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1974 Arrow-Hart Murray Load Center, any possible way to ID?

1974 Arrow-Hart Murray Load Center, any possible way to ID?


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Old 02-01-23, 02:32 PM
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1974 Arrow-Hart Murray Load Center, any possible way to ID?

Getting solar quotes. Far too expensive to do a panel replacement and service upgrade as I'd have to:

1. Rip up 30 feet of driveway to trench and get into my neighbor 2 doors down's yard to hit the power company's transformer. SDGE is also about 12 months out to even talk to you about service upgrades.
2. Move the panel location as it's 12" above the gas meter. Good ol' 70s. That is not allowed anymore and would need moved and all house circuits relocated to the new location.

SOO... I am trying to make my current panel work. It's a 1974 Arrow Hart Murray with nothing but that and a UL label. No part #, no serial number, no ratings. nothing. How the hell was this the "way it was done" back then? I checked 4 of my neighbors - same panel - no markings. It has a 100a feed from the meter, two 1 gauge wires. What I really want to find out, and what I am hoping for, is that this is a 125a panel. Reason being I could get 50a of solar on that. Reasoning:

125a panel, 120 rule = 150a.

100a main breaker. 50a solar breaker. 9.2kw system.

9200/240 = 38.33a. 80% rule = 38.33*100/80 = 47.91a.

That's adequate for my needs and would really cut down the complexity of this job.

I have Googled everywhere, I cannot find any documentation, pictures, ways to ID, anything on these old panels. It has been trouble free the 20 years I have owned the home.






 
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Old 02-01-23, 04:48 PM
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Since you have a 100 amp main breaker and #1 wires (assuming aluminum) your panel is likely rated for 100 amps.

A solar system does not add a load to a panel so I am not sure what your concerns are. 120 rule??
 
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Old 02-02-23, 12:30 AM
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Yes, for clarification please explain what a "120 rule" is so we can better follow you..
 
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Old 02-02-23, 01:09 PM
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NEC 120 rule for maximum amps in the load center:

https://www.kumukit.com/pdf/support/...ule.pdf?200707

If the load center is 100a serviced by a 100a main breaker, the 120 rule states you can add up to 20a of PV solar. 100*1.2 - 120a - 100a main = 20a for solar.

If the load center is 125a serviced by a 100a main breaker, the 120 rule states you can add up to 50a of PV solar. 125*1.2 = 150a - 100a main = 50a for solar.

Huge difference. The #1 wires coming up riser are copper. There is absolutely nothing anywhere to ID this panel. I have heard most of the centers used back then in this area are 125a, but the inspector won't care - needs proof.

 
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Old 02-02-23, 04:28 PM
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Could you extend the service (perhaps outside) and run it to a new 125 or larger main panel. Then convert your existing panel to a sub and feed it with a 100A breaker from the new main. Run the solar into the new main.

Of course that would require convincing the AHJ to approve leaving the current panel where it is.
 
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Old 02-03-23, 06:02 AM
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The label is supposed to be on the back of the door. Maybe it hasn't lasted the years.

The 1972 catalog might give you a start. It shows a 125 amp rating for main lug 12 space and larger outdoor panels. LW112DR is a likely part number for yours as it is the only traditional 100 amp main breaker outdoor panel listed.
 
 

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