Dos contenedores circulares en un estante pueden parecer iguales desde abajo pero contener cantidades completamente diferentes de cable. Por eso, un cambio fácil de luces a menudo se convierte en una visita adicional al mayorista. El diámetro aquí suele ser el mismo número, pero es la profundidad donde se toman decisiones importantes. Hemos compilado una lista de tipos disponibles de contenedores, sus capacidades según las regulaciones de llenado de cajas NEC, y cómo la situación es diferente en el resto del mundo.
Para resumir, una caja de salida de techo redonda se identifica típicamente por el estándar norteamericano de 4” (10.2 cm) de diámetro. Sin embargo, las variaciones en términos de profundidad y volumen de la caja pueden variar mucho, desde profundidades de 0.5” para la bandeja redonda de 4” (que tiene un volumen total de 6 in3) hasta 1.5” o 2.125” (octágonos de 4”) para un volumen de 15.5 in3 o 21.5 in3 respectivamente. Una caja octagonal aún más grande de 4-11/16” (11.8 cm) tiene un volumen de 25.5, 29.5 o 42.0 in3. También están disponibles bandejas poco profundas de 3.5” (8.9 cm). Sin embargo, en el Reino Unido e Irlanda, lo mismo se llama ceiling rose que tiene un diámetro que varía de 85-90 mm, o caja común empotrada que mide 100×100×45 mm.
Un diámetro, cuatro profundidades: la familia de cajas redondas
Las cajas de techo vienen en dos variedades y varias profundidades. Los tipos pueden ser redondos (tipo bandeja o caja redonda con poca profundidad) y octagonales (tipo octagonal, que es usualmente a lo que se refieren cuando hablan de cajas redondas de techo). Hay diámetros en opciones metálicas de 3-1/2 in o 4 in para las bandejas, un diámetro de 4 in para los octágonos, y 4-11/16 in para los tipos octagonales más grandes usados en aplicaciones comerciales. Los siguientes en la línea son las cajas para ventiladores de techo y cajas para luminarias que tienen un diámetro de 5 pulgadas y 6 pulgadas respectivamente, ya que pertenecen a una clasificación diferente de productos.
La profundidad es lo que importa en cuanto a capacidad según la tabla NEC 314.16(A), y la diferencia entre un octágono de 1-1/2 pulgadas y uno de 2-1/8 pulgadas equivale a 6 pulgadas cúbicas, que es la cantidad aproximada para alojar 2.5 conductores más de calibre 14 AWG en ellas. Generalmente, esta es la diferencia entre el éxito y el fracaso en términos de verificación de llenado de caja.
| Tipo de caja | Diámetro | Profundidad | Volumen | Máx. conductores 14 AWG |
|---|---|---|---|---|
| Bandeja redonda | 4 pulg (102 mm) | 1/2 pulg (13 mm) | 6 in³ | Solo conductores, sin dispositivo |
| Octágono | 4 pulg (102 mm) | 1-1/4 pulg (32 mm) | 5 in³ | 6 |
| Octágono | 4 pulg (102 mm) | 1-1/2 pulg (38 mm) | 5 in³ | 7 |
| Octágono (clasificado para ventilador) | 4 pulg (102 mm) | 2-1/8 pulg (54 mm) | 5 in³ | 10 |
| Octágono, grande | 4-11/16 pulg (119 mm) | 1-1/4 pulg (32 mm) | 5 in³ | 11 |
| Octágono, grande | 4-11/16 pulg (119 mm) | 1-1/2 pulg (38 mm) | 5 in³ | 13 |
| Octágono, grande profundo | 4-11/16 pulg (119 mm) | 2-1/8 pulg (54 mm) | 0 in³ | 21 |
Junto a estas cajas están las no metálicas que tienen números ligeramente diferentes debido a que son moldeadas en lugar de estiradas. Por ejemplo, una caja de techo no metálica de 4 de diámetro y 2-1/4 de profundidad tendrá un volumen alrededor de 20 pulgadas cúbicas, mientras que la que tiene 2-3/4 de profundidad varía entre 18 y 20 pulgadas cúbicas, dependiendo de cómo esté moldeada y si tiene abrazaderas incorporadas. La tabla NEC no considera el uso de cajas plásticas ya que se requiere que tengan su capacidad escrita.
Por qué existen las cajas redondas y octagonales
Una caja eléctrica con forma rectangular generalmente se instala en un techo, lo que significa que la forma de la caja tiene más que ver con el diseño del accesorio. Para un accesorio de iluminación, una caja redonda u octagonal proporciona una superficie uniforme para que el dosel repose. Al usar una caja octagonal, es posible rotar el dosel y aún cubrirá la abertura de la caja. Otro beneficio es el hecho de que la caja octagonal o redonda tiene tornillos para accesorios 8-32 que se colocan en el espaciamiento estándar de 2-3/4 pulgadas o 3-1/2 pulgadas que se puede encontrar en cualquier soporte de montaje para luminarias.
La forma octagonal tiene una ventaja adicional, ya que tener lados planos permite knockouts que dirigen los cables en la dirección correcta al trabajar con rejillas de techo. Esto significa que los cables pueden entrar en la caja desde el lado más apropiado sin crear tensión en los cables.
La profundidad es el número que importa, y el llenado de la caja es cómo se verifica
Todos los cables, al pasar por la caja para conexiones, empalmes o terminaciones, ocupan espacio. Además de los cables, los dispositivos, tierras y dispositivos de sujeción también ocupan espacio. Un dispositivo se cuenta por su yugo en lugar del número de tomas que tiene, por lo que un duplex receptáculo eléctrico es una asignación de un dispositivo, no dos. De acuerdo con la Tabla 314.16(B), se debe tener en cuenta que un cable 14 AWG ocupa 2.00 cu in, mientras que un 12 AWG ocupa 2.25 cu in. El yugo cuenta como dos cables del tamaño más grande que se esté usando, todos los cables de tierra juntos cuentan como un solo cable, y los dispositivos de sujeción cuentan como otro.
Ahora tome una caja de techo real. Una caja octagonal de 4 pulgadas por 2-1/8 pulgadas tiene un volumen de 21.5 cu in. Con un solo cable 14/3 entrado y solo un cable 14/2 salido para una línea de interruptor. Así, tiene cinco conductores que llevan corriente a 2.00 cada uno, un cable de tierra a 2.00, y un dispositivo de sujeción a 2.00, sumando un total de 14 cu in, dejándole 7.5 cu in libres. Pero, ¿qué pasa si usa un cable 12 AWG y añade un atenuador en el yugo? Entonces todo el volumen sobrante se usará muy rápidamente. Los mismos cálculos que son adecuados para un colgante pueden no ser suficientes para un interruptor inteligente si la caja resulta insuficiente para el cableado utilizado.
Justo al lado de esta consideración se encuentra otra pregunta, que se refiere a la siguiente sección del vocabulario. El término “gang” se refiere a dispositivos y no a aberturas, lo que significa que incluso si hay muchos cables dentro de la caja, una caja con un atenuador ya se cuenta como un gang.
| Ítem contado | Asignación de volumen | Notes |
|---|---|---|
| Cada conductor 14 AWG | 00 cu in | Contado si termina o empalma en la caja |
| Cada conductor 12 AWG | 25 cu in | Misma regla, asignación mayor |
| Todas las tierras de equipo | 1 asignación al tamaño más grande | El NEC 2020 y posteriores añaden una asignación adicional después de cuatro |
| Each device yoke | 2 × largest conductor on that yoke | A dimmer or GFCI counts as one yoke |
| Internal cable clamps | 1 asignación al tamaño más grande | External connectors need no allowance |
| Pigtails under 6 in | 0 | Both ends inside the box |
Octagon, round or square: telling them apart on sight
Study the corners carefully. A true octagonal structure features eight straight edges and angular breaks in angles; however, a round shape is called a circle featuring a shallow flange for not having any flat edges; a square box has four corners and is usually of size 4 × 4 inches. The square shape is the one very often confused with ceiling box, and it should not be neglected because the volume of the 4-inch square box is equal to 30.3 cubic inches, which is actually one and half times bigger than in octagon with the same diameter and height.
The mounting hardware also makes the distinction clearer. The octagons and pans made for ceilings are supplied with brackets or captive nails for mounting onto a beam or between the beams. The square boxes are installed usually through the sides and are mostly used as junction boxes or junction device boxes along with the mud ring that is also the method of converting square box to round mounting point for the fixture in case of need.That is where the single-gang and multi-gang distinction starts to bite, because a two-gang ring will not fit a 4 in octagon no matter how the wiring is arranged.
Fixture-rated versus fan-rated: the weight numbers
The fixture box must be capable of supporting the weight, and there are weight ratings that apply as well. Standard ceiling light fixture pans and octagonal boxes are rated only to support a 50 lb (22.7 kg) load, and they are explicitly not designed for ceiling fan use unless labeled otherwise. A ceiling fan installation box is either securely fastened to a joist or securely mounted and rated for fan support, usually at a maximum weight of 35 lb (15.9 kg) for fan weight plus 50 lb for fixture weight.
This is one of the most common mistakes in ceiling work: putting a fan in a box that was designed for a light fixture. The box may look the same and be the same size, but its classification is not the same, and careful wiring does not make up for the fact that the support weight rating was never assigned to the box.
Outside North America: metric sizes and the ceiling rose
In the United Kingdom and Ireland, they utilize the same device known as a ceiling rose. The standard size of loop-in is 85 or 90 mm in diameter and is made of white plastic material. It is regulated by BS 67, which takes care of the arrangement mechanism of terminals and strain relief, and it is rated with a 6 A capacity at 250 V — a sum that is suitable for a domestic light circuit heavily served by LEDs. Decorative ceiling roses made of brass, chrome, or plaster come with diameter ranging from 150 mm to 250 mm and are selected based on design and appearance rather than capacity.
There is another family of boxes that are characterized as metric flush boxes. One of the European round boxes can be marked with 100 mm of diameter and depth measurement of 46 mm having ten entrances for 25 mm conduit and are labeled as complying with BS EN 60670-1 and BS EN 60670-22. Such square boxes usually measure 100 x 100 x 45 mm. It is the same constraint that shapes UK wall socket and switch plate formats, which are built to a shallow back profile precisely because the boxes behind them are shallow.
| Región | Typical part | Diameter / size | Profundidad | Norma |
|---|---|---|---|---|
| US / Canada | Octagon ceiling box | 4 pulg (102 mm) | 1-1/2 in or 2-1/8 in | NEC 314.16, NEMA OS-1 |
| US / Canada | Non-metallic ceiling box | 4 pulg (102 mm) | 2-1/4 in or 2-3/4 in | NEMA OS-2, marked capacity |
| Reino Unido / Irlanda | Loop-in ceiling rose | 85-90 mm | Approx. 25-30 mm | BS 67, 6 A / 250 V |
| Reino Unido / Irlanda | Decorative ceiling rose | 150-250 mm | Fixture-dependent | Angled as decorative |
| Europe | Round flush connection box | Ø 100 mm | 45-46 mm | BS EN 60670-1 / -22 |
Brands, and why the dimensions differ anyway
All companies that we are familiar with, RACO (Hubbell), Carlon (Thomas & Betts), Steel City (Eaton), Arlington, Appleton and Southwire are happy to provide dimensional data, and for the case of steel octagons their dimensions are virtually in line with the above table since this box must fit the standard fitting canopies. The exception is with the moulded boxes since a 4 inch PVC ceiling box developed by one company can have a volume of 20 cu in at a depth of 2.5 in while another will have volume of 18 cu in, where the variation will depend not on the standard, but on the thickness of the box walls, integral clamps and knockout count.
This is the reason why the marked capacity is more important than the nominal size, and that is the reason NEC 314.16(A) dictates the necessity for marking boxes of less than 100 cu in by their volume. If the marking has been covered with paint use measuring tools instead: the internal diameter, the internal depth to the back of box should be measured and then compared against the values published in the manufacturer’s catalog.
How to size the replacement before you buy
The best way of achieving a successful installation is to perform four measurements and then make one verification. Begin by measuring the hole opening at the ceiling. This will give you the required size diameter for the fixture canopy. Next, measure the depth of the hole, using either a ruler or a stiff piece of wire. Next, have a count of electrical conductors that are already exiting the junction box, along with the wire gauge. Lastly, check the volume or the catalog volume. Based on this information, you can decide if you are dealing with a light or a fan, and it is essential to know this, as the choice will depend on the device type rather than its size.
In case of replacing the shallow pan with one that requires increased space for wiring, a 4 in octagon at 2-1/8 in provides 21.5 cu in of space while preserving the standard fixture spacing. If more space is actually required, a 4-11/16 box with a plaster ring will do the job.
Why we care about box sizes when we make switches and sockets
An important context about the position of GOG. We produce wiring devices i.e. switches, sockets, dimmers and smart switches in wholesale and OEM quantities, but we don’t manufacture junction boxes. Nevertheless, we are interested in the dimensions of junction boxes, because the majority of complaints of installers refer not to the device malfunction, but to the fact that the device does not fit the box. Smart switches are deeper than mechanical ones, and the dimmers are deeper than smart switches as they have rather large heat sinks. A 12 AWG installation occupies the box space faster than a 14 AWG one.
Hence, when someone asks us which dimmer to choose for a room with shallow ceiling boxes, we begin the answer with the volume.Pick the enclosure first, then the device that fits it — the same order we would suggest when selección de dispositivos de cableado across a whole building, where box depths are rarely uniform from floor to floor.
Preguntas frecuentes
What are the standard sizes for ceiling junction boxes?
The recognized standard for steel boxes is the 4 in round pan box that is half an inch deep (approximately equal to 6 cubic inches), the 4 in octagon box that can be 1-1/2 in, 2-1/8 in and finally, the 4-11/16 in octagon box which could have dimensions from 1-1/4 in, 1-1/2 in and then finally 2-1/8 in (having capacities of 25.5, 29.5 and 42.0 cubic inches) Non-metallic equivalents exist, which around 20 cubic inches in size while being 2-1/4 inch deep.
What is the diameter of a round junction box?
4 in (102 mm) is most frequently found in home and light commercial ceiling electromagnetic induction systems. Flattop lighting plates are also available in sizes of 3-1/2 in (89 mm). Larger commercial octagonal boxes measure 4-11/16 in (119 mm), while heavier installations can utilize boxes sized at 5 in and 6 in. The metric standard for circular boxes is Ø 100 mm.
What are the standard dimensions of a ceiling box?
The dimension corresponds to the product of diameter and depth, but depth would be variable. Thus, a 4 x 1.5″ octagon, a 4 x 2.125″ octagon, as well as a 4.6875 x 2.125″ octagon do exist to hold 15.5, 21.5, and 42.0 cubic inches correspondingly. Siempre provide both figures, because saying that it “is a 4 inches box” does not tell you how much wire it is able to hold.
Can you mount a ceiling fan in a round ceiling box?
Provided the rated box is used. Standard octagon boxes and the currently standard models are rated for fixtures up to 50 lbs (22.7 kg), which means they are not suitable for fan installations. The fan-rated boxes are designed to be braced and carry their own ratings for fans and fixtures, so be sure to check the labels first before attaching any ceiling fixture.
How do I find the volume if the box is not marked?
You can take a reading of the internal diameter and of the internal depth and then look this up in the manufacturer’s catalogue; it is the same 4 in diameter, which is present in several depths, and the volumes do not result from the measurements done from the outside. Non-metallic boxes are normally marked so a missing mark usually suggests paint, and then a quick scrape on the inside surface shows it usually.
Referencias
- NFPA — NFPA 70 National Electrical Code, Article 314 Outlet, Device, Pull and Junction Boxes
- NEMA — OS-2 Nonmetallic Outlet Boxes, Device Boxes, Covers and Box Supports
- NEMA — OS-1 Sheet-Steel Outlet Boxes and Box Supports
- UL Solutions — UL 514A Metallic Outlet Boxes
- BSI — BS 67 Specification for ceiling roses
- Hubbell RACO — Steel boxes and covers catalogue data
- Carlon — Non-metallic box capacity tables
Conclusión
Regular round ceiling outlet boxes measure 4 inches in diameter and the key to this is their depth. Half Inch pan makes adjustments for only lighting fixtures and items of minor importance; 1 1/2 Inch octagon has volume of 15.5 cubic inches and is appropriate for pendant and downlight installations; standard octagon measuring 2 1/8 inches is good for 21.5 cubic inches and is usually applied when installing a fan and possibly any IoT-device and 4-11/16 boxes with a maximum volume of 42.0 cubic inches are needed when a ceiling outlet turns into a junction.Select a box to get started, determine the number of wires inside of it, state whether you have a fixture or a fan at hand and only then pick an appropriate box. Performing these steps in such order is the reason why some ceiling installations last not more than 20 minutes while others require three attempts.
