In typical North American residences, a pole transformer supplies two leg 120 V electrical service to a home. My service is rated at 200 A. If I were to draw excess current from the triplex service cable running from the pole to the meter socket, what device protects the cable from overcurrent?

  • Are you talking about a fault upstream of the main breaker? (say if the meter socket shorted out) or if you pulled excess load? Also, are we talking about a single main breaker configuration, or a "rule of six" service setup? – ThreePhaseEel Sep 8 '19 at 19:01

There isn't one

Just your main breaker. That's it.

The presumption is that the area prior to the main breaker is exceedingly simple, with nothing tapped there, so a problem there would be unlikely.

That's also why this is a bad idea.

Collecting data about electrical accidents is what the NFPA does for a living, so clearly, "unlikely" is proven out by statistical analysis. If incidents were occurring at a rate of concern, they would be acting to deal with it, and their NEC codebook or UL standards would soon be updated to reflect it.

It wouldn't take a whole lot to add a fusible link to the input bussing of a meter, so that's an easy action UL can simply mandate. If it were necessary. The fact that they haven't says a thing or two.

  • All makes sense. But that means that with, for example, 100A service, you could have a (or more than one!) subpanel with a 200A main breaker because it is "just a convenience shutoff switch" but the main panel main breaker must be no larger than provisioned service. – manassehkatz-Moving 2 Codidact Sep 8 '19 at 19:21
  • 1
    Right. The "multiple main breakers that add up to greater than provisioned service" went out with the Rule of Six. But subpanels can be as big as you please. – Harper - Reinstate Monica Sep 8 '19 at 19:43
  • @JimmyFix-it Are you saying the answer should be phrased "there isn't one"? Or is there a protective device to which I am unaware? If the former, I'll cheerfully edit. If the latter, please write an answer! – Harper - Reinstate Monica Sep 8 '19 at 22:56
  • @Harper can you cite or explain the "multiple main breakers that add up to greater than provisioned service" comment? Is this somehow allowed? – ndemarco Sep 10 '19 at 13:11

Twenty-five years ago I was panicked by a forecast of an ice storm and decided to cut off a tree branch hanging over my service drop. I was standing on a fiberglass ladder up against the power pole and working a manual pole saw.

I barely nicked the insulation of my service drop--loud bang, big spark, one or two teeth burned off the saw blade, but no interruption in service. The fuse on the transformer did not blow.

The branch was incompletely cut and now hanging on the wire. With a heavy heart I called the power company. One guy appeared later that afternoon in a bucket lift truck. He detached my service at the pole, cut away the branch, examined the service drop after separating the wires, and pronounced it still usable (applied tape IIRC), then reconnected it at the pole. No charge.

I have always wondered if the conductor was damaged.

  • I think you may have answered the OP's question here: "If I were to draw excess current from the triplex service cable running from the pole to the meter socket, what device protects the cable from overcurrent?", answer: "fuse on the transformer". – Jimmy Fix-it Sep 8 '19 at 22:47
  • I may have introduced a red herring. We have aerial distribution lines and aerial service drops (on our side of the alley). The houses across the alley have underground distribution lines from the poles on our side of the alley going under the alley to underground transformers in the front yards. The only times I have seen the fuse on our pole blow is when the 40-year-old underground line began failing by shorting. I don't know if the fuse on my pole is related to my service. I think that fuse is in high voltage line (8000 V ?). – Jim Stewart Sep 8 '19 at 23:02

I have had the luck , a few times, of finding a short which knocked out 1 of the phases to a service. I have learned that there are fuses on the backside of the meter to protect their equipment. However, I am unfamiliar if the new digital meters have this protection in the same way or if it is internal. Their rating on residential units are 10,000 amp short circuit protection which translates into around 1 million watts of flash and heat in that moment of the short. Commercial typically has 50,000 amp short circuit protection. The grid has other points that will open as needed to protect the infrastructure.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.