MOTOR CALCULATIONS

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shockrocker68

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Hello, I am installing 15 - 3/4HP single phase motors, The application is for non continuous loads.The branch ccts will come from a NQOD Square D panelboard, 208/120v, the motors will run off 120v 8.4A each, How would i calculate the load for theses motors, to size the correct panelboard and transformer.
 
Re: MOTOR CALCULATIONS

How about you show us your calculations, and we then can critque them?

First find the FLC of the motors.

Use 430.22 to determine the branch circuit conductor sizes.

Use 430.52 to determine the ground-fault and short-circuit protection needed.

Use 430.24 to determine the feeder requirements.

Use 430.62 to select the feeder OCD.
 
Re: MOTOR CALCULATIONS

the motors will be installed with a #10AWG Branch CCt on Seperate 20 amp Breakers. they will Also have there own seperate O.L.
 
Re: MOTOR CALCULATIONS

Don't for get to read 430.22E the Table if the load is other than continuous.

Charlie P
 
Re: MOTOR CALCULATIONS

You could probably go as small as #12 conductors and as large as a 35-ampere breaker for the branch circuits.

You should use the FLC rating as provided by the XIII. Tables in 430 and not the nameplate rating.
 
Re: MOTOR CALCULATIONS

This will be a new installation of 480-208/120v xformer and a new panelboard, I calculated 1 motor by nameplate 8.4 X 1.25, then the rest of the motors {14} by the nameplate,I should have went by 430-150, never the less, I Came up with 128 amps. Non continous load, but does the panelboard have to be rated for non cont load.
 
Re: MOTOR CALCULATIONS

408.13 states that, "all panelboards shall have a rating not less than the minimum feeder capacity required for the load...."

215.2(A)(1) states that, "Feeder conductors shall have an ampacity not less than required to supply the load... and that it "shall have an allowable amapacity not less than the noncontinuous loads plus 125 of the continuous loads."

You really should use the FLC of the motors. It changes things considerably with 15 motors. The table lists 13.8-amperes for a 3/4, 115-volt motor.

13.8 x 1.25 = 17.25 + (14)13.8 = 210-amperes. Will all 15 motors operate at exactly the same time?
 
Re: MOTOR CALCULATIONS

Try using VA for minimum 208Y/120V. 3-phase feeder.

(14) (13.8A) (120V.) = 23184VA
(1) (13.8A) (120V.) (125%) = 2070VA
25254VA / (208V. times SQ Root of 3) = 70.1A minimum feeder.

A 30KVA xfmr will easily do the job. Afraid a 15KVA might be cutting it close. Of course balance these 120V. loads on the (3) legs of the panelboard.

How bout a (24) pole space NQOD panelboard with
#3 wires from the xfmr to a 100A/3P main bkr. in the panelboard. The single poles could be up to 35A if you wish.

There are some design issues in all this. Just my quick 2-cents with the info given.
 
Re: MOTOR CALCULATIONS

BPH:

I think you have the load right, but you forgot to convert to 3 phase for the current. I also get about 70 amps. I would use a 100A panel and a 30KVA xformer.

Steve

[ February 24, 2005, 09:28 AM: Message edited by: steve66 ]
 
Re: MOTOR CALCULATIONS

Boy, that is one highly efficient 3/4 HP motor to operate at 8.4 amps at 120-volts. You sure about that? Just make sure that amp rating is correct.
 
Re: MOTOR CALCULATIONS

OK. Maybe I'm missing something. He has 15 non-continuous loads. I can also see calculating for 15 continous loads. I'm conservative, I can deal with that.

But why 1 motor at 125% and 14 at 100%???

BPH quotes the code: it "shall have an allowable ampacity not less than the non-continuous loads plus 125% of the continuous loads."

To me this reads:
(# of non-continuous motors)*(load)+(# of continuous motors)*(load)*(125%)=total ampacity

Shockrocker68 states:"The application is for non continuous loads"

If they are ALL non-continuous, then why not 15*13.8A???
 
Re: MOTOR CALCULATIONS

See section 430.24

In order to determine the size of the feeder and feeder OCD, we need to calculate the load based on this section.

I am still interesting in knowing if all 15 motors will operate at once, and what the duty-cyle service is if not continuous.

[ February 24, 2005, 03:14 PM: Message edited by: bphgravity ]
 
Re: MOTOR CALCULATIONS

BPH:

430.24 states: Conductors that supply several motors must have an ampacity of not less than:
(1) 125% of the highest-rated motor FLC [430.17], plus
(2) The sum of the FLCs of the other motors (on the same phase) [430.6(A)].

Shockrocker used a plural for the branch circuits. He also mention using #10. Therefore I assume he is using 1 branch per motor. 430.24 is for branch circuit rating. Therfore would it not be 125% of just one motor. Then add up all the VAs of the 15 branches for the feeder?

Modifying Kilo's calcs:
(13.8A)(120V)(125%)for one branch....125% of the highest motor...
(15) (13.8A) (120V) (125%) = 31050VA
31050VA / (208V. times SQ Root of 3) = 86.2A minimum feeder to the panel

My other take on this is where it might get confusing, the codes states motors "(on the same PHASE)". Shockrocker says he is using a 3phase panel (208/120)

So should it be 1@125% +4@100% per phase (5 motors per phase)??

Forgive my ignorance if I'm way off base..I'm trying to get an education here...
Bob
 
Re: MOTOR CALCULATIONS

Sorry, I should have sent you to 430.62

So I guess the calculation should go as followed:

One motor = 13.8 x 250% = 35-ampere OCD.

Per section 430.62, 35 + (14)13.8 = 225-ampere

But, I don't think this is an acurate way to go about this calculation considering these are single phase motors on a three phase system.

I think a better way to go about this is to determine the load on each phase:

13.8 x 1.25 = (4)13.8 = 72.45-ampere feeder

35-ampere + (4)13.8 = 90-ampere OCD.

Now I have confused myself!!

[ February 24, 2005, 04:21 PM: Message edited by: bphgravity ]
 
Re: MOTOR CALCULATIONS

Wow, all those reply's got me a little confused too. Well as I see it you have (15)*13.8 (120V FLA) which equals 207 amps at 120V. If you calculate for VA you get 120*207 = 24.8kVA. Or this is 69A at 208V, 3-phase.

A minimum of 30kVA transformer will be needed. A 30kVA transformer will supply 83A at 208V. If you use the 60 deg table 310-16 4#1, 1#6G can be routed from the xfmr to the 100A, 120/208V, 3-phase panelboard.
 
Re: MOTOR CALCULATIONS

Sizing of an xfmr is a design issue. There are rules-of-thumb like 1KVA per HP for 3-phase & 1.25KVA per HP for 1-phase. But for most general industrial/commercial 3-phase applications you can size the xfmr like you would size the NEC minimum feeder.

As far as "continuous loads" go, do not make too much of it on motor ckts. because NEC minimums treats them as continuous anyway.

It is accepted practice to use the NEC tables FLA's for sizing conductors & branch ckt. OCP as per 430.6(A)(1). That is what my post of 2/23 above is based on.

shockrock's application almost sounds like the (15) motors have "fake 3/4 HP ratings".
You've seen what I'm talking about. Usually the "home workshop-type appliances" - table saws, drill presses, etc.

I guess that NEC was eventually forced to acknowledge these "fake HP's" with Exception 3 to 430.6(A)(1).

Also, most modern components of the type that we are speaking of, have terminals rated for 75 Degree C.
 
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