disconnecting means test question

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mannyb

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Electrician
Question on Prep Exam

Example test Question. When an inverse time circuit breaker is used as a disconnecting means only for a 7.5 HP Single phase 240v ac motor the CB must have a standard amperage rating of at least?

My answer is 50amps

7.5 hp fla =40a table 430.248
40a x 115% = 46a next highest 50a

SO you size disconnect by 115% per 430.110 separate from MCP for motor than can be adjusted for higher settings if needed ? I am just wondering if the disconnect breaker would trip before the mcp if sized this way. I always assuned you used 430.52 to size OCP.
 
mannyb said:
Question on Prep Exam

Example test Question. When an inverse time circuit breaker is used as a disconnecting means only for a 7.5 HP Single phase 240v ac motor the CB must have a standard amperage rating of at least?

My answer is 50amps

7.5 hp fla =40a table 430.248
40a x 115% = 46a next highest 50a

SO you size disconnect by 115% per 430.110 separate from MCP for motor than can be adjusted for higher settings if needed ? I am just wondering if the disconnect breaker would trip before the mcp if sized this way. I always assuned you used 430.52 to size OCP.
Click to expand...

Your answer is correct. That is the minimum size for a disconnecting means.

430.52 gives the max for GFSC protection.

What is MCP? And how are they two breakers involved? You lost me there.

Similar thread that explains better than me.

http://forums.mikeholt.com/showthread.php?t=187443
 
jumper said:
Your answer is correct. That is the minimum size for a disconnecting means.

430.52 gives the max for GFSC protection.

What is MCP? And how are they two breakers involved? You lost me there.

Similar thread that explains better than me.

http://forums.mikeholt.com/showthread.php?t=187443
Click to expand...

I am trying to understand is sizing for disconnecting means and sizing a MCPs for motor starter basically the same? Would I size the feeder breaker at 115% then size the motor starter using 430.52. Or like you may already have explain I use 430.52 for MAX protection? Thanks for all replies
 
mannyb said:
I am trying to understand is sizing for disconnecting means and sizing a MCPs for motor starter basically the same? Would I size the feeder breaker at 115% then size the motor starter using 430.52. Or like you may already have explain I use 430.52 for MAX protection? Thanks for all replies
Click to expand...

Errr...gimme a few, more like tomorrow.

I will respond, or hopefully someone smarter then me first; you bring up multiple issues intermixed.
 
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