[Washington1]

Anybody want to take a crack at this?
What is the flc of a motor with a nameplate rating:
-1 ph
-240v
-Fla=16.5
- 3 hp

[tkrussell]

If you mean for branch circuit wiring and device and OCPD sizing, then must use Table 430.148 rating of 17 amps.

For overload protection devices then use the nameplate rating of FLA 16.5 amps.

[Washington1]

I guess I'm bored!

Am I violating site rules :)

Any how, is that your last and final answer (any one accept TK)? ;)

[ballengerb1]

None of us should get any closer to this one. No one except Kevin can answer this.

[Washington1]

If you mean for branch circuit wiring and device and OCPD sizing

Branch circuit conductor-->Sure do!

[Washington1]

Outside of the above being a requiremnt of 21.25A min requirement-- (17)(1.25%)

Tk,

If this unit was located in an environment, where it was considered continuous. Have you noticed anything in the NEC that says we would have to multiple an additional 1.25%?

[tkrussell]

No, this 17 * 1.25 would be continuous rating and set the minimum rating of a OCPD. But depending on the type of OCPD and motor, refer to Table 430.52 for max ratings. Some of them may be surprising.

For example, a single phase motor and an instantaneous CB the max rating is 800%!

[Washington1]

Exactly! Excellent feed back!

[tkrussell]

Ok, so your bored huh?

I have one for you.

Size the disconnect, branch circuit conductors, and overload devices, for a 25 HP 480 volt 3 phase fire pump.

Key word is "fire pump".

This ought to blow some minds

[Washington1]

Wow!
Talk about a confusing section. Seeing that this is something I will never install, I have an excuse for lack of knowledge. :) Yet, here it goes:

Based on the information:
-Conductor size=#8 CU per T.310.16 and T.430.250 (34)(125%)=42.5A >assuming a 75 degee termination<Also See 695.6(c)(2) <-assuming no VD
-Overload Protection=None required per NEC 695.6(d)--yet short circuit protection required for branch and feeders. Based on the information you provided. A 25HP 480v 3 ph max lock-rotor current disconnect=365 amps per 430.257(b)---so 400amps per 240.6

:) or :(

[tkrussell]

So your saying a #8 wire on a 400 amp breaker and disconnect?

Sounds right to me.

[Washington1]

Where is my prize?

I'm sure this will have some people wondering... :)

Good question!

[tkrussell]

Here you go...

I am surprised I did not get any comments about a #8 wire on a 400 amp breaker.

[Washington1]

I am surprised I did not get any comments about a #8 wire on a 400 amp breaker.

I was surprised as well. I was waiting on someone to ask how this was allowed. I guess they are out shopping or don't care! :)

[Cobraguy]

I know better than to question either of you! And I also know there are some strange exceptions out there for things like you exampled. So... in this case... even though I don't understand completely why it is what it is, I accept your answer!

Now... tell us why! A good teacher teaches... they just don't give tests! :)

[tkrussell]

This is no test, just a discussion between a couple of tradesmen, one that may not have had a need to look for this situation. Better to be aware of something now to be prepared in the future if this situation ever arises.

Washington did well, hit the nail.

And to illustrate that there are exceptions, albeit, this is probably one of the most extreme.

A fire pump is a critical unit needed for several reasons, usually when there is not enough pressure or capacity in the normal water system to provide both to fill a sprinkler system, and to continue supplying water at the point of combustion.

Even thou there is mandatory testing and maintenance performed on the pump and system, it has been determined that due to Murphy's Law, the damn thing may have a problem at the time of a fire.

So provisions are in place to allow the motor to start, or allow it to try to start, and keep running, at any cost.

So all breakers, fuses, switching, transformers, etc, must be sized to handle the locked rotor amps of that motor. As you can see from the example, a 25 HP motor will have 365 amps of locked rotor current. The distribution system will be sized to handle this current.

The wire, while small to handle the normal starting and running current, is known to handle high current for a short period, and is consumable, is only sized to be 125% of the running current.

It is not practical to size wire to handle the LRA, so FLA is used.

If the pump has a problem, this will allow the pump to run, at any cost, even if it burns up. Suppressing the fire is the goal.

[Washington1]

This is no test, just a discussion between a couple of tradesmen, one that may not have had a need to look for this situation. Better to be aware of something now to be prepared in the future if this situation ever arises.

Washington did well, hit the nail.

And to illustrate that there are exceptions, albeit, this is probably one of the most extreme.

A fire pump is a critical unit needed for several reasons, usually when there is not enough pressure or capacity in the normal water system to provide both to fill a sprinkler system, and to continue supplying water at the point of combustion.

Even thou there is mandatory testing and maintenance performed on the pump and system, it has been determined that due to Murphy's Law, the damn thing may have a problem at the time of a fire.

So provisions are in place to allow the motor to start, or allow it to try to start, and keep running, at any cost.

So all breakers, fuses, switching, transformers, etc, must be sized to handle the locked rotor amps of that motor. As you can see from the example, a 25 HP motor will have 365 amps of locked rotor current. The distribution system will be sized to handle this current.

The wire, while small to handle the normal starting and running current, is known to handle high current for a short period of time, and is consumable, is only sized to be 125% of the running current.

It is not practical to size wire to handle the LRA, so FLA is used.

If the pump has a problem, this will allow the pump to run, at any cost, even if it burns up. Suppressing the fire is the goal.

Couldn't have said it better!

[Cobraguy]

And it makes perfect sense to me. Thanks guys!

[Washington1]

Ok, I just proof read my answer and found a mistake (typo) :(

It's actually 183A, and not 400A:(

Yet, still amazing! :)

[tkrussell]

Damn, I did not double check that number, but still the answer is amazing.

Again, knowing the code can be your friend. I have met electricians that use wire rated full boat. What a waste of copper and labor, and money.