Guest House Electrical Wiring

Question: I own a 1960s home with a 100 amp service panel. Behind the house is a detached guest house. There is a 40 amp 220 feed going from the main panel to a sub panel outside the guest house. They used number 8 wire in metal conduit through the attic going into the ground. © By: Dave Rongey

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Electric Service for a Guest House

Question:
I own a 1960s home with a 100 amp service panel.
Behind the house is a detached guest house.
There is a 40 amp 220 feed going from the main panel to a sub panel outside the guest house.
They used number 8 wire in metal conduit through the attic going into the ground.

At one time the guest house sub panel had a spa hooked up to it but is long since been removed.
It was a old 70s spa that used swimming pool pump and blower and sucked down a lot of power.
Only a lighting and outlet circuit is required for the guest house now as cooking and heating are all gas.

A 1.5 inch metal pipe runs down from the attic at a corner of my house into a access box and then goes underground to the guest house.
This side of the main house has two bedrooms, and they each require more power then the current circuits allow.
They both have small window ACs in them, one uses max of 5 amp and the other 10 amp which I have measured.

My Question:
Can I go into the 40 amp junction box with EMT, splice in 12 gauge wire to the 8 gauge = hot, neutral and green ground and then surface mount outside my wall coming in to my bedrooms with a single outlet 20 amp GFI in each bedroom to plug the two ACs into?

Can I safely do this just by splicing into the 40 amp 8 gauge wire or am I going to need to put another sub panel.

Thanks

Additional Comments: good resource

Electrical Question from Winterlight about Electrical Wiring

Received from Winterlight a Contractor in Santa Barbara, California

Dave’s Reply:
Thanks for your electrical question.

Guest House Electric Power

NO – Do Not Splice the Wiring as you have described – This would be very unsafe.

The right way would be to install a NEMA 3R sub-panel on the wall where the existing junction box is where circuit breakers could provide power to the bedroom air conditioner units or any load.
The correct wire type and size must be used along with circuit breaker protection.

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