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Introduction

I am doing some research for a project. I have an unfinished detached two story garage. The second story space 400 square feet. Inside the house is a 200 AMP main panel with a feeder to the garage. The detached garage has a 125 AMP sub-panel. The breakers in the main panel and sub-panel are 50 AMPS.

I believe the wiring from the main panel to the sub-panel transitions based on where it is located. For example, part of the feeder line is buried and pops into the sub-panel. The marking on this wiring are:

AWG (33.62 mm2) AA-8000 AL Type use-2/RHH/RHW-2 600V SUN RES XLPE

Here are my questions

  1. Even though the breakers are sized at 50 AMPS, how much power can this feeder line supply safely?

  2. Will the current configuration provide the power I need? The following is a list of devices/appliances I think I need for the space.

    • Hot Water Heater (30 AMPS 2-pole)
    • HVAC (outside compressor) (?)
    • HVAC (inside air-handler) (?)
    • HVAC (electric heat) (?)
    • Refrigerator (20 AMPS)
    • Microwave (20 AMPS)
    • Washer (20 AMPS)
    • Dryer (30 AMPS 2-pole)
    • Two Electric Cars (?)

I believe electric cars will become more popular in the near future. So I want to plan for enough power to recharge two electric cars. For example, Tesla has two types of wall chargers:

  • Standard: 60-amps (48A draw)
  • High Amperage: 90 amps (72A draw)
  1. The sub-panel is 125 AMP. Can I just swap out the 125 AMP sup-panel with a larger panel like 200 AMP for the moment until I can upgrade the service? The reason is I want to wire the sub-panel one time. If I find out that I need more power in the future, I do not want to have to reinstall and rewire a sub-panel.

Based on the table below this 33.62 mm2 converts to 2 AWG Aluminum wire. Based on the second table below, this shows an allowable Ampacity of 75 amps @ 60 degrees C. I presume that the cable is rated for 125% of the estimated need (50 * 1.25% = 62.5 amps), rounded up to 75 amps.

I believe the full length of the feeder line is less than 80 feet.

Images

200 AMP Main Panel. The feeder for the garage is in the lower left side with the 50 AMP breaker. The feeder lines are red and black. Main Panel

125 AMP Sub-Panel. I believe the feeder lines come in as Aluminum and then I think they transition to copper. Garage Sub-Panel

These are the markings on the feeder line in the garage subpanel. Markings on feeder line Markings on feeder line 2

Converts mm2 to AWG. Conversion Table

NEC Allowable Ampacities NEC Allowable Ampacities

  • Your Sub Panel is a Eaton Main Lug, but since it is in an detached structure from the main Panel, they used a 50 Amp Breaker to power the bus. You mention a lot of what ifs... you were questioning upsizing Sub panel to a larger size, what is the size/ accessibility of the feeders going to the sub-panel....they would need to be changed for any but a small boost. If you are worried about Sub Panel size. put in a much larger panel now, before filling. I would put in a Main breaker panel, not a main lug panel, decide how much you are realistically going to upgrade, then go for it. – Limo DRIVER Jul 3 '18 at 19:23
  • Ran out of space. By accessibility of the feeders, I mean the conduit they are in, string overhead, how much would you have to have dug up to replace them to allow 200 Amps. Sometime, I find myself applying too much future proofing, costing extra time and materials to guarantee I can fit any and all future wants with what I am doing today. IT I will put in a sub panel capable of 200 Amps. because I want to get a welder sometime in the future, but have never ended up getting one yet. – Limo DRIVER Jul 3 '18 at 19:31
  • I take it your project is installing an electric car charger (or two)? Can you get us how much current the A/C compressor and the electric heat pull? – ThreePhaseEel Jul 3 '18 at 21:25
  • Also, how many circuits in your main panel feed kitchen counter receptacles? – ThreePhaseEel Jul 3 '18 at 21:26
  • AWG is not a type of wire. It is a unit describing a previous figure, like mm or gallons or volts. You should add the figure. From the mm2 I think it is "2". – Harper Jul 4 '18 at 3:13
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Based on the information you posted on question #2. I refer you to NEC Article 100 Definitions. In particular "Dwelling Unit" A single unit, providing complete and independent living facilities for one or more persons, including permanent provisions for living, sleeping, cooking (microwave), and sanitation. Point being your description of what you want to install qualifies it as a dwelling unit.

NEC Article 230.79 (C) will tell you that a one family dwelling minimum service size would be a 100A minimum. I would say that it would meet your requirements except for the charging stations. To be safe you would probably want to calculate them in at 100% of the load (2 @ 48A draw = 96A). So that would give us a 200A panel in this garage area.

Using that information I would suggest you look into providing a separate 200A service for you detached garage.

This would also fix a few of the questions I would have about your feed to the garage from your main panel.

Such as:

I know you mentioned the USE-2 conductor size but your main shows a different cable being connected to the 50A breaker you designated as the garage feed. It is the amperage size of that conductor that you need to be concerned about.

The sub-panel is connected to this feed by a 50A breaker and right there in front is a nice nick in the use-2.

And the split bolt connectors tying in the use-2 to the conductors attached to the 50A breaker. Are they AL/CU rated?

Finally, from a design standpoint, I never liked the idea of running about half of the load through a load center into another panel. If nothing else set two main breakers at the meter and run a separate feed to the panel in the garage. This prevents over heating and if that leads to a failure you now have to replace you main panel while your main house is without power. In other words diversify your loads.

So you said you were doing some research, I hope these are a few ideas to kick around. Good luck.

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