Studying

hoody32

Member
Location
norwalk, ct
I plugged in some new data.
480 volt 3 phase
45* ambient temp
75* terminals
90* insulation

3 phase motors of: 1.5hp, 3hp, 3hp, 7.5hp

2-15 Kw, 480 volt 3 phase 3 wire continuous duty heaters.
 

mikeames

Senior Member
Location
Germantown MD
Occupation
Teacher - Electrician - 2017 NEC
You need to size your overloads? Do you know what size your overloads need to be? I don't see them in your calcs.

You Take the FLA not the the FLC. The service factor affects the overloads sizing as I indicate din the Article and section above.
 

hoody32

Member
Location
norwalk, ct
You need to size your overloads? Do you know what size your overloads need to be? I don't see them in your calcs.

You Take the FLA not the the FLC. The service factor affects the overloads sizing as I indicate din the Article and section above.
I’m not sure what size the overloads need to be. I was thinking I could just use the HP chart in the code book.
 

kwired

Electron manager
Location
NE Nebraska
I thought feeder calcs got rounded up as well but then I was taught that its rounded down because going above 250% is only allowed as an exception if breaker is tripping. My calcs are above in post #29
Transformer secondary conductors and feeder taps must have overcurrent protection equal or less than conductor ampacity as a general rule and anything over 800 amps as well. Otherwise nearly all other cases you must have conductor ampacity no less than calculated load (including 125% factor where necessary) but can use next standard higher overcurrent device.

General rule with motor circuits and thermal mag breakers is 2.5 times motor current and next standard size higher being maximum unless it won't allow the motor to start.
 

kwired

Electron manager
Location
NE Nebraska
Few questions. Where does the service factor come into play? To find the current of the heater how do we know we have to decide by 1.732? What table should I be using for the breaker?
Service factor of motor means nothing to branch circuit and feeder calculations. Usually will make a difference in selecting or setting motor overload protection though. Most motor overload units with a "set the dial" are set up for service factor of 1.15 and all you need to do is set to motor nameplate amps (which is why I also gave motor nameplate amps in the information of the setup) but if you have a motor with a service factor of 1.0 you usually need to multiply motor nameplate amps by .90 and use that value for setting overload. Same usually applies when selecting overload thermal elements from a selection table - use nameplate amps selection table for 1.15 SF but derate nameplate amps by .90 if a 1.0 SF. This information is usually on the selection table for thermal elements or in the instructions for electronic overloads.

The 1.732 factor is not a code thing, this comes from electrical theory studies. VA (which is same as watts for resistance loads) divided by volts gives you amps - but only on a DC or single phase AC circuit. With fully balanced three phase you still need to divide by square root of three to find amps as those amps get distributed across all three lines instead of just two.
 

mikeames

Senior Member
Location
Germantown MD
Occupation
Teacher - Electrician - 2017 NEC
I’m not sure what size the overloads need to be. I was thinking I could just use the HP chart in the code book.
No you don't use the book for overloads. That's the difference between FLC and FLA. Think of FLC as being " Full load current in the CODE" and think of FLA and "Full load amps ACTUAL" The overloads are sized on the actual not the code. The wire size and protection is sized on the FLC.
 

hoody32

Member
Location
norwalk, ct
I plugged in some new data.
480 volt 3 phase
45* ambient temp
75* terminals
90* insulation

3 phase motors of: 1.5hp, 3hp, 3hp, 7.5hp

2-15 Kw, 480 volt 3 phase 3 wire continuous duty heaters.
i want to figure out the feeder size and the overload protection. Is there more information needed to finish the problem? My answer was : #4 aluminum on a 90 amp breaker.
 

mikeames

Senior Member
Location
Germantown MD
Occupation
Teacher - Electrician - 2017 NEC
You need to know the FLA of those motors to calc overloads, so you would need to look at the name plate rating.
 

kwired

Electron manager
Location
NE Nebraska
i want to figure out the feeder size and the overload protection. Is there more information needed to finish the problem? My answer was : #4 aluminum on a 90 amp breaker.
You need to know nameplate current rating to determine motor overloads.

For 75C terminations you must have minimum of 3 AWG aluminum conductor. I had a minimum feeder conductor ampacity of 71.35 amps.

With 90 C conductor and ampacity adjustment for 45 degrees (C?) aambient 3 AWG is still acceptable.
 
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