Molly Ayala
In electrical engineering, MLO stands for 'Main Lug Only'. It is one of two main types of panelboards, the other being the MCB or 'Main Circuit Breaker' panelboard. The key difference between the two is that while an MCB panelboard contains as many circuit breakers as it can hold, an MLO panel is only allowed to contain two to six circuit breakers. This distinction is important as it affects the cost of electrical construction projects.
MLO (Main Lug Only) Electrical Panel Characteristics and Values
| Characteristics | Values |
|---|---|
| Overcurrent protection | Provided on the supply side ahead of the panelboard |
| Number of circuit breakers | Limited to 2-6 |
| Cost | Lower than MCB panelboards |
| Safety | Requires opening the next upstream device to de-energize |
| Use cases | Subpanels, facilities with separate overcurrent devices upstream, or as a cost-saving measure |
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MLO and NEC
MLO stands for Main Lug Only. In a Main Lug Only panelboard, the feeder wires land on lugs that are connected directly to the panelboard bus. Overcurrent protection for an MLO panelboard is provided on the supply side ahead of the panelboard as permitted by the NEC. To de-energize an MLO panelboard, one would open the next upstream device.
The NEC, or National Electric Code, permits the use of MLO panelboards. NEC article 408 covers the installation of panelboards, including where overcurrent protection is required. NEC article 408.36 requires that an overcurrent protective device be installed anywhere on the supply side of the panelboard. Based on this requirement, the panelboard overcurrent protection is permitted to be provided by the upstream device/feeder breaker serving it, unless that panelboard is supplied by a transformer. If the panelboard is supplied through a transformer, NEC article 408.36 (B) requires overcurrent protection on the secondary side of the transformer, with some exceptions.
Additional information and clarification for transformer secondary conductors and required overcurrent protection is provided in NEC article 240.21 (C). NEC article 240.21 notes that the secondary conductors cannot exceed 10 feet in length for most installations, and 25 feet in length for industrial applications without overcurrent protection. Where the conductors would exceed this length, the overcurrent protection is required to be located on the secondary side within the requirements given, typically provided by an enclosed circuit breaker located at the transformer. Where the conductors do not exceed this length, there is an additional option for the secondary conductors to be protected by the primary side overcurrent protection.
MLO panelboards are often used in construction projects to minimize the material costs associated with electrical distribution equipment. Providing an MCB panelboard instead of an MLO panelboard adds between 30-100% to the cost of the panelboard depending on the bus rating. For lower-rated panelboards, it is closer to the 30% range, however, as the panelboard sizes increase, the additional cost also increases. Therefore, utilizing MLO panelboards in lieu of MCB panelboards, where permitted, can greatly reduce the cost of electrical construction.
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MLO vs MCB
MLO stands for Main Lug Only, while MCB stands for Main Circuit Breaker. In an MLO panelboard, the feeder wires land on lugs that are connected directly to the panelboard bus. An MLO panelboard can contain a maximum of six circuit breakers. Overcurrent protection for an MLO panelboard is provided on the supply side ahead of the panelboard as permitted by the National Electric Code (NEC).
On the other hand, an MCB panelboard contains as many circuit breakers as it can hold. The feeder wires land on a breaker, and the output of that circuit breaker supplies the panelboard's bus. An MCB panelboard provides overcurrent protection and a means of cutting power to the panelboard if work is required inside of it. However, this is not considered best practice as the line side of the circuit breaker will still be energized, which can be a hazard.
MCB panels are typically used for main panels in residences and downstream of step-down transformers in commercial applications. MLO panels are typically used when there is already a separate overcurrent device upstream or for a subpanel fed directly from another breaker in a different panel.
Providing an MCB panelboard instead of an MLO panelboard can add between 30-100% to the cost of the panelboard, depending on the bus rating. Therefore, utilizing MLO panelboards instead of MCB panelboards, where permitted, can help to minimize material costs associated with electrical distribution equipment in construction projects.
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MLO panelboards
MLO stands for "main lug only". In a main lug only panelboard, the feeder wires land on lugs that are connected directly to the panelboard bus. Overcurrent protection for an MLO panelboard is provided on the supply side ahead of the panelboard as permitted by the National Electric Code (NEC). To de-energize an MLO panelboard, the next upstream device must be opened.
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MLO and overcurrent protection
MLO stands for 'main lug only', and it is a type of panelboard that can be used in electrical design and construction. In an MLO panelboard, the feeder wires land on lugs that are connected directly to the panelboard bus. This is in contrast to a main circuit breaker (MCB) panelboard, where the feeder wires land on a circuit breaker, and the output of that circuit breaker supplies the panelboard's bus.
MLO panelboards are typically used where there is already a separate overcurrent device upstream or for a subpanel fed directly from another breaker in a different panel. Overcurrent protection for an MLO panelboard is provided on the supply side ahead of the panelboard itself, as permitted by the National Electric Code (NEC). This is in contrast to an MCB panelboard, where the overcurrent protection is provided by the MCB within the panelboard.
The NEC sets requirements for the installation of panelboards, including where overcurrent protection is required. While an MLO panelboard can have a maximum of six breakers, an MCB panelboard can contain as many circuit breakers as it can hold. Providing an MCB panelboard instead of an MLO panelboard can add between 30-100% to the cost of the panelboard, depending on the bus rating. Therefore, using MLO panelboards instead of MCB panelboards where permitted can help to reduce the cost of electrical construction.
It is important to note that in an MCB panelboard, the MCB also provides a means of cutting power to the panelboard if work is required inside of it. However, this is not considered best practice as the line side of the circuit breaker will still be energised, which can be a hazard. In an MLO panelboard, to de-energise the panelboard, you would open the next upstream device. Proper lock-out tag-out procedures should be followed to ensure the safety of personnel and anyone coming into contact with the electrical distribution equipment.
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MLO and cost
MLO stands for 'main lug only' in the context of electrical panels. It is a type of panelboard, which is a component of an electrical distribution system that divides an electrical power feed into branch circuits, providing a protective circuit breaker or fuse for each circuit.
MLO panelboards are often used downstream of a main circuit breaker (MCB) panelboard and are referred to as secondary or add-on panels. They are typically used when there is already a separate overcurrent device upstream, or for a subpanel fed directly from another breaker in a different panel. Unlike MCB panelboards, which can contain many circuit breakers, MLO panelboards are only allowed to contain two to six circuit breakers.
MLO panelboards are a more cost-effective option than MCB panelboards. Providing an MCB panelboard instead of an MLO one adds between 30-100% to the cost of the panelboard, depending on the bus rating. This additional cost increases as the panelboard size increases. Therefore, using MLO panelboards instead of MCB panelboards, where permitted, can significantly reduce the cost of electrical construction.
It is important to note that MLO panelboards do not have a main overcurrent device in the panel. Instead, overcurrent protection for an MLO panelboard is provided on the supply side ahead of the panelboard, as permitted by the National Electric Code (NEC). To de-energize an MLO panelboard, one must open the next upstream device, and proper lock-out tag-out procedures should be followed to ensure safety.
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Frequently asked questions
MLO stands for Main Lug Only. In a main lug-only panelboard, the feeder wires land on lugs that are connected directly to the panelboard bus.
MCB stands for Main Circuit Breaker. In an MCB panel, the wires land on a breaker, the output of which supplies the panel bus. An MLO panel, on the other hand, does not contain a breaker.
MLO panels are typically used when you already have a separate overcurrent device upstream, or for a subpanel fed directly from another breaker in a different panel. Using an MLO panel instead of an MCB can also help to reduce costs.
To turn off an MLO panel, you need to go to wherever the panel is being fed from and open the next upstream device.
