Burying multiple circuits in one trench

Question

I'm in the process of running a circuit from the panel in my garage to an equipment area for a koi pond I built. Because of some rock at about 16 inches depth in part of the run, I'm taking advantage of the code allowance for 12" depth for the circuit which will be 20A and GFCI protected. In addition, I'm running it in nonmetallic conduit. There will be a receptacle at the equipment location but I'll also use this opportunity to add one at a nearby patio.

I've dug the trench and installed the conduit, no wires pulled yet. Currently I have an outdoor extension cord providing service to the pond and patio. While traveling, a contractor came by to do some work and plugged in on the patio. They ended up tripping the breaker and couldn't reset it (locked garage) so they left. One of my neighbors later noticed that the pond equipment (life support for the koi, really) was off. He called and we got it started. But this got me thinking that it would be pretty easy to have something in the other outlet trip my pond off.

So, I've decided I probably need to forego the patio receptacle. But I was curious. The way 300.5 is written is says max 20A overcurrent protection and GFCI. I've interpreted this as the total of all wires in the trench/conduit. But, is it possible to run 2 circuits in that conduit if each is less than 20A? I'm guessing not, but I don't really understand the full interpretation of that.

Thanks

Answer 1

I am not a code expert, but my understanding of reading 300.5 is that multiple circuits would be permitted, provided:

• Each circuit <= 20A and GFCI protected
• Conduit fill requirements are met (not a big deal for small circuits)
• Conduit multiple circuit derate requirements are met

The last can be an issue. As I understand it from many previous posts on DIY SE, generally speaking 2 circuits are not a problem due to the way wire current ratings are defined but once you have 3 there can be problems and 4 or more is pretty much guaranteed to require large wires than usual.

Assuming all that to be the case, you should be able to run 2 15A and/or 20A circuits in one conduit. Based on this fill table even 1/2" PVC 80 can handle 5 wires = 2 x hot/neutral + 1 x ground.

You may want to consider metal conduit. With the right type of metal conduit, you can go to 6" depth instead of 12".

As far as GFCI, install it inside the building. That can be a GFCI/receptacle or GFCI only or GFCI/breaker. The GFCI electronics will last far longer (and likely fewer nuisance trips) if installed inside (temperature controlled, no rain) than outside. Every receptacle outside must be GFCI protected, no ifs, ands or buts. Another answer noted the issue of exceptions. Those exceptions are for things like fire alarms and, in some jurisdictions, refrigerators. You don't want to risk a ground-fault problem in an outside water feature - the risk to a human (instant injury or death) is far greater than the risk to your koi, as much as you may love them, if their equipment is temporarily off.

If you do get frequent nuisance trips, try to figure out where/how to better protect the equipment. But at least putting the convenience receptacles on a separate circuit will avoid a nuisance trip on that circuit from affecting the koi pond equipment.

Answer 2

Since this is a life safety device for the koi, it should not be on a shared circuit and should not be on GFCI. How the heck do you do that to a water feature?

Low voltage. You knock it down to 24 volts for transmission to the koi pond. 24 volts is not dangerous and can be around water installations.

I understand that 24 volt aerators are readily available; some are even designed to be fed off solar. (an interesting option coupled with battery; after all what do the poor dears do during a power outage?)

If you can only get a 120V aerator, then you bump it back up from 24V to 120V as close as possible to the pump, and locally derive that 120V, meaning you need a ground spike right there. But make a real effort for everything out at the pond to be 24V.

Note that low voltage is MUCH more sensitive to voltage drop, so be careful about wire sizing. If cost becomes prohibitive, bump up to #6 or even #2 aluminum. The cheapest way to splice aluminum to copper (in a low-voltage regime) is with bare lug-style terminals (look at a service panel accessory ground bar for an example of those, in fact you can get one and cut it up!) You can get them with plastic molded cases around them (Polaris, ILSCO Mac Block, Alumiconn) but that considerably more than doubles the price, and you can just wrap them a whole bunch of layers of electrical tape.

Answer 3

There is nothing wrong with running multiple line voltage circuits in the same conduit. You can't run low voltage in the same conduit. You didn't mention the size of the conduit, so you'll have to learn about fill capacities. Having 2 circuits individually protected by GFCIs would minimize the chances of the "life support" for your pond failing. But being realistic, outdoor circuits often have nuisance trips on GFCIs. I'm not sure, but believe it's code legal on critical circuits to swap out a GFCI breaker with a normal breaker if you are getting nuisance trips. I'm not sure if a Koi pond would count as a critical circuit.