Panelboards: MCB vs. MLO
Updated: Dec 8, 2021
Every construction project begins with a budget and staying within those budgets begins at the design process. For electrical design that means there needs to be a focus on what is required by the National Electric Code (NEC) and what may not be necessary for a functional and safe system. For panelboards, many times designers are utilizing main circuit breaker (MCB) panelboards where they are not required, and in some instances for every panelboard in the facility. In many cases, however, the NEC permits the use of main lug only (MLO) panelboards. 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 the electrical construction.
MCB Vs. MLO Panelboards:
In a main circuit breaker panelboard, the feeder wires land on a circuit breaker and the output of that circuit breaker supplies the panelboard’s bus. With a main circuit breaker panelboard, the overcurrent protection is provided by the MCB within the panelboard, which also provides 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 that circuit breaker is not energized, the line side will still be energized. Ideally to work inside any panelboard, you should ensure both the line and load side of the main circuit breaker are deenergized by opening the next upstream device.
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 deenergize an MLO panelboard you would open the next upstream device.
In either case, for any maintenance required at panelboards, proper lock out tag out procedures should be followed to ensure the safety of personnel and anyone coming in to contact with the electrical distribution equipment.
National Electric Code Requirements for 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 albeit with an exception. Where supplied by a transformer typically the panelboard will be provided as an MCB panelboard to meet this requirement. 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’ in length for most installations, and 25’ 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. You can actually use an MLO panelboard but unlike a MCB panelboard which contains as many circuit breakers as it can hold, the MLO panel is only allowed to contain two to six circuit breakers. Most dry-type transformers are separately derived systems which per NEC 100 is defined as, “An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections.” In essence, the primary and secondary voltages don’t share the same neutral. It is also like having another service to the facility. When this happens, we can use an MCB panelboard or an MLO panel with a maximum of six breakers (Reference NEC 230.71(B)).
Providing main circuit breaker panelboards only where required and utilizing main lug only panelboards where permitted can help to minimize the material costs associated with electrical distribution equipment in construction projects. Additionally, ensuring proper lock-out tag out procedures can help ensure the safety of personnel and others who may come into contact with the electrical equipment.
Kori Terray, PE
Electrical Group Leader