Sizing Direct Burial Wire

[Dan Freeman]

I have to run a direct burial cable from the sub panel in my shed (20 amp breaker that is currently not used) to my pond in my garden about 120 feet away. My pond pump/aerator draws 3 amps, but if I use a heater in the winter to keep a hole in the ice, that will add an additional 7 amps for a total of a 10-amp draw. Can anyone with electrical knowledge tell me:
1. What size direct burial cable I need if I just go with the pump/aerator at 3 amps?
2. What size direct burial cable I need if I decide to size it for the pump/aerator and heater at 10 amps (which is the way I am leaning)?
Thanks.

 

[EbS-P]

Wire must be sized to breaker capacity. The actual load does not figure into wire size. My completely amateur and only slightly informed opinion is you need #12 with 2 plus ground.

Any chance this ever would need 240v? As it would be cheaper in the long run to lay a 3 plus ground cable.

(This is where I’m at at some point they put a 240v well pump in but left the 2+g wire but ran one of the hot legs as 120 v for heat tape, using the ground as the neutral. So I have a hot ground. More information than you asked for but it might be relevant. ).

 

[Ashful]

EbS-P got it right. But in addition to that, I'd also down-size the breaker to 15A.

Connecting AWG-12 to a 20A breaker will always be legal, but long runs will have voltage drop issues, which will affect the efficiency and lifetime of the load. For this reason, it is very common to oversize wiring for a given breaker for circuits with long runs.

Specifics will vary (eg. motors vs. lighting), but for general mixed load circuits, the rule of thumb is 2% voltage drop target. In your case, if it is really only 5 amps on this circuit, you would hit that around 140 feet on AWG-12. Allowing 20A on 120 feet of AWG-12 would generate a voltage drop of more than 2x3.5%, no bueno.

AWG-12 on a 15A breaker would be my recommendation.

Note that in saying all of this, I am an electrical engineer, who has never worked a day as a residential electrician. My experience is all industrial high power electronics, and observing what past electricians have done in each of the dozen houses I've lived in or worked on. So, if a true licensed electrician came along and disagreed with me, I'd defer to them, but I don't think they will disagree with my assessment here.

edit: After posting, I noticed that Dan had said 10A, not 5A... not sure why I had the wrong number in my head while responding. At 10A, you will see 1.8% drop on hot and return each, 3.6% apparent voltage. Not fantastic, the standard published de-rating tables for 2% drop list AWG-10 for 115 feet at 10 amps, and only 70 feet on AWG-12. Depending on the type of load, you will want to make a decision between these two, as per what level of voltage drop is acceptable. For example, if the load is rated to run on 110V-120V, and your panel holds steady at 120V, then you're covered at AWG-12.

 

[GrumpyDad]

I have to run a direct burial cable from the sub panel in my shed (20 amp breaker that is currently not used) to my pond in my garden about 120 feet away. My pond pump/aerator draws 3 amps, but if I use a heater in the winter to keep a hole in the ice, that will add an additional 7 amps for a total of a 10-amp draw. Can anyone with electrical knowledge tell me:
1. What size direct burial cable I need if I just go with the pump/aerator at 3 amps?
2. What size direct burial cable I need if I decide to size it for the pump/aerator and heater at 10 amps (which is the way I am leaning)?
Thanks.

Edit: If you read my previous response, I was confused scratch what I said.

You should check with local code. It will be likely that they 1. will require that the subpanel breaker is GFCI leading to the pond 2. that you obviously have some sort of shut off at the pond and not just direct wire in and 3. that you have two circuits at the pond not one feeding the pond appliances.

 

[festerw]

For that distance you need at least a 10 gauge wire, 8 would be better.

 

[Ashful]

AWG-10 for 115 feet at 10 amps

For that distance you need at least a 10 gauge wire, 8 would be better.

Glad the first electrician to come along didn't say I was off my rocker. I am more an authority on theory than convention.

What I've always found interesting is that, at least in homes which I've lived (including the current abode), is that I see electricians taking drop over length into account when running branch circuits to sheds and outside outlets, but universally ignoring it for circuits of similar length inside the house.

 

[begreen]

If minimal voltage drop is desired then going to #10 UF will suffice. If done in #12 UF, the tables say a 4 v drop with a 10 amp load for 120 ft of wire. Our line voltage is 125vac so I would find that tolerable for the small pump motor, but if the local line voltage at the panel is more like 110V then definitely go up a size.

 

[Dan Freeman]

Thanks for all the feedback, guys. Just so happens my electrician is stopping by today to look at something else for me. I am going to bounce this project off of him and see what he says. I'll let you know.

 

[Dan Freeman]

My electrician was here today. He said at a max of 10 amps (with 3 amps for the pumps and an additional 6.5 amps for the deicer during the winter) I will be fine with 12/2 UF-B electric cable at 110 feet to the pond. He said the 20amp circuit breaker in the sub panel box is fine also. I will put in 2 gang gfci receptacles down near the pond for the two pumps and the deicer.

 

[begreen]

Agreed, though I might protect the whole circuit with a GFCI breaker.

 

[Dan Freeman]

Agreed, though I might protect the whole circuit with a GFCI breaker.

No need to if the receptacles at the pond are GFCI.

 

[ABMax24]

The NEC recommends voltage drop be limited to no more than 5%.

The CEC (Canada) recommends no more than 3% voltage drop, and must not exceed 5%.

 

[EbS-P]

No need to if the receptacles at the pond are GFCI

I agree nothing up wire and much quicker to reset than a walk to the breaker.