Fuse sizing of a disconnect

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t_ray7

Member
Location
US
Occupation
Designer
If you have a fused disconnect that is acting as a branch overcurrent device, how do you size the fuse based on all of the loads involved? I have a control panel where we have a few motor and non-motor loads in a single branch circuit. I have to size the main panel disconnect fuse size but this would be sizing as a branch overcurrent device, I think.

Consider the following as components of a single branch circuit, the total load is 134A
  • 1 100HP VFD, 124FLA - there is no branch circuit protection provided, max per documentation is 200A fuses (no minimums listed), and this I'm hoping can be covered by the main disconnect
  • 2 Motors, 2HP, 3.4FLA, each protected by an MPCB set at 3.4A
  • 1 1000VA 480x120 single phase transformer protected by its own 4A breaker, 2.1FLA
  • 1 480Vac to 24Vdc power supply protected by its own 10A breaker, 0.9FLA
Can I protect this circuit with 200A fuses and still be legal? The motor group section in NEC and UL 508A leave me with a lot of questions on how to apply each scenario. I'm thinking since each individual circuit other than the VFD is protected and sized correctly, then there isn't a problem with it.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
If you have a fused disconnect that is acting as a branch overcurrent device, how do you size the fuse based on all of the loads involved? I have a control panel where we have a few motor and non-motor loads in a single branch circuit. I have to size the main panel disconnect fuse size but this would be sizing as a branch overcurrent device, I think.

Consider the following as components of a single branch circuit, the total load is 134A
  • 1 100HP VFD, 124FLA - there is no branch circuit protection provided, max per documentation is 200A fuses (no minimums listed), and this I'm hoping can be covered by the main disconnect
  • 2 Motors, 2HP, 3.4FLA, each protected by an MPCB set at 3.4A
  • 1 1000VA 480x120 single phase transformer protected by its own 4A breaker, 2.1FLA
  • 1 480Vac to 24Vdc power supply protected by its own 10A breaker, 0.9FLA
Can I protect this circuit with 200A fuses and still be legal? The motor group section in NEC and UL 508A leave me with a lot of questions on how to apply each scenario. I'm thinking since each individual circuit other than the VFD is protected and sized correctly, then there isn't a problem with it.
You need to be aware of the tap rules here.
 

t_ray7

Member
Location
US
Occupation
Designer
where did you come up with #6
Using 31.4.1 (b), it looks like the tap conductors have to comply with 31.4.3, which says that they can't be less than 1/3rd the ampacity of the branch circuit conductor, which in my case is calculated to be 2/0 (163A). So 1/3rd of that is 54A, or 6 AWG. Nothing in this design is marked suitable as tap conductor protection so I can't apply the 1/10th rating.
 

t_ray7

Member
Location
US
Occupation
Designer
Looks like that's the new problem. The largest size they can accept is 10 AWG.
 
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