Yes, tandem breakers are absolutely allowed in most cases. Tandem breakers are typically permitted in panelboards that are designed to accommodate them and are listed for tandem breaker use. Tandem breakers are useful for adding additional circuits in panels where space is limited. However, the National Electrical Code (NEC) and local building codes may have specific requirements and limitations regarding their installation. Always consult the panel’s labeling and documentation, as well as local codes, to ensure compliance.

Tandem breakers, also known as twin or double breakers, do not inherently share a neutral. Each circuit connected to a tandem breaker should have its own separate neutral wire. However, if a tandem breaker is used in a multi-wire branch circuit (MWBC), which typically consists of two hot wires sharing a single neutral, then the neutral would be shared. It’s crucial to ensure proper wiring practices and code compliance when installing tandem breakers to maintain electrical safety and system integrity.

Square D Homeline circuit breakers are not designed to be interchangeable with other brands because of specific mechanical and electrical design requirements. Using a non-Homeline breaker in a Homeline panel can void warranties, fail inspections, and pose safety risks. Only Square D Homeline breakers should be used in Homeline load centers to ensure proper fit, function, and compliance with UL listings and the National Electrical Code. Always refer to the panel’s documentation and manufacturer recommendations for compatible breakers. 

To determine if a panel can accept tandem breakers, follow these steps:

• Check the Panel Label: Look inside the panel door for a label or diagram. It will usually provide information about the types of breakers that can be installed, including if tandem breakers are allowed.
• Identify the Model and Series: Verify the panel’s model and series number, usually found on the label. This information can help confirm compatibility with tandem breakers.
• Look for Tandem Breaker Slots: Some panels have specific slots designed for tandem breakers, usually marked with a “T” or similar indicator.
• Consult Manufacturer Documentation: Refer to the manufacturer’s documentation or website for detailed specifications and allowable configurations for your panel model.
• Compliance with NEC: Ensure that the use of tandem breakers complies with the National Electrical Code (NEC) and local codes.

The number of tandem breakers allowed in a 100 amp panel depends on the specific panel model and its design. Generally, this information can be found on the panel's label or in the manufacturer’s documentation. Here's how to determine the allowance: Panel Label: Look for a label inside the panel door. It will indicate the maximum number of tandem breakers allowed and identify which slots can accept them. Panel Model Specifications: Refer to the manufacturer’s documentation for your specific panel model. This documentation will provide detailed information on the capacity and configuration. Slot Markings: Inspect the panel for markings that identify which slots are designed for tandem breakers. These slots are typically marked with a "T" or a similar indicator. NEC Compliance: Ensure compliance with the National Electrical Code (NEC) and local codes, which might have additional restrictions. For example, some 100 amp panels may allow up to 10 tandem breakers, effectively providing 20 circuits, but this varies by panel design and manufacturer. Always follow the specific guidelines for your panel model to ensure safe and compliant installation.