Breakers of the same size in series

[RCHaySr]

I have a client that insist on installing one breaker in series with a second breaker of the same size; i.e., 150 A feeder breaker in one panel feeding a 150 A main breaker in a second panel. Is there any code issues to prevent them from doing this?

 

[dkidd]

No. The first one would be overcurrent protection, and the downstream could be a local disconnect.

 

[RCHaySr]

They have both as functioning breakers. I tell them that either or both may trip in a fault condition, but they insist on doing it this way. Is there any code issue that I can site which will prevent this?

 

[dkidd]

They have both as functioning breakers. I tell them that either or both may trip in a fault condition, but they insist on doing it this way. Is there any code issue that I can site which will prevent this?

No. It is not a code issue. Yes, either one may trip.

 

[ActionDave]

Some folks feel the need to have a main on every panel, it's a personal preference. It adds nothing to the safety, or reliability of the installation as long as the wire is properly protected at the source.

Another similar install that gets people worried sometimes is a smaller breaker, say 100A, feeding a larger, 200A main breaker panel.

 

[charlie b]

I tell them that either or both may trip in a fault condition, but they insist on doing it this way.

True, and that is perfectly acceptable.

Is there any code issue that I can site which will prevent this?

No, as others have already said. My question: why would you want to prevent this?

 

[Jraef]

Technically, the downstream breaker COULD just be a main switch, i.e. no trips. But what you will find is that the only "switch" available in a panelboard is a Molded Case Switch, which is a breaker with no trip elements. However, if you try ordering it that way, you find out that it's a special so you actually pay more, even though technically the MCD could be cheaper because it has fewer parts. It's really all about volume.

But an MCS is available as an option in many cases, specifically BECAUSE you can't predict which one will trip first, and in a situation where down time is thousands per minute, taking 1 minute longer to figure out which breaker tripped is going to cost you more than the difference in the cost of the Main device.

 

[jaylectricity]

Why don't you want twice the protection?

Also, that is not considered "in series".

 

[kwired]

Why don't you want twice the protection?

Also, that is not considered "in series".

Why do you think it is not in series?

It definitely is not in parallel.

 

[kwired]

I once had a fault in a 30 amp circuit, my helper at the time thought tightening the clamp on the romex connector meant turn the screws until they don't turn anymore I guess:blink:. Not only did my helper need clean underpants, it tripped the 30 amp breaker as well as the 200 amp main breaker in the panel.

 

[jaylectricity]

Why do you think it is not in series?

It definitely is not in parallel.

Because there's no appreciable resistance. It's not in parallel or series. It's not even a complete circuit. If you put the neutral from the first panel on the main on the second panel, then hooked the branch circuit ungroundeds to the breakers of the second panel while landing the neutrals to the breakers on the first panel, then maybe you could call that in series.

 

[GoldDigger]

Looking at it a different way: If opening either breaker cuts off current to the load, then the three are in series.

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[jaggedben]

Because there's no appreciable resistance. It's not in parallel or series. It's not even a complete circuit. If you put the neutral from the first panel on the main on the second panel, then hooked the branch circuit ungroundeds to the breakers of the second panel while landing the neutrals to the breakers on the first panel, then maybe you could call that in series.

Dude, they're in series. You're overthinking it.

 

[dkidd]

Looking at it a different way: If opening either breaker cuts off current to the load, then the three are in series.

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Where did three sneak in?

 

[kwired]

Where did three sneak in?

Two breakers and the load, there is at least one other thing in the series - the source.

 

[jaylectricity]

The breaker is a splice. A splice with a handle. A feed-in/feed-out. It's not a series. YOU'RE overthinking it.

 

[GoldDigger]

The breaker is a splice. A splice with a handle. A feed-in/feed-out. It's not a series. YOU'RE overthinking it.

You, on the other hand are under thinking it.
A closed loop for applying Kirchoff's voltage law is a set of series elements, while the wires connecting to a node to apply the current law are in some sense in parallel.

If I open a splice (with or without a handle), the current stops flowing, so it must be in series.

You, on the other hand, seem to think that series and parallel only apply to loads and sources.

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[kwired]

The breaker is a splice. A splice with a handle. A feed-in/feed-out. It's not a series. YOU'RE overthinking it.

If you don't put the breaker in series with the path of current it won't stop current flow when it opens, it just opens whatever parallel path(s) are connected to it.

 

[ggunn]

The breaker is a splice. A splice with a handle. A feed-in/feed-out. It's not a series. YOU'RE overthinking it.

Electrically, the breakers are in series. "In series" might mean something else special to you, but all the current from the source to the load passes first through one breaker and then through the other. They are by the definition of the word in series.

 

[Jraef]

Why don't you want twice the protection?

Aside from the strange side trip into the definition of "series", I'll also point out that having two breakers in series is not "twice the protection", it's the SAME protection, twice. Or in other words, it's redundant, not additive. There is a difference.

To reiterate what I said earlier, the argument against it is that you will now spend more time trying to determine WHICH of the redundant protection devices acted on the fault current, increasing your troubleshooting time and thus, down time losses if it's any kind of a production or processing facility.