Neutral size for 200A feeders to residential shop, detached structure, 3/0 CU ungrounded conductors?

[Apprenti]

Neutral size for 200A feeders to residential shop, detached structure, 3/0 CU ungrounded conductors?

400A Meter Base -> 3/0 CU ungrounded conductors from MB to 200A disconnect -> 3/0 CU ungrounded conductors from 200A disco to underground to 200A Main Breaker panel in shop.

Please and thank you!

 

[roger]

What do you think and how did you come about your answer?

 

[Apprenti]

What do you think and how did you come about your answer?

Table 250.102(C)(1), 2/0 or 3/0 CU, # 4 CU grounded conductor?

 

[suemarkp]

That would be the absolute minimum size if there was a minimal unbalanced load. But there is more to it than that. What is the load in the shop (is there a load calc sheet?), and are any of those loads 240V only with no unbalanced (120V) component? Are there any loads in that shop that provide a specific neutral reduction factor such as an electric range or clothes dryer?

 

[Apprenti]

Oof, I'll have to get back to you on a lot of that.

No load Calc, minimal loads, small square footage. All equipment 120V, balanced load. A drill press here, belt sander there. Never on at the same time. Maybe a 30A or 50A two pole for a welder.

Two 20A general receptacle circuits. 15A lighting, one circuit, corded LED's into receptacles, four to six fixtures.

Thank you for the constructive replies!

 

[suemarkp]

120V loads can't be considered balanced because you never know which ones will be on. It would be possible (but highly unlikely) that all 120V loads on phase A are on and all on phase B are off. That puts the same current on the feed to A and the neutral. A welder doesn't connect to the neutral. SO if it contributed 30A to the load calc, you could subtract 30A off the neutral current.

Typically people install a neutral that is either the same size as the ungrounded conductors or maybe a size or two smaller. Full size reduces voltage drop and allows for all 120V circuits in the panel. If this panel was supplying something like a bunch of 240V heaters and one or two lighting and receptacle circuits, you could consider dropping the neutral way down. It is a way to save cost, but hampers future plans if they want to repurpose into more 120V loads and less 240V ones. If much of the 200A panel is not yet used (e.g. a load calc of 100A with a feeder rating of 200A), then it would be a better design to assume all future growth is unbalanced 120V loads.

 

[Apprenti]

120V loads can't be considered balanced because you never know which ones will be on. It would be possible (but highly unlikely) that all 120V loads on phase A are on and all on phase B are off. That puts the same current on the feed to A and the neutral. A welder doesn't connect to the neutral. SO if it contributed 30A to the load calc, you could subtract 30A off the neutral current.

Typically people install a neutral that is either the same size as the ungrounded conductors or maybe a size or two smaller. Full size reduces voltage drop and allows for all 120V circuits in the panel. If this panel was supplying something like a bunch of 240V heaters and one or two lighting and receptacle circuits, you could consider dropping the neutral way down. It is a way to save cost, but hampers future plans if they want to repurpose into more 120V loads and less 240V ones. If much of the 200A panel is not yet used (e.g. a load calc of 100A with a feeder rating of 200A), then it would be a better design to assume all future growth is unbalanced 120V loads.

Thank you!!!

 

[electrofelon]

Most people just use a two size reduced neutral and call it a day. It's not in the code, but is near universally accepted for single phase.