heiko.stugg
2011-02-24, 02:10 PM
If I am not mistaken, REVIT MEP is sizing the branch circuit conductors based upon the overcurrent protection device 'Rating' and the 'Wire Type' selected for the individual circuit chosen under the 'Electrical Circuit' tab. The 'Rating' is the essentially, the minimum ampacity found under the 'Wire Size' tables under the electrical settings menu. This selection works well as long as you have a receptacle branch circuit, but doesn't work for motor circuits selection.
The National Electrical Code allows the minimum motor conductor size to be selected based upon 125% of the motor full load amps, not the 'Rating' or OCPD. For example, If I have a 20 Horsepower, 460V, 3 Phase Motor served from a motor control center, the OCPD at the MCC, in my case an MCP, can be 50 Amps. Based upon the current selection process of REVIT, the wire size selected will be #6 AWG phase conductor; based upon the FLA of a 20 HP Motor, 27 FLA * 1.25 = 33.75 Amps; #8 AWG phase conductor would be adequate. For low HP motors, this difference in copper is that much, but for larger HP motors, the cost difference can quickly escalate.
Now I have tried to add a 'wire size' call "THWN-MTR(MCP)" and modify it such that if I select for example a 30A MCP that it selects the appropraite conductor size basd upon the 'Rating', but that doesn't work because you could have a 30A MCP for a 10 HP Motor that uses #12 AWG conductor and a 30A MCP for 15 HP that uses #10 AWG conductor; the REVIT MEP "wire size' table prevents you from having duplicate conductor sizes; otherwise it would work quite easily. Unless anybody knows another work around; I am not able to use this portion of the design tool for motor conductor selection and will have to go back to conventional design and drafting means.
I would hope that this is something that could be fixed in future revisions or versions of REVIT MEP.
The National Electrical Code allows the minimum motor conductor size to be selected based upon 125% of the motor full load amps, not the 'Rating' or OCPD. For example, If I have a 20 Horsepower, 460V, 3 Phase Motor served from a motor control center, the OCPD at the MCC, in my case an MCP, can be 50 Amps. Based upon the current selection process of REVIT, the wire size selected will be #6 AWG phase conductor; based upon the FLA of a 20 HP Motor, 27 FLA * 1.25 = 33.75 Amps; #8 AWG phase conductor would be adequate. For low HP motors, this difference in copper is that much, but for larger HP motors, the cost difference can quickly escalate.
Now I have tried to add a 'wire size' call "THWN-MTR(MCP)" and modify it such that if I select for example a 30A MCP that it selects the appropraite conductor size basd upon the 'Rating', but that doesn't work because you could have a 30A MCP for a 10 HP Motor that uses #12 AWG conductor and a 30A MCP for 15 HP that uses #10 AWG conductor; the REVIT MEP "wire size' table prevents you from having duplicate conductor sizes; otherwise it would work quite easily. Unless anybody knows another work around; I am not able to use this portion of the design tool for motor conductor selection and will have to go back to conventional design and drafting means.
I would hope that this is something that could be fixed in future revisions or versions of REVIT MEP.