Re: Electrical Safety

Posted by John T on November 29, 2019 at 09:40:37 from (174.207.18.143):

In Reply to: Electrical Safety posted by David g on November 29, 2019 at 06:21:50:

David, great post, AS USUAL when any electrical or legal question is posed:

A) There is "some" (NOT saying all mind you don't anyone have a calf) misinformation and misunderstanding out there.

B) There are a TON of opinions, some lay some professional, some ignorant some educated, some right some wrong.

That being said FWIW here's my rusty recall based on the NEC as it was way back when I practiced power distribution but no warranty, things change and my memory cells are fading lol. I was also an electrical engineer for Century Electric Motor Company but that was like 50 years ago

1) When you design a branch circuit you FIRST compute the "maximum continuous current"
2) You size the conductors to have a MINIMUM ampacity of 125% of the MCC
3) You provide and size overcurrent protection TO PROTECT THE WIRE which was sized per the above !!!!!!!!!
4) The Motor (wire already protected per the above) itself is protected by internal thermal overload protection or external thermal overload devices such as found in motor starters or combination starters etc aka "heaters" or "thermals" properly sized to avoid excess current and overheating and motor damage.

OVERCURRENT PROTECTION

When a motor starts there's an initial high surge current that may be 3 to 6 times as much as full load current and a typical general purpose Thermal Magnetic circuit breaker might therefore trip SOOOOOOOOOOOO when I practiced the code allowed the use of a circuit breaker as much as 175% above the value as computed above to allow the motor to start. Their reasoning as I recall was based on the fact if a dead short were present the "magnetic" portion of the thermal magnetic breaker would still cause it to trip PLUS the motors thermal protection device (as above) would still protect the motor.

HOWEVER other methods were available such as instead of using circuit breakers use Dual Element Time Delay Fuses orrrrrrrrrrr I think they made HACR circuit breakers which served a similar purpose as Dual Element Time delay Fuses to allow the motor to start and not trip.

Sooo you compute the maximum continuous current,,,,,,,,,,size the conductors to have a minimum ampacity of 125%,,,,,,,,,size the overcurrent protection to protect THE WIRE,,,,,,,,,,,Make exception to allow motor to start (Time delay fuses or bigger or specialized breakers) ,,,,,,,,,provide motor thermal overload protection,,,,,,,,,,

DISCLAIMER Im long retired n rusty so if any professional electrical engineers or electricians can add to this or correct it PLEASE DO SO so we might all learn.

NOTE Anytime fire or life safety is concerned consult local authority or trained professional electricians or engineers and DO NOT risk your home or life on opinions (lay or professional mine included) posted here, too much is at risk !!!!

John T BSEE,JD Long retired n rusty power distribution design engineer so noooooooo warranty don't bet your life or home on it

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