BREAKER SIZE TO LOAD SIZE

[ohmhead]

Question we have a factory water pump unit it has three motors 7.5 hp each the nameplate on unit shows 37.5 fla total load this is a 480 v 3ph packaged unit .

The nameplate shows a 60 A required breaker. Can I put this on a 45 amp breaker in panel if UL listed for 100% operation in encloser .

If the unit has a duty cycle possible or can a 45 amp breaker be rated at 100 % possible what could be a rule or what might be a way to use that 45 amp breaker in this case .

Breaker rated at 80 % or 100% does the code state this what article ?

My wire size to this unit is 6 copper with 8 ground .

 

[augie47]

I would think you would be o.k. on a 45 amp breaker.
NEC (430.62) would call for a 45 amp feeder over-current (for (3) 7.5 hp excluding any control load)
With your #6 you would be fine on a 60 especially with the factory input, but I see not problem if you want to use a 45.

 

[ohmhead]

I would think you would be o.k. on a 45 amp breaker.
NEC (430.62) would call for a 45 amp feeder over-current (for (3) 7.5 hp excluding any control load)
With your #6 you would be fine on a 60 especially with the factory input, but I see not problem if you want to use a 45.[/oes the breaker TE]



Augie does the breaker used need to be 100 % rated at 37.5 which is over the 80% load or is that just for plug and cord branch per code .

 

[augie47]

Are you cord & plug connecting the unit ?
Keep in mind we are discussing a feeder so 210 is not applicable.

The 45 amp breaker meets the requirements of 430.62 which, IMO, is the applicable Article.
IF the load was continuous (all three motors loaded fully for 3 hours or more), the 45 amp breaker would still meet the requirement of 215.3 for a feeder.

 

[Smart $]

The minimum SC/GFP under Code, i.e. disregarding nameplate, is 3.25*11A + control equipment load. 37.5A is likely pretty accurate, without control load being given. There is no difference between the minimum SC/GFP whether using a 100%-rated breaker or not. Motors do not fall into the categories of continuous or non-continuous, but for all other purposes are totaled with the non-continuous loads when determining breaker ratings (with 125% for largest motor included in the total).

 

[Smart $]

PS: What is really be in question using a 45A-rated breaker, is whether the pump motors will start without tripping the breaker. Perhaps they will if interlocked such that only one motor starts at a time. If all three can start at the same time, there's a good chance a 45A breaker will trip on that occasion.

 

[iceworm]

... The nameplate shows a 60 A required breaker. ...

.... Can I put this on a 45 amp breaker in panel if UL listed for 100% operation in encloser . ...

I'm not seeing a code issue.. This is a design issue. And the code is not a design guide. Using the code to justify design decisions generally does not work well.

I'm guessing you don't have an AHJ looking over your shoulder. If you did, the AHJ would look at the nameplate and say, "You don't have a 60A CB". And that's pretty well the end of that discussion.

Or you could justify your design decision:

We don't have a 60A CB available, but we do have a 100% rated 45A - which would be the same as 56A non-100% rated.
And if we calculate the required feeder size per NEC 430, a 45A CB is acceptable.
I consider the 100% rated 45A CB exceptable for this installation.​

So, If no AHJ:

Declare yourself the Engineer of Record.
Is your proposed install unsafe? Nope, it's safe. It won't start a fire if circuit/equipment fails.
Will it work per customers expectations/specification? Maybe. You, as the EoR get to decide.​

However, consider the Worm's First Law of Installation:

"If you don't install it per the mfg direction, don't complain if it does not work per the mfg specifications."​

Personally - I think you will be okay. But that is internet engineering from my side of your monitor - and worth every bit of what you paid.

ice

 

[ohmhead]

Well I enjoyed the last post the best but thanks for every ones response by the way we do have a inspector on site but some times we like to engineer our work we don't ask the inspector what to do hes there to inspect and his time is a special time . Were not really designing this plans are design by engineer then two years later equipment comes to job site its not what the engineer had planned but thats normally on every project and normal in this trade .

Theres a note on the contract drawings that states to verify crt size ampacity voltage phase with anything that's new and installed on the project so when its close and the breaker needs to change we have to work it out or replace it that's why I asked the question .

Not much engineering on this problem but I like to see what options are given with the rules and I was under the impression that the factory had a 80 % limit on most breakers . Or anything in the NEC code to limit how far one could go down in size so what your saying is if it holds were good to go . Thanks my thoughts were it would hold if only one or two motors ran . But if all three do it will trip .

 

[iceworm]

Well I enjoyed the last post the best but thanks for every ones response by the way we do have a inspector on site but some times we like to engineer our work we don't ask the inspector what to do hes there to inspect and his time is a special time . ...

I'm glad you enjoyed my post. It was susposed to be non-adverserial/ interesting/enjoyable.

I'm with you on not asking the AHJ. It isn't fair to pop them with a question that they have to spend hours researching to get a good answer. You're right, their time is special, they don't have that kind of time - and maybe not the specific background knowledge. And you have been staring at it hours on end - you already know the answer as well as anybody.

... plans are design by engineer then two years later equipment comes to job site its not what the engineer had planned but thats normally on every project and normal in this trade. ...

Yes - sigh - true

... so what your saying is if it holds were good to go . Thanks my thoughts were it would hold if only one or two motors ran. But if all three do it will trip.

Yes (cringe - hope I'm right)

If it is important (as in, has to absolutely work first time out of the starting gate - can't wait for new parts) I'd lay out the motor starting currents on a time-current curve for the CB. If there is daylight between the curves - that's the definition of coordinated. It's golden.

Let us know how it goes.

ice

 

[GoldDigger]

One thing which is clear is that if equipment lists a Minimum Circuit Ampacity, you cannot use either smaller wire or a smaller breaker than that number. But there is no guarantee that there will not be nuisance trips at that level.

Tapatalk!

 

[ohmhead]

I'm glad you enjoyed my post. It was susposed to be non-adverserial/ interesting/enjoyable.

I'm with you on not asking the AHJ. It isn't fair to pop them with a question that they have to spend hours researching to get a good answer. You're right, their time is special, they don't have that kind of time - and maybe not the specific background knowledge. And you have been staring at it hours on end - you already know the answer as well as anybody.


Yes - sigh - true


Yes (cringe - hope I'm right)

If it is important (as in, has to absolutely work first time out of the starting gate - can't wait for new parts) I'd lay out the motor starting currents on a time-current curve for the CB. If there is daylight between the curves - that's the definition of coordinated. It's golden.

Let us know how it goes.

ice

Yes ill keep you posted after we turn it on were still waiting on the mechanical contractor to do his piping so it will be weeks . LOL