The Number Of Slots Per Pole Should Be
What is the maximum number of circuit breakers allowed in an electric panel?
Machines the number of slots per pole usually lies a) between 2 to 4 b) between 6 to 8 c) between 9 to 16 d) between 20 to 30. Assuming number of slots per pole per phase = 4 1/2 Total number of slots = 4.5 x 8 x 3 = 108 Slot pitch = πD/s = π x 149/108 = 4.35 cm (quite satisfactory) Number of conductors per slot = 660/108 ≈ 6 Total number of conductors revised = 108 x 6 = 648 Number of turns/phase = 108. E = emf induced per coil side m = number of slots per pole per phase, n = number of slots per pole β = slot angle = 180/n The emf induced in concentrated winding with m slots per pole per phase = mE volts. In practical machines the windings will be generally short pitched and distributed over the periphery of the machine. It depends on the required speed. N (rpm) = (60 x f) / N where:- f = frequency and N = number of pole pairs. The 60 is there to convert from revolutions per second to revolutions per minute as the frequency is in cycles per second. If the number of slots per pole per phase 𝒒𝒒= 𝑺𝑺 𝒎𝒎𝑷𝑷. Is an integer, then the winding is called an. Integral-slot winding in case the number of slots per pole per phase, q is not an integer, the winding is called. Fractional-slot winding. For example a 3-phase winding with 36 slots and 4 poles is an integral slot.
Saturday, June 16, 2018
Not What It Used To Be
The answer to this question was once very simple. The National Electrical Code (NEC) specified that a lighting and appliance branch circuit panelboard could not contain more than 42 overcurrent devices (circuit breakers). That was the absolute maximum, unless the manufacturer specified a lower number, which was often the case for smaller panel boxes.
But the 2008 edition of the NEC deleted the maximum number and the new standard became whatever the manufacturer specifies. That number can be found in the fine print of the data plate on the inside of the panel door and is often near the schematic diagram. Some panels only allow one breaker per slot, while others allow some or all of the slots to accept a tandem breaker, which fits two breakers in one slot. So a “20/30” panel, for example, has 20 slots, but will accept up to 30 circuits, allowing up to half of the slots to be tandems.
CTL Breakers Also Important
The NEC has had an additional requirement since 1965 that “a panelboard shall be provided with a physical means to prevent the installation of more overcurrent devices that than that number for which the panelboard was designed, rated, and listed.” This is known as Circuit Total Limitation panel or “CTL.” Unfortunately, non-CTL breakers are still manufactured for repair of pre-1965 panels, and are sometimes used in newer panels to get around this limitation. They lack the “rejection” feature that keeps CTL-breakers from being installed improperly. Both CTL and non-CTL breakers are clearly marked, although in very fine print, on the side of the breaker.
Maximum Amps Per Stab
One additional limitation for the breakers in a panel is the “maximum amps per stab” that a manufacturer allows. A stab is another name for a bus bar finger. Panels have half as many stabs as total breaker slots, so a panel with forty slots has twenty stabs, and a typical 150-amp rated panel requires that the sum of the amperages of both the breakers on one bus bar finger not exceed 200 amps. So this means you cannot have two breakers side-by-side with a total amp rating that exceeds 200 amps. This maximum is also listed in the data plate, which may also list limits on the number, rating, or arrangement of the breakers in the panel. To learn more, see our blog post What is a 'bus stab' in an electric panel?
So the short answer to the maximum number of breakers question is 42 for pre-2008 panels, and whatever the manufacturer specifies for newer panels—while not exceeding the maximum amps per stab or using non-CTL breakers in a CTL panel.
Also, see our blog posts Should I buy a house near a high-voltage power line? and What causes copper wires to turn green or black in an electric panel? and Can you add circuit breakers by different manufacturers to an electric panel if they fit?
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Here’s links to a collection of our other blog posts about ELECTRIC PANELS:
• My circuit breaker won't reset. What's wrong?
• What is a split bus electric panel?
• What is the maximum number of circuit breakers allowed in an electric panel?
• When should a corroded or damaged electric panel cabinet or disconnect box be replaced?
• What is a tandem circuit breaker?
• When did arc fault circuit interrupter (AFCI) breakers first become required?
• Can an electric panel be located in a closet?
• Can an electric panel be located in a bathroom?
• How do I identify a combination AFCI (CAFCI) circuit breaker?
The Number Of Slots Per Pole Should Be Paid
• What does a circuit breaker with a yellow or white test button indicate?
• What is the maximum gap allowed between the front of a recessed electric panel box and the wall surface surrounding it?
• What are the requirements for NM-cables entering an electric panel box?
• Why is a fuse box/panel an insurance problem for homebuyers?
• Why is bundled wiring in an electric panel a defect?
• What is the difference between GFCI and AFCI circuit breakers?
• Why are old electrical components not always 'grandfathered' as acceptable by home inspectors?
• What happens when you press the 'TEST' button on a circuit breaker in an electric panel?
• What is a Dual Function Circuit Interrupter (DFCI)?
• What is the difference between a Combination Arc Fault Circuit Interrupter (CAFCI) and an Arc Fault Circuit Interrupter (AFCI) circuit breaker?
• What is the difference between 'grounded' and 'grounding' electrical conductors?
• What does it mean when a wire is 'overstripped' at a circuit breaker?
• Why is an old fuse panel dangerous?
• Who is the manufacturer of those 'bad' electric panels?
• Why is the circuit breaker stuck in the middle?
• What is a double tap at a circuit breaker?
• What is the right electric wire size for a circuit breaker in an electric panel?
• What is the life expectancy of a circuit breaker?
• My circuit breaker won't reset. What's wrong?
The Number Of Slots Per Pole Should Bet
• Why do some breakers in my electric panel have a 'TEST' button on them?
• What is the right size electric panel for a house?
• What do I need to know about buying a whole house surge protector?
• What is the maximum allowed height of a circuit breaker (OCPD) above the floor?
• What is the maximum height you can mount an electric panel above the floor?
• What is the code required clearance in front of an electric panel?
• What is the main bonding jumper and where do it find it in an electric panel?
Visit our ELECTRIC PANELS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.
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