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 (retired)
How big is the motor? Larger disconnect switches will have a HP rating.
- Location
- New Jersey
- Occupation
- Journeyman Electrician (retired)
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 (retired)
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.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I run into where fused switch needs to be next frame size because of the fuses necessary to hold during motor starting, where otherwise unfused switch from same product line can be the lower amp rated switch. Example 20 HP @ 480 volt three phase can use 30 amp Square D unfused disconnect as it is rated 20HP at that voltage but if you need fused switch you have to go to the 60 amp switch as you will need 45 or 50 amp fuses if it is started across the line. Their 30 amp fused switch has the same switch but the 30 amp max fuse is a limiting factor and those are only marked 15 HP @ 480 volts.
JoeStillman
Senior Member
- Location
- West Chester, PA
- Occupation
- Electrical PE
Don't forget that non-fused disconnects are only rated for 10 kAIC, but fused disconnects can get you to 100 kAIC or more. If you have high available fault current, you might need to go with a fused disconnect and maybe a larger size as kwired says.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Yes, good point.
But in many cases, the fuses can be upstream... and in a few cases, the NF disconnect might even be series listed with a circuit breaker (of the same manufacturer, Siemens does this for sure and I believe Square D does too). So for instance the NF disconnect stand-alone is only good for 10kA, but if ANYWHERE upstream there is a Class R, T, L or J fuse, it is series rated at 100kA. Breaker series ratings (if any) are typically less, but in the Siemens referenced sets, a 480V NF disconnect can be series rated at 65kA with a specific Siemens breaker, and 65kA is pretty good for most installations.