Sub Panel Electrical Wiring

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Sub panel usage situations....

1. You need additional circuits and you have run out of space on the main electrical panel. Although if it a space only issue and you have a breaker panel as opposed to older style fuse panels - then check to see if any of the single full slot breakers could be be substituted with a twin breaker which is a two breakers than only take up a single slot on the panel. This if possible may solve your issue without installing a sub panel.

2. You want an elaborate workshop area either as part of the main dwelling or as a separate building on the property and you want to have access to both circuit protection and isolated cut-offs in the same area as the workshop.

3. You want to isolate in a separate panel circuits that are to be serviced by a backup generator, although this can be done at the main panel without a sub panel, remember the transfer switch! Read the article - 'Backup Generators / Transfer switches'

In the picture above the main electrical service panel is on the left and the sub panel is on the right. The sub panel can be located right beside / close by the main panel or in separate areas, if it is not for an accessory building or for use in a workshop then it will most likely be close to the main panel. Although it is difficult to see in the picture there is an equipment grounding wire coming from the main panel to the sub-panel.

The size of the breaker at the main panel that feeds the sub-panel must be less than the main breaker at the main panel, for example if the your service is 200 amps you will have a main breaker of 200 amps at the main electrical service panel so the breaker feeding the secondary sub panel must be less than 200 amps. 60 amps to 100 amps seems to be common to feed the secondary sub panel. You have to figure out what the load demand is in the secondary sub panel and size accordingly. Our experts on the forums can help determine this - MEGA DIY HELP

Make sure the wire size used to feed the secondary sub panel is rated equal or higher than the breaker selected at the main panel used to feed the sub-panel. Also read the article called 'Wire Types', where a chart is included on the amp rating of wire sizes.

The sub panel load rating must be equal to or higher than the feeder breaker selected at the main panel.