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I am currently reviewing and redesigning the electrical installation of my property which came with a 100A SquareD QOD6S MLO load center very badly abused and about 130' away from the service drop which is a single phase 120V. There are plenty of problems in this circuit and one is this center being the only (malfunctioning) station between appliances and the incoming feed. I need to rewire this as a sub panel and install a main disconnect at my properties entrance. I am a layman in electrical installations, I've had my share, but rewiring the whole circuit is a step up in my history. I am aware that this is dangerous and will therefore call the provider to have them cut the service before pulling/installing anything.

My confusion is about the fact that most articles written today call for 100A or 200A main panels/disconnects to provide the necessary load, which I understand, but applies mostly to directly connected main panels. I hardly read anything about spread out configurations where the service drop is way separated (like my 130') from the living space. There are some but not enough.

My power company (Mexican national power) has its specifications for the basic installation (totally ignored on my property, probably due to dating) which is the following (translated and redrawn original illustration with comments in italic font): Translated original illustration for meter installation from provider in Mexico So here's my doubt:

I redesigned the existing panel to become a sub panel of 100A (we don't have a lot of continuous draw). Its breakers will add up this much (4x15A & 2x20A). The underground wire run from the panel to the property entrance is #8 or #6, cannot figure anything out because it has no marking at all, so that would be rated somewhere between 40A and 75A depending on the insulation. Now the power company recommends a main disconnect of 30A connected to the meter with THW#8 which is rated 50A.

How do these numbers add up in reality?
Or in other words if my continuous load was greater than 30A how would a main disconnect of that size not trip that?

I read this answer to another question where it's said that

Don't bother trying to coordinate the trip rating of this "shutoff switch" breaker with the actual supply breaker in the main panel. That must be sized to protect the wire to the subpanel.

Can I be fine with a 30A breaker as a main disconnect for a 100A sub panel 130' feet away?
If yes then why so?

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    Will the run from meter to house be in conduit? – Harper Apr 2 at 4:11
  • Yes, are you planning to run the wire from meter-pole to house in conduit, or with direct burial cable? – ThreePhaseEel Apr 2 at 4:14
  • The 130' run is an already existing one and most probably in conduit, yes. I'll have to check on that tomorrow. – Marian Apr 2 at 4:20
  • 30 amp is quite small for the 6 breakers you have. You might be able to get by with it depending on the actual load. When volunteering many years ago I worked on a home that only had 10awg as the main feed, I was surprised but it worked for the limited lighting and hot plate they used for cooking.+ – Ed Beal Apr 2 at 13:59
  • @EdBeal That's where my question is pointed at...In northern territory cities (Texas climate) a 200A main panel is probably an adequate choice (old energy wasting A/Cs, fridges and all kinds of old appliances). But the power company does not specify different installation instructions for these areas. So with a continuous load of let's say 31A (huge house, A/C on all day, bunch of old bulbs, heavy fridge with ice generator, electric mower for big patio, washers etc. etc.) how will a 30A disconnect not trip that? So I DO need to calculate my continuous load for the main disconnect at all times? – Marian Apr 2 at 16:25
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It's not clear to me why they're limiting you to 120V @ 30A. Everything about this suggests that the transformer capacity available up at the pole is very limited, and they would be unable to supply 100A-200A @240V to each residence.

Interestingly, they are instructing you to wire your meter with the neutral through the meter. Normally, the meter does not measure neutral, only the hots. As such, this meter will be effectively recording double the current. It is literally measuring current coming and going, akin to a theater charging your entry and your exit as two admissions. As such its current readings should be divided in half. If it's a smart-meter which measures voltage across the supply, then it should be able to accurately compute KWH, but watch out.

120V is very sensitive to voltage drop, hitting 3% drop at only 60 feet or so. So you should upsize the wires as much as you can bear. The two-#8 setup they are requiring is perfectly capable of supporting 240V with the voltage drop in mind, assuming you are forever stuck at 120V @ 30A. However we certainly hope they will provide better service someday. The max the 3/4" plastic conduit can hold is three 6 AWG THwN wires, which will support 60A @ 240V in the future. For today, install two of them and add the third on that future happy day.

If you want to be cheap, still use #6, but use aluminum wire. That is the equivalent of #8 copper, and is perfectly fine in service/feeder applications like this. Make sure the 30A breaker is AL-rated, if not, stick 6" pigtails of #10Cu on it.

30A @ 120V is very, very small service. I have it myself, in my US apartment (didn't even know this til just recently, and the master never tripped, but it is FPE so that is to be expected). 30A@120V provides 3600W, and will allow you to run no more than two heat-making appliances at once: hair dryer, microwave, portable heater, toaster oven, hot plate, laser printer, gaming PC, etc.

Don't let the very small service deter you from putting in a full and proper service panel with many circuits. It doesn't matter how many circuits are in your panel; it matters how much actual load you draw on them. For instance if I owned this place I'd slap in a 30-space (3PE, a 42-space won't fit lol) and probably put in 25 circuits, all 20A. This wouldn't have any effect on the actual load, because we'll still use the same appliances the same ways. For us it would eliminate the "kitchen dance" where we can only run 1 heat-making appliance at once because the kitchen only has 1 20A circuit.

And then of course in the happy day they upgrade your service, you are ready to make the most of it.

Again, the draw on your service isn't decided by how many breakers are in your panel. It's decided by what's plugged in and turned on.

Oh, speaking of that, your "main" panel inside your house does need to have a main breaker of its own, or some other obvious master shutoff switch nearby. That is because it's an outbuilding. Since your "main panel" only has 6 spaces, you might slide by with the old "Rule of Six" (you must be able to de-energize the whole house with no more than 6 breaker throws), but I thought that rule was outlawed.

It does not matter if the 30A supply feeds a 60A, 100A, 200A or even 400A "main panel" in the house. The important job is protecting the wire to the house, and the 30A breaker does that just fine.

As far as grounding, you definitely need grounding rods, water pipe ground or Ufer ground at the house. Because of that, I am advising to treat the long conduit run like it's a service lateral, and ignore the fact that the meter and main breaker are back at the pole. That will remove the need to run ground in the conduit (rather silly, IMO, a ground rod at the pole contributes nothing to safety, and you need the house ground rods in any case).

  • Oh man, now I'm back to my original doubt hahaha. If my continuous load was, let's say 50A (talking about future), which is fine with my panel at the house, the main disconnect could still be the 30A because it's only protecting the 130' #8/6? Wouldn't this make the 30A breaker trip or at least heat up the wire running to the main? I don't understand why not? The existing panel has an existing ground rod. So I'd run the 130' phase and neutral from the 30A breaker to my panel and bond neutral with ground in that panel as if it was a main? I may open a separate question for that one... – Marian Apr 2 at 20:34
  • Also the 130' feed already exists, I don't want to pull that really. It seems to be #8 THWN, I measure an outer diameter of the cable including insulation of 6mm exactly on different measurements. – Marian Apr 2 at 20:43
  • My meter is this cfe.mx/industria/PublishingImages/… just that it's "CL 11KU" instead of "CL 10KU" – Marian Apr 2 at 20:46
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    The 30A breaker won't allow you to pull 50A continuous load. It will stop you at 30A and prevent #8 (or even smaller #10) wire from reaching worrisome temperatures. #8 wire is perfectly adequate, even considering the voltage drop, so if it's already in the conduit that'll be fine until you increase service size significantly. @Marian – Harper Apr 2 at 21:05
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    @Marian yeah, my figures were from THWN-2, which has fairly slim insulation. Older wire means thicker insulation, so #8 is far more likely. That's good enough for now. If it ever comes to it, your conduit can fit three #6 THWN-2. – Harper Apr 2 at 21:39

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