not right for you, given your constraints.

But you are better off still with either 6/3Cu or #1Al.

6/3 will let you breaker the feeder at 60A, and provide two legs to the tiny house. Make your local main 30A which then feeds a NEMA 14-30, which is a larger RV socket. That all but guarantees local trip first on modest overuse overloads. Inside the tiny house you have its panel with any number of breakers.

If you run 1Al you can feed breaker at 100A (or if your main breaker is 100A, lug feeds will suffice) and again with a local breaker at 30A, or 50A with a NEMA 14-50.

...Or have a power monitoring system warn you

Given the constraints you just shared, parking your tiny house at friends' homes, where you can't upservice the tinyhouse pad, you are probably better off with some sort of a power monitoring system. It would be able to warn you when you are exceedi workable load or remaining near maximum load for long enough for a breaker to possibly trip. Breaker trip curves are grossly consistent, for instance 30A for 5 minutes and 25A indefinitely are probably pretty safe on any make of breaker. You just need it to sound a buzzer as you get close,