MWBC with different ratings

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BUYER246

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Location
Lubbock, TX
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Residential Wireman
I have a customer with multiple storage units. He wants me to install a 20 amp circuit for lighting and a convenience outlet and a 30 amp 120 volt RV outlet in the unit. It is over 225' from the panel. Can I run a hot and neutral #8 THHN for the 30 amp and another single #10 THHN for the 20 amp and share the #8 neutral? The EMT will be the ECG. I would use handle ties between the single pole 20 and single pole 30 breakers.
 
Since there are multiple storage units, I have two observations to offer:
  • The good news is that I believe you could run three of your MWBCs in the same conduit from the panel to a junction box near one unit, and continue the run to the two next units, and not have a problem with derating for more than three current-carrying conductors in the same conduit. For example, with three MWBCs in the same conduit, the #8 ampacity would be 70% of 55, or 38.5 amps, and this is higher than 30, and the #10 ampacity would be 70% of 40, or 28 amps, and this is higher than 20. So you can still use your 30 and 20 amp breakers.
  • The bad news is that I believe you are undersizing the wires. Assuming an 80% load on either the 20 amp circuit or the 30 amp circuit, I calculate a voltage drop of 7%.
 
Thanks, Charlie. I will increase to #6 & #8 respectively. Never know what people will decide to put in the units later on down the road.
 
BUYER246 said:
Thanks, Charlie. I will increase to #6 & #8 respectively. Never know what people will decide to put in the units later on down the road.
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That will get you down to 4 or 5%, assuming an 80% loading. But as you pointed out, it all depends on what the owner/tenant plugs into the receptacle.

 
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