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Breaker slots per amperage rating


Bain
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Today I looked at an atrocious electrical service and panel that require a major overhaul. Everything you can imagine was wrong, and I told the buyer an electrician needed to check out the entire configuration and effect repairs/replacement.

The panel in question was a 40-slot, 200 amp box that someone had stuck a 100-amp main in, after which every available slot was equipped with a breaker. We all know that most 100-amp panels have twenty slots, while most 200-amp panels have forty slots. Is there anything in the NEC that mandates how many circuits can be connected to a given main-size, or is common sense supposed to prevail? And to go a step further, is there a code-imposed limit on how many tandem breakers can be installed in a given panel?

It's always struck me as strange that tandem breakers aren't permitted within new services, but they're perfectly legit once a panel has been installed and inspected.

John

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Just to test what I have learned so far.

Better to have a 100 amp main in a panel rated 200 amp, then the other way around.

Though it is common to have 20 slots on that hundred , the only way to know for sure would be to do a load calculation.

Red flag may be raised , but if you are not sure a regular electrician should check it out.

As we all know the totals of breaker amp ratings regularly exceed the main breaker , due to the fact they are not all being used at the same time.

Now I step back for my chew down.

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The NEC doesn't regulate the number of circuits for a given panel rating (100 amp, 200 amp, etc.). The manufacturers' specs do. The label will show how many circuits the panel was designed and tested for, and there's no way to legitimately go beyond that.

Up to this point, here's the concensus maximums as far as I know:

100 or 125 amps - 20 circuits

150 amps - 32 circuits

200 amps - 40 circuits

225 amps 42 circuits

That includes all manner of tandem/half-size/you name it breakers.

Brian G.

Surely Among the Most Trampled Specs [:-crazy]

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Originally posted by Bain

. . . The panel in question was a 40-slot, 200 amp box that someone had stuck a 100-amp main in, after which every available slot was equipped with a breaker. We all know that most 100-amp panels have twenty slots, while most 200-amp panels have forty slots. Is there anything in the NEC that mandates how many circuits can be connected to a given main-size, or is common sense supposed to prevail?

I believe that the 100 amp/20 breaker, 200 amp/40 breaker rule comes from UL. The NEC only says that a single panel can't have more than 42 breakers.

And to go a step further, is there a code-imposed limit on how many tandem breakers can be installed in a given panel?

No, that limitation will be printed on the panel's schematic along with the acceptable and unacceptable locations for the tandems.

It's always struck me as strange that tandem breakers aren't permitted within new services, but they're perfectly legit once a panel has been installed and inspected.

John

Why do you think they aren't allowed in new services?

- Jim Katen, Oregon

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Originally posted by chicago

Just to test what I have learned so far.

Better to have a 100 amp main in a panel rated 200 amp, then the other way around.

Though it is common to have 20 slots on that hundred , the only way to know for sure would be to do a load calculation.

No. A load calculation won't tell you how many breakers to put in a panel.

It's easy to know exactly how many breakers are allowed. Just read the schematic that's attached to every panel.

- Jim Katen, Oregon

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John, assuming you are talking about new construction rather than just a new replacement panel, then it makes sense that the panel should be large enough to handle the homes circuits without resorting to half-size or tandem breakers. BUT...if a hard and fast rule actually exists anywhere, I suspect it's a local AHJ thing. I certainly can't find anything in the NEC that would prohibit any particular listed breaker in a listed panel.

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Originally posted by Chad Fabry

The NEC only says that a single panel can't have more than 42 breakers.

Is it breakers or circuits? Theoretically one could have have 80 circuits with

forty breakers

The actual code wording is "overcurrent devices."

I don't believe it's possible to have more than one circuit on a breaker.

- Jim Katen, Oregon

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While I have the good book open I might as well post the whole relevant section.

408.15 Number of Overcurrent Devices on One Panelboard. Not more than 42 overcurrent devices (other than those provided for in the mains) of a lighting and appliance branch-circuit panelboard shall be installed in any one cabinet or cutout box.

A lighting and appliance branch-circuit panelboard shall be provided with physical means to prevent the installation of more overcurrent devices than the number for which the panelboard was designed, rated, and approved.

For the purposes of this article, a 2-pole circuit breaker shall be considered two overcurrent devices; a 3-pole circuit breaker shall be considered three overcurrent devices.

Note that it's the number of OCPDs that is restricted, not circuits. Assuming you have some 240-volt circuits, that number would actually be less.

So, basically, you can count the poles or lugs to get to the 42 (or smaller) number. Doesn't really matter if they are full, tandem or half size. Each lug counts as one "overcurrent device".

Before you ask...a Square-D lug that can take two conductors only counts as one.

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Originally posted by Richard Moore

A lighting and appliance branch-circuit panelboard shall be provided with physical means to prevent the installation of more overcurrent devices than the number for which the panelboard was designed, rated, and approved.

Unfortunately this is where almost all manufacturer's have failed miserably. With most brands it's very easy to alter a tandem to fit a slot where none was meant to go; so people do. People are people.

Brian G.

Convienence Trumps Specs [:-paperba

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Before you ask...a Square-D lug that can take two conductors only counts as one.

Richard Moore

Rest Assured Inspection Services

Seattle, WA

It may only count as one, but damnit, it's two circuits.

The fact that it originates from the same over current device is all

good , but by that theory, everything originating from that bus is one circuit.

I read carefully what the NEC says a circuit is; I've been working w/ electrical circuits my entire adult life... they're circuitous, they have a beginning and an end and in the case of a double lugged breaker, there are two beginnings and two ends. Why is it one circuit?

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Originally posted by Chad Fabry

Before you ask...a Square-D lug that can take two conductors only counts as one.

Richard Moore

Rest Assured Inspection Services

Seattle, WA

It may only count as one, but damnit, it's two circuits.

The fact that it originates from the same over current device is all

good , but by that theory, everything originating from that bus is one circuit.

I read carefully what the NEC says a circuit is; I've been working w/ electrical circuits my entire adult life... they're circuitous, they have a beginning and an end and in the case of a double lugged breaker, there are two beginnings and two ends. Why is it one circuit?

One beginning...

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The 8 is not the shape of the cables running through the house, it's the path of the current running through the cables. Snip the eight where the loops are common. Tie one end to the OCPD. Tie the other end to the neutral bus bar in the panel. I'm simplifying by neglecting the EGCs.

Consider wiring a normal branch circuit. Run a cable from the panel to a junction box. From the j-box run two cables, each one to a different outlet. This is done all the time and you'd call this one circuit, right? If the OCPD trips, both outlets and the wiring between them and the panel should be dead. The portion of the circuit from the j-box to the outlets represents the two loops of the eight (snipped, and tied back to the hot and neutral bus through the cable that runs from the panel to the j-box).

Now, take away the j-box and the wiring but keep the original outlets. This time run two cables, each one from an outlet back to the panel. Instead of tying the two cables together in a j-box, wire the hots to an OCPD that is approved for more than one hot and wire the neutrals to the neutral bus. OCPD trips, and two outlets and wiring between them and the panel should be dead. One circuit or two? I say one.

(I'm not convinced I'm right ... I'm trying to figure it out too)

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Without getting at all technical, I agree with Chad. That one breaker is feeding two circuits, even if it gets counted as one because of the breaker. I've always thought those breakers were a bad idea to start with. Keep it simple on the residential side. One breaker, one wire; period.

Brian G.

"Because We Can" Is Not a Good Reason [:-dunce]

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Originally posted by Brian G

Without getting at all technical, I agree with Chad. That one breaker is feeding two circuits, even if it gets counted as one because of the breaker. I've always thought those breakers were a bad idea to start with. Keep it simple on the residential side. One breaker, one wire; period.

Brian G.

"Because We Can" Is Not a Good Reason [:-dunce]

If that one wire is six inches long and then is spliced to two wires that serve different devices, is it one circuit or two?

If that one wire leads to a 4-gang box and is split off to control two different switch loops, is it one circuit or two?

- Jim Katen, Oregon

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