100 amp garage sub panel


Old 04-04-07, 12:46 AM
Thread Starter
Join Date: Nov 2005
Location: pennsylvania
Posts: 14
Question 100 amp garage sub panel

Have some elec. knowledge and got talked into helping freind.Underground feed already installed for 100 amp. but only 3 wire 2 hot 1 nuetral for 240 supply to sub.Sub. is square D.My question is can i use 100 amp double for main disconnect in garage.Also since no ground wire is run from panel in house to sub panel can i marry ground buss to nuetral buss in sub panel and run ground to a grounding rod or two to create bonded circuit?Garage is approx. 50 ft. from main panel in house.
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Old 04-04-07, 04:03 AM
Join Date: Oct 2006
Location: Wilmington
Posts: 4,219
The ground and neutral buss in the sub panel are to be kept seperate. The ground buss does get connected to a grounding rod, in most areas(check local code), but you need a ground wire between the main panel and the sub panel for a ground.
Old 04-04-07, 05:01 AM
Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
Just Bill is incorrect.

If there are no other metallic paths between the structures (no water pipes, no telephone wires, no television wires, etc.) then a three wire feed is allowed. It is not ideal, but it is allowed. The downside is that it prohibits adding those other items later (water via metal pipes, telephone, etc.).

Regardless of whether this is a three wire or a four wire feed, you need a ground rod (or probably two) at the garage.

If this is a three wire feed then the ground and neutral get connected at the sub panel. if this is a four wire feed then they remain separate.

You can only use a 100 amp breaker in the sub panel if the sub panel is rated for 100 amps or more. This breaker protects the panel.

The breaker in the main panel protects the wire feeding the sub panel, and must be sized for both the wire feeding the sub panel and sub panel itself (the smaller of the two).
Old 04-04-07, 06:54 AM
Thread Starter
Join Date: Nov 2005
Location: pennsylvania
Posts: 14
Smile 100 amp garage sub panel

Sub panel is 100amp,there are no water lines but there is a telephone and cable TV line to garage.Will run 4th wire somehow to assure ground loop. Thanks for the info,wish my freind would have talked to me before running feed line!

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