a question about multiple motors on the same starter...

[Fulthrotl]

a question......

i want to put five 3 hp motors on the same motor starter....
480 volt 3 phase motors driving exhaust fans in a factory.

so, i go with 125% of the full load current of one motor plus
the sum of all the motors is my understanding....

so if i size the contractor for that ampacity, the running overcurrent
protection will still be in place for each individual motor, as any one
of them pulling excessive current flow will interrupt the starter the
same as if it was only a single motor, with appropriately sized heaters.

am i correct in this belief?

my other question is, i have two ways to run this... i can go with
pipe on the roof, teeing off and feeding fused disconnects for
each motor, or running it underneath the roof of this tilt up,
off a lift, with EMT. the over the roof method most likely will be
GRC. i have to check with the AHJ what their local rules are.

i also have to deal with derating the conductors if they are
close to the roof, which they would be..

the plus side is it would go a lot faster putting it in on the roof,
on blocks, than running underneath off a scissor lift.

the total run is 250', so voltage drop would require #8's for
the voltage drop.

so, what 'choo guys think? over the roof or under the lid on a
lift?

the other thought is that MC cable in #8 would go up pretty damn
fast, but isn't gonna get a photo shoot for beauty.

how would you do it?

 

[Volta]

My thought is that if one or more of the five were disconnected for some reason you would not have the desired overload protection at that time.

Otherwise, if I could avoid pullboxes or splices that were difficult to reach later I would try hard to do that. Even with a 25 or 30 degree ambient adder, it is likely I'd go with the roof, assuming the fan connections are there. If the motors are inside, I'd probably keep the wiring in too.

 

[GoldDigger]

a question......

i want to put five 3 hp motors on the same motor starter....
480 volt 3 phase motors driving exhaust fans in a factory.

so, i go with 125% of the full load current of one motor plus
the sum of all the motors is my understanding....

so if i size the contractor for that ampacity, the running overcurrent
protection will still be in place for each individual motor, as any one
of them pulling excessive current flow will interrupt the starter the
same as if it was only a single motor, with appropriately sized heaters.

am i correct in this belief?

That may be OK by the calculation required by Code, but since all motors are by definition going to be starting at the same time if run from the same starter, I would design for 125% or the total motor load, even it turned out not be mandatory to do so. There is also no diversity factor in whether the other motors are actually running to allow extra capacity for starting one motor.

Depending of the rest of the circuit, and including how often the motors will be started as well as just what the calculated voltage drop at the motors will be during starting, I might even size the wire and the starter above 125%.

And it is, IMHO, never a good idea to have multiple motors on one circuit without individual overload protection. There is no internal overload protection at each motor? If there is, then you do not need to worry about overload protection for the motors at the contactor or upstream.

 

[iwire]

You can use one contactor but the code requires individual motor overloads for each motor. You would just run the contactor coil circuit in series through each overload units contacts.

 

[augie47]

I think you are going to hit another Code stumbling block. iwire addressed the overload protection, but you must also address short-circuit/gf protection. For one 3 HP 480v motor the maximum breaker would be a 15 amp (maximum fuse 10 amp) which would be too small to handle all motors.

 

[Fulthrotl]

I think you are going to hit another Code stumbling block. iwire addressed the overload protection, but you must also address short-circuit/gf protection. For one 3 HP 480v motor the maximum breaker would be a 15 amp (maximum fuse 10 amp) which would be too small to handle all motors.

looks like i'm gonna end up bidding it as a nema 1 box large enough to handle
5 din rail 3 phase fuse blocks, 5 din rail starters with running overcurrent protection,
5 disconnects up on the roof, i can feed all of them with one set of #8 wires on a single
breaker, and put the box on a column in the center of the building, so it limits the
length of the individual motor leads. i'll run individual 1/2" emt to each motor, with #12 in it,
not on the roof, without j boxes in the runs, just transitioning to GRC to penetrate
the roof, with a transition to sealtight, going to the disconnect. i'm looking at using
motor rated switches inside the mushroom fan housings. i can mount the T stat controlling
the whole thing on the nema one box, and call it good. i could use MC cable for the motor
leads, but that would just look cheezy, imho.

is there anything else i have to derate for, like the political situation in the middle east?

btw, thanks for the feedback.

 

[Fulthrotl]

forum it seems is stuck... pardon me while i push start it....

forum it seems is stuck... pardon me while i push start it....

Want to post reply but can't!
"Hey man, I was trying to reply to your thread about one starter sharing multiple motor loads, but it won't let me. Do you mind putting up a new post on that thread so it will show as being unread? Apparently that is the problem. Never had this happen before. Thanks."

 

[CFL]

Want to post reply but can't!
"Hey man, I was trying to reply to your thread about one starter sharing multiple motor loads, but it won't let me. Do you mind putting up a new post on that thread so it will show as being unread? Apparently that is the problem. Never had this happen before. Thanks."

Ahh, finally. Thanks. OK, when you talk about putting your box on a column, is this an obvious location for future maintenance? Is there a big cost savings doing it that way versus putting the starter at the fan location to serve as both disconnect and starter?

Personally, I prefer having all starters in one location, and multiple penetrations directly to the fans vs lots of conduit running across the roof, so long as they are in a convenient, obvious location.

 

[cadpoint]

I've been trying to play along.:happyyes:

What you haven't said was how or where your going to run all the low voltage wires up?

Maybe I missed it.

 

[CFL]

I've been trying to play along.:happyyes:

What you haven't said was how or where your going to run all the low voltage wires up?

Maybe I missed it.

The low voltage wires would only have to run to box with the starters which could be under the roof not on top.

If the starters were on the roof, the low voltage wires could still be ran in the same conduit as the 480v power.

 

[cadpoint]

The low voltage wires would only have to run to box with the starters which could be under the roof not on top.

If the starters were on the roof, the low voltage wires could still be ran in the same conduit as the 480v power.

Understood, so now they have five wires in 1/2"? Or per second OP's post now twelve wires plus controls wires in a single penetration.

I would think they need bigger and more roof penetrations.

 

[Volta]

The low voltage wires would only have to run to box with the starters which could be under the roof not on top.

If the starters were on the roof, the low voltage wires could still be ran in the same conduit as the 480v power.

What low voltage are you talking about, the thermostats? I think it is unlikely that what I assume are Class 2 T-stat wires are allowed in the power conduit per Article 725.

 

[CFL]

What low voltage are you talking about, the thermostats? I think it is unlikely that what I assume are Class 2 T-stat wires are allowed in the power conduit per Article 725.

I wouldn't run a class 2 circuit to the contactor coil. I would run a 120v control circuit through contacts on tstat.

 

[CFL]

What low voltage are you talking about, the thermostats? I think it is unlikely that what I assume are Class 2 T-stat wires are allowed in the power conduit per Article 725.

I should not have used the term low voltage, I was quoting cadpoint, my bad.

 

[Volta]

I should not have used the term low voltage, I was quoting cadpoint, my bad.

No worries, it's all good. I'm the first to say that 'low' and 'high' are very relative: 5vdc can be high on a circuit board and 136kV can be low compared to 1MV.

120 as low voltage is fine with me.

 

[cadpoint]

Ok i'll assume the OP is talking about something like this

http://www.galco.com/images/symc/777-lr-p_wd.jpg

 

[Fulthrotl]

I've been trying to play along.:happyyes:

What you haven't said was how or where your going to run all the low voltage wires up?

Maybe I missed it.

here is what i have in mind for this:

conduit from main panel about 200', with 3 hots and a ground to provide power
to all the starters. emt conduit.

nema 1 enclosure mounted on a column 6' above the floor, center of the building,
with five 1/2" conduits going up the column to five mushroom exhaust fans.
the fans, it turns out, have a tube up thru the fan to the motor platform, so you
don't need roof penetrations. it's a 3/4" emt tube, so a 1/2" emt will sleeve thru it.

it goes into the back of a j box with a motor rated switch for a disconnecting means
under the mushroom head on the exhaust fan. nothing exposed on the roof.

controls for this is a T stat mounted on the nema 1 box, 5' above the floor. 24 volt
coils on the starters, and a control xfmr in the nema 1 enclosure supply power,
and the stat controls it. a hand-off-auto switch above it provides overrides.
or, i can use a t stat with 120 volt contacts, and a higher control voltage.
i'd like to use a hoffman box with a disconnect lever to open the power
to this. there is no telling who will have access or decide to fiddle with it.

now, i gotta figure out how many hours this thing will take to pipe together.
seems 4 days labor, a week for a scissor lift, and the material, and the
buisness license and permit fees should do it, but i'll put in 5 days, 'cause there
is gonna be a bunch of wheel spinning, i suspect.

whatchoo guys think? :dunce: am i high or low? i've gotten into pipe runs like this
in warehouses before, and it can eat buckets of time.

 

[Besoeker]

You can use one contactor but the code requires individual motor overloads for each motor. You would just run the contactor coil circuit in series through each overload units contacts.

I think the OP was planning to do that:

the running overcurrent
protection will still be in place for each individual motor, as any one
of them pulling excessive current flow will interrupt the starter the
same as if it was only a single motor, with appropriately sized heaters

I've run up to 80 small motors off a single VSD. Each had its own thermal overload.

 

[iwire]

I think the OP was planning to do that:

He was not.

 

[Besoeker]

He was not.

How do you interpret:

protection will still be in place for each individual motor