Beatrice Lang
In electrical terms, MLO stands for 'Main Lug Only'. It is a type of panelboard or load centre that connects directly to the panel bus via lugs. MLO panelboards do not contain a main circuit breaker, which can reduce costs in electrical construction. They are often used when an additional overcurrent device is not required, such as for subpanels or downstream of transformers. MLOs are commonly used in residential load centres to expand the number of circuits and create independent electrical zones.
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What You'll Learn
MLO stands for 'Main Lug Only'
MLO stands for Main Lug Only. It is a type of panelboard or load centre that does not contain a main breaker. Instead, the feeder wires are connected directly to the panelboard bus.
Main Lug Only panelboards are permitted by the National Electrical Code (NEC) in certain circumstances. They are often used to reduce costs in electrical construction projects. MLO panelboards 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.
In a home, a main lug load centre is sometimes installed to expand the number of circuits. It creates independent electrical zones, which are especially valuable when building an accessory dwelling unit or ADU. However, not all homes have a main lug load centre.
The opposite of an MLO is an MDP, or Main Disconnect Panel, which provides overcurrent protection for branch circuits.
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MLO panels are installed downstream from a main breaker load centre
MLO stands for "Main Lug Only". MLO panels are installed downstream from a main breaker load centre, which is the home's main load centre, receiving electricity from the power lines. A main lug load centre, or subpanel, does not have a main breaker. Instead, it has a single lug, which is a copper or aluminium connecting terminal that clamps down the bare end of an electrical wire. Incoming power from the main breaker load centre feeds into a central lug, which then feeds into the line side of the lugs attached directly to the bus bar.
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. They are also used to expand the number of circuits in a home and to create independent electrical zones, especially when building an accessory dwelling unit (ADU). Not all homes have a main lug load centre, and they are only permitted by the National Electrical Code (NEC) in certain cases.
One advantage of using MLO panels instead of main circuit breaker (MCB) panelboards is the cost reduction. Providing an MCB panelboard can add between 30-100% to the cost of the panelboard, depending on the bus rating. MLO panels also provide a safer option for maintenance, as to de-energize an MLO panel, you simply open the next upstream device, whereas with an MCB panelboard, the line side will still be energized even if you open the main breaker.
However, it is important to note that MLO panels are limited to having only two to six circuit breakers, while MCB panelboards can contain as many circuit breakers as possible. Additionally, MLO panels may not be suitable for visual disconnect requirements, as some people find a breaker easier than a pull-out plug.
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MLO panels are used when a separate overcurrent device is upstream
MLO stands for Main Lug Only, and it is the opposite of an MDP, or Main Disconnect Panel. MLO panels are used when a separate overcurrent device is upstream. In other words, MLO panels are used when there is already a device in place to protect against overcurrent, so another one is not required. This is in contrast to MCB (Main Circuit Breaker) panels, which include their own overcurrent protection.
MLO panels are typically used as subpanels, which are installed downstream from a main breaker load center. They are used to expand the number of circuits in a home and to create independent electrical zones. However, they do not increase the home's amps, as they are limited by the overall amperage supplied by the main breaker load panel.
MLO panels are often used in construction projects to minimize material costs associated with electrical distribution equipment. They are also used when there is a need to work inside the panelboard, as it is safer to do so when both the line and load side of the main circuit breaker are de-energized, which can be achieved with MLO panels by opening the next upstream device.
MLO panels are also used when there is a separate overcurrent device upstream because they have fewer circuit breakers than MCB panels. MLO panels are allowed to contain only two to six circuit breakers, while MCB panels can contain as many circuit breakers as possible. This makes MLO panels a more cost-effective option when a separate overcurrent device is already in place.
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MLO panels are cheaper than MCB panels
MLO stands for "main lug only" in electrical terms. A main lug load center, or subpanel, is installed downstream from a main breaker load center. It is sometimes used to expand the number of circuits in a home and to create independent electrical zones.
Now, MLO panels are cheaper than MCB panels because they do not have a main breaker. In a main lug load center, the incoming power from the main breaker load center feeds into a central lug, which then feeds into the line side of the lugs attached directly to the bus bar. The main breaker load center's main breaker provides the main overcurrent protection, and the circuits in the MLO have their own individual overcurrent protection with the circuit breakers.
On the other hand, a main circuit breaker (MCB) panelboard provides overcurrent protection within the panelboard and a means of cutting power to the panelboard if work is required inside of it. This is not considered best practice as, although the load side of the circuit breaker is not energized, the line side will still be. Ideally, both the line and load sides of the main circuit breaker should be de-energized by opening the next upstream device.
The absence of a main breaker in the MLO panel results in cost savings. 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, the additional cost is closer to 30%, but as panelboard sizes increase, so does the extra cost. Therefore, using MLO panelboards instead of MCB panelboards, where permitted, can significantly reduce the cost of electrical construction.
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MLO panels are limited to two to six circuit breakers
MLO stands for 'main lug only' and refers to a type of electrical panel or load centre that does not have a main breaker. Instead, it has a single lug, which is a copper or aluminium connecting terminal that clamps down the bare end of an electrical wire.
The six-breaker rule applies when a building is detached, and more than six breakers are installed in the panel. In this case, a main breaker is required in addition to the 90-amp breaker at the main panel. This is because, with more than six breakers, it would take more than six hand movements to shut off all the breakers in the panel, and so a main breaker is necessary to simplify the process of disconnecting the supply.
MLO panels are often used when there is already a separate overcurrent device upstream, or for a subpanel fed directly from another breaker in a different panel. They are also used to reduce costs, as providing an MCB panelboard instead of an MLO panelboard adds between 30-100% to the cost of the panelboard, depending on the bus rating.
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Frequently asked questions
MLO stands for Main Lug Only. It is a type of panelboard or load center that does not have a main breaker. Instead, it has a single lug, which is a copper or aluminum connecting terminal that clamps down the bare end of an electrical wire.
MCB stands for Main Circuit Breaker. In an MCB panelboard, the overcurrent protection is provided by the MCB within the panelboard, which also allows for the cutting of power to the panelboard when work is required. On the other hand, an MLO panelboard does not have a main breaker, and the overcurrent protection is provided on the supply side ahead of the panelboard.
Utilizing MLO panelboards instead of MCB panelboards can help to reduce costs in electrical construction projects. An MLO is typically used when there is already a separate overcurrent device upstream or for a subpanel fed directly from another breaker in a different panel.
