Alwayslearningelec
Senior Member
- Location
- NJ
- Occupation
- Estimator
What is the size of a motor disconnect switch based off of? How would one calculate? Is it the breaker size feeding it?
Motor Disconnect Switch
- Thread starter Alwayslearningelec
- Start date
- Location
- New Jersey
- Occupation
- Journeyman Electrician
How big is the motor? Larger disconnect switches will have a HP rating.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Has anyone ever seen a disconnect switch with a HP rating that was actually less than the 115% that Augie mentioned?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Not necessarily. As mentioned 115% is the general rule. Inverse time breakers can be 250% of FLA to allow motor starting.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
No, I have never seen that.
Which begs the question as to why the NEC allows it in the exception to 430.110(A)! I imagine that at one point in the past, someone must have made a "HP rated switch" that did not meet the 115% rule, i.e. it was rated somewhere between 100% and 115% of the specific motor FLC. Most likely some dedicated OEM part where the manufacturer was shaving pennies on high volume production.
Side note: that 115% rule does NOT apply below 2HP (430.109(C)). If you want to use a "general purpose" snap switch that does not have a specific HP rating. In that case, the switch must be 2x the motor FLA, or if identified as "only for use on AC", it is 125% (inverse of 80% rule).
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Isn't there a top end for HP switches? I haven't looked but in my mind bolted pressure switches didn't have HP ratings nor did MV switches.
TwoBlocked
Senior Member
- Location
- Bradford County, PA
- Occupation
- Industrial Electrician
I am used to looking at the voltage and HP rating of the disconnect . 430.110 gives more info.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I asked because I seem to remember a scenario in the field years ago where the HP rating of the switch was less than the 115%. If I had to guess the numbers were very close as far as being just under the 115%.
ptonsparky
Tom
- Occupation
- EC - retired
Yes, I remember a similar situation. IIRC, it was a 30 Amp disconnect. I had to move up to a 60. IDR if it was 1 or 3 phase.