Garage Subpanel

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Dave781

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Location
Massachusetts
Occupation
Contractor
Wishing to install 100 amp subpanel in a detached garage, in Massachusetts. Existing conditions inside the residential house are: 200 amp service panel (w/circuit breakers) with an exsisting 100 amp subpanel immediately adjacent to 200 amp panel. The 200 amp panel is 90% filled, but there is still reasonable room to install a double pole breaker and associated wire to feed the detached garage. I was told by one electrician that I cannot have 2-100 amp subpanels off the 200 amp panel. He said this within seconds of opening the panel door and by taking a glance at the number of breakers in the 200 amp panel. He did no load calculations or tried to assess the amount continuous loads on the 200 amp panel. I trying to understand if there is a specific code issue at hand or is this just an overly conservative opinion/assessment. There is no electric hot water heater, no electric heating or other large continuous loads on the existing panel. The detached garage will have a welding plug outlet, hard wired 20 amp/2-pole air compressor, 20 amp outlets, 15 amp lighting circuits etc. A 60 amp service could work in the garage, but I have all the stuff (wire & main breaker panel) for a 100 amp configuration. Is he concerned that future work/additions to the panel will over load the 200 amp panel. I gotten feedback from another guy just based on a quick conversation that said “multiple subpanels gets fuzzy, just stay with the 60 amp.” Just trying to understand the technical reason for his statement.
 

infinity

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Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Wishing to install 100 amp subpanel in a detached garage, in Massachusetts. Existing conditions inside the residential house are: 200 amp service panel (w/circuit breakers) with an exsisting 100 amp subpanel immediately adjacent to 200 amp panel. The 200 amp panel is 90% filled, but there is still reasonable room to install a double pole breaker and associated wire to feed the detached garage. I was told by one electrician that I cannot have 2-100 amp subpanels off the 200 amp panel. He said this within seconds of opening the panel door and by taking a glance at the number of breakers in the 200 amp panel. He did no load calculations or tried to assess the amount continuous loads on the 200 amp panel. I trying to understand if there is a specific code issue at hand or is this just an overly conservative opinion/assessment. There is no electric hot water heater, no electric heating or other large continuous loads on the existing panel. The detached garage will have a welding plug outlet, hard wired 20 amp/2-pole air compressor, 20 amp outlets, 15 amp lighting circuits etc. A 60 amp service could work in the garage, but I have all the stuff (wire & main breaker panel) for a 100 amp configuration. Is he concerned that future work/additions to the panel will over load the 200 amp panel. I gotten feedback from another guy just based on a quick conversation that said “multiple subpanels gets fuzzy, just stay with the 60 amp.” Just trying to understand the technical reason for his statement.

Time to find another electrician. As you stated correctly only a load calcuation can accurately determine whether or not the 200 amp service is adequate.
 

roger

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Location
Fl
Occupation
Retired Electrician
I agree with the others, you need to find new EC's to consult with, the ones you have talked to are not in the know.

Roger
 

Dave781

Member
Location
Massachusetts
Occupation
Contractor
Thank you. One other question: I choosing whether to pull the new garage service off the 200 amp panel or the existing 100 amp panel - is that soley based on which panel is easier to access and install/configure the conductors? or are there other considerations
 

roger

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Staff member
Location
Fl
Occupation
Retired Electrician
Thank you. One other question: I choosing whether to pull the new garage service off the 200 amp panel or the existing 100 amp panel - is that soley based on which panel is easier to access and install/configure the conductors? or are there other considerations
When you find a knowledgeable EC they will be able to assist you with this decision.

Roger
 

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I am guessing the ec is probably referencing 408.30 except 2 and has it confused


Exception No. 2: Individual protection shall not be required for a
panelboard protected on its supply side by two main circuit breakers or
two sets of fuses having a combined rating not greater than that of the
panelboard. A panelboard constructed or wired under this exception
shall not contain more than 42 overcurrent devices. For the purposes of
determining the maximum of 42 overcurrent devices, a 2-pole or a
3-pole circuit breaker shall be considered as two or three overcurrent
devices, respectively
.
 
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