Check my math and design please.

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Strahan

Senior Member
Location
Watsontown, PA
Scenario: Installing a motor control panel housing the controls for 3 pumps rated at 30hp each.

Each pump will be protected with an inverse time breaker by looking at 430.250 it is determined to use 40amp in calculations so:
pump 1 40 x 250%=100A
pump 2 40 x 250%=100A
pump 3 40 x 250%=100A
So each will be using a 100A inverse time breaker for protection. The 250% came from 430.52.

Next main disconnect located in panel will also be an inverse time breaker so;
100A + 40A + 40 = 180A
This is not a standard size so next size up rule applies giving me a 200A breaker.
Feeder conductor size: 125% of full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors so:
40A x 1.25 = 50 + 40 + 40 = 130Amps
Using table 310.16 75deg column size 1AWG copper.
Now for my question how or where do I find how to determine the size of breaker supplying the feeder conductors? This run will be approx. 200' from the service and I'm not exactly sure how to size the breaker at the service.
Thanks for your help!!!:)
 

Strahan

Senior Member
Location
Watsontown, PA
Ok I believe I found a problem. Per 430.62 the feeder protective device must not be larger than the highest motor protective device rating plus the FLC of the other motors so 200amp main breaker will not work. I will size this to175A and I believe the breaker back at the service could also be sized to 175A as well but I'm not sure can you guys help? And if it can be 175 then I'm a little confused because the wire will only have a amperage rating of 130a.
 

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
You didn't mention anything about overload protection. Will this be in starters supplied by others?
 

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
The 175A breaker is the feeders short circuit/ground fault protection only. Feeder overload protection is supplied by starters/VFD's.
 

Rockyd

Senior Member
Location
Nevada
Occupation
Retired after 40 years as an electrician.
Strahan,

Mike's newsletters are a great place to mine information - such as explanations of proper math for feeder protection.

Check these two letters out -

Part 1

Part 2

The letters may be old, but the numbers, and conceptuality, are still right.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
One question I have is why do you use the maximum rating for the IT breaker (250%)? Did you actually calculate the available short circuit current or did you just decide to go with max value allowed?
 
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