6

Facts:

  • The national electrical supply company of Mexico called "Comisión Federal de Electricidad" (CFE) oficially instructs users to install:

    • A two pole one trip main interrupt behind the service drop of "250V/30A" capacity for single phase 120V service.
    • A two pole one trip main breaker behind the service drop of "250V/30A" capacity for split phase 240V service (Controversially called dual-phase in Mexico)
    • A three pole one trip main breaker behind the service drop of "250V/100A" for three phase 240V service. (Most probably open delta)
    • #8 AWG type THW copper wire from the meter lugs to the premises service interrupt for both 120/240V service (Rated 50A by the NFPA 70 NEC edition 2020 - See Table 310.16)
    • Properly sized THW copper wire "as needed by demand" and "in accordance to sizing tables" for three phase 240V service.
  • The NEC is being translated almost word by word and published in Mexico under the name NOM-001-SEDE-2018 (2018 is the latest I can find)

  • There is no real enforced implementation of this "Norma Oficial Mexicana", neither any inspection at service connection anywhere throughout the nation which makes it more of a guide for the educated electrician.

  • The actual service drop of the utility is usually #6 aluminium wire. (As found in their own published specs - See Table 1; Rated between 40-55A depending on the type by the NFPA 70 NEC edition 2020 - See Table 310.16)

  • The actual meter (if it is digital and up to date [there's still a lot of analog meters used from old installations]) is rated 15(100)A in any case.

  • Most homes in Mexico use mainly gas for any heat appliances and only cold water for laundry although there is common use of air conditioning units in the north.

  • A common household load would include appliances such as a relatively large fridge, several washers w/o dryers, a steam iron, a microwave, one or several blenders and lots of consumer electronics. Occasionally a power tool for a short period of time.

  • Most shops with extensive use of high demanding electrical tools including high capacity welders will nevertheless have a simple 120V or 240V single phase service contract.

  • Lots of, if not most, electricians will install a 50A main breaker nonetheless knowing that that's what a #8Cu THW and a #6Al can carry.

Doubts:

So there's quite a few things that don't add up here and it makes one wonder about the quality of the provided service. The disconnect means requirement of 30A is way lower than what the wiring instructions would be capable of (50A) for a pure 120V service and very boldly limits the end users consumption.

The fact that the instructions for a 240V service are not altered in any way other than requiring an additional wire and a different meter base is just outrageous and makes it per their own definition completely useless unless you want to permanently trip the main service interrupt with your 240V appliances.

It appears that the only power providing utility in the country does not necessarily adhere to the nationally published electrical code or even less promote it. (In fact in reality most of their personell wouldn't even know a code existed and any educated layman would cringe over many of the installations done by the utility)

Questions:

  1. Why would a utility instruct the premises to install lower rated main interrupts than their service actually provides?
  2. Taking into account that inspection does not exist, would it be ok to just go with documented standards and up size the main interrupt to something within specs? (For example 50A main interrupt for a 120V single phase service fed with proper wire size)
  3. If I was to rent a 240V split phase service and ran adequate wire would a 100A main service be realistic since it would be a balanced load between the two #6 ungrounded aluminium conductors resulting in 50A each?
  • This service is being run to a standalone, single-family dwelling (house), correct? – ThreePhaseEel Sep 29 at 22:16
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    In fact all of the linked material and mentioned instructions are for that case indeed (although not necessarily standalone, could be side by side dwellings following their documentation). There are different documents regarding commercial installations and multi-dwelling units (apartments etc.) that are not taken into consideration in this question. – Marian Sep 29 at 22:18
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    2 conductors each carrying 50A in a split-phase system does not make it a 100A system. It's still a 50A system, except that you have 240V instead of 120V. – brhans Sep 30 at 0:31
  • Just checking - there's nothing here to stop you uprating the installation on the homeowner's side of the breaker, ready for a 50A or 60A service in the future, right? Other than initial costs. – Criggie Sep 30 at 12:19
12

Why 30A service?

Because of provisioning. They are not able to provision 50A service at this time at this location. That is probably due to transformer or pole-line wire capacity in the neighborhood. Giving everyone 50/60A service means they would be oversubscribing their transformers and pole lines. This would necessitate a big capital expense upgrading those, and that is not commercially feasible right now.

If you want more service, tell them you need more service and pay whatever additional they charge you for the additional "share" of the transformer.

Safety regs

Given that you think inspection does not exist, can you just ignore safety regs altogether and do what you want? Sure, what could possibly go wrong?

  • If the power company charges extra for fitting a 50/60A breaker, and you haven't paid the upcharge, then if you fit a 60A breaker behind their back, that is plain theft of service. I don't know how that's handled in Mexico, but it would be well within their right to remove their service drop and refuse to provision to your address any power at all.
  • If the transformer blows up, they are going to quickly walk the street and see which neighbors have illegally changed their 30A breaker to 50/60A. They will be deemed responsible for the transformer blow, and they will buy the neighborhood a new transformer and wiring.
  • If there is a house fire in any way relating to this, the fire insurance will balk at paying, and because an illegal act was behind it, the mortgage lender will come after you personally for the value of the house.
  • It may be a challenge to sell this house in the future.

Getting 100A out of 2x50A

Split-phase power doesn't work that way.

If you had 50A@240V service, which you do not, you do not have any way to get 100A@120V. What you could get is 50A @ 120V in two instances.

So if you have a single 100A@120V load (not likely) you could not power it off 240V/50A service. However, if you have eight 12A@120V loads, you could put four on one side of the service (48A) and four on the other side (48A) and you'd be solid. Derating notwithstanding, of course.

Electric large appliances out of this 30A service

Not gonna happen.

  • If you attempt to run an 30A electric dryer, it would have to be the only large appliance running at the time.
  • A 30A electric water heater is impracticable because you can't control when it cycles.
  • An electric range is Right Out; they draw too much.
  • A 240V/30A air conditioner is also out of the question, since it runs too often for "turning everything else off" to ever work.

If you have any of these coming in, talk to the power company, or Craigslist.

If you have a 20A-breakered (16A) 240V appliance, that might have a shot of working.

  • Great answer, I like. Take note that I'm writing about common practice throughout a whole country. This is more of an overall issue on this side of the river. The same documents apply to the northern region of Mexico where A/C units are common. Burned receptacles and power drops are also common. So the sole provider of electricity is just ignoring the fact that their installations are wildly underdeveloped for modern use. And don't get the wrong idea, there's a lot of rich kids in Mexico and lots of working people with heavy equipment. It's not funny if you think about it. – Marian Sep 30 at 1:43
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    Having said the above the utility actually officially claims to not be able to provision 30A or more at this time in ANY location in the WHOLE country for residential use... – Marian Sep 30 at 1:45
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    Definitely the best argument for solar panels + PowerWalls that I have ever heard. Ignore the PoCo; let your solar + batteries carry your big loads. – Harper Sep 30 at 2:02
  • I like the idea indeed. Although based on the comments below on ThreePhaseEel's answer I suspect that the utility is really just trying to artificially keep their network load low. Shady to me but maybe not too bad of a move seeing how lots of installations here heavily lack any standards. – Marian Sep 30 at 2:53
4

Your utility would violating Code up here in the USA

While the US National Electrical Code does not apply to utility wiring or operations for the most part (these fall under the US National Electrical Safety Code, or NESC, instead), the NEC does contain requirements for electrical service sizing, and your utility does not follow them for their split-phase (3-wire) services. In particular, NEC 230.79 requires that single residences get a 100A, 3-wire service, and that commercial structures or apartments with more than two circuits get a 60A service of some sort:

230.79 Rating of Service Disconnecting Means. The service disconnecting means shall have a rating not less than the calculated load to be carried, determined in accordance with Part III, IV, or V of Article 220, as applicable. In no case shall the rating be lower than specified in 230.79(A), (B), (C), or (D).

(A) One-Circuit Installations. For installations to supply only limited loads of a single branch circuit, the service disconnecting means shall have a rating of not less than 15 amperes.

(B) Two-Circuit Installations. For installations consisting of not more than two 2-wire branch circuits, the service disconnecting means shall have a rating of not less than 30 amperes.

(C) One-Family Dwellings. For a one-family dwelling, the service disconnecting means shall have a rating of not less than 100 amperes, 3-wire.

(D) All Others. For all other installations, the service disconnecting means shall have a rating of not less than 60 amperes.

However, split-phase service doesn't work the way you think it does

A split-phase service is two 120V services at a given current "stacked atop" each other to produce a 240V service, basically, so given a 50A capacity, you only use a 50A breaker as you are getting 50A of 120V on each leg of the service.

  • 1
    Nice, thanks for pointing this out. They are apparently not completely stupid ;) I looked up the translation and it was transcribed accordingly for their service. You can even see they cherry picked that one cause the formatting is completely off. Stating that a "popular living space up to 60m²" should be provided with "not less than 30A" hahaha. Which is funny, one could argue that under that condition a larger popular living space would fall under (D) which is translated as is and would call for 60A making my 50A breaker no theft at all... – Marian Sep 30 at 2:12
  • Actually it's even broader than that. The translation reads: PROY-NOM-001-SEDE-2018 article 230-79. c) "One-family dwellings. In one-family dwellings, the disconnect means of the conductors receiving service shall have a capacity according to the load connected. In popular living spaces of up to 60m² it shall not be less than 30 amperes." --- I think that's pretty clear then. Just curious now what they'd actually do/charge if my load was actually requiring bigger service wires than #6Al...(not the common case here but still intriguing) – Marian Sep 30 at 2:33

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