Load Calculations And Industrial Machines

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industrial1003

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I am looking to add an industrial machinery load (multiple machines) to an existing facility, and am finding the code (NEC 670) good at spelling out the supply conductor and Feeder OCPD portions for industrial machinery installation. My issue is that I need to calculate the proposed new building load for a permit submission package. I have nameplate data but am wondering if that FLC needs to be multiplied by 125% since the machine will be considered a continuous duty load on the buildings electric system. NFPA 79 only requires the FLC of all devices operating at the same time for nameplate data so not sure how they set their values assuming minimum requirements. Any advice would be appreciated as I have spent a few hours now going back and forth between 670 and 220 without really making a solid determination.

Example machine data would be:

1 machine 480V 40A FLC nameplate 3 phase
2 machines 480V 15A FLC nameplate 3 phase
1 machine 480V 25A FLC nameplate 3 phase

KVA at 100 % listed comes out to:
1 X 33255 VA
2 X 12471 VA
1 X 20785 VA

Total KVA = 78982 VA = 78.982 KVA

KVA at 125% listed comes out to:

78.982KVA X 1.25 = 98.7275 KVA
 
Motors are not calculated as either continuous or noncontinuous (C & NC)... they're calculated simply as motors... but if you had to list them, NC... but you do have to throw in an extra 25% for the largest motor (by FLC, not by HP).

220.14(C) directs us to 430.22, 430.24, and 440.6 for branch circuits.

220.50 directs us to 430.24, 430.25, 430.26, and 440.6 for feeders and service, but you may also end up at 430.62 or 430.63.

Calculating for multiple motors and other loads can be one of the most difficult calculations to perform, because of all the nuances involved with motors.
 
Since the machines all have fused disconnect switches, I am treating those as the branch circuit OCPD's (they are the last OCPD in the line with the entire machine), and the conductors from the breaker panel to the machine would then be feeders. 430.25 lists that the conductors be sized for the "minimum circuit ampacity marked on the equipment" 670.4 appears to give a different take on the same set of feeders (supply conductors) with them being the 125% of highest rated motor, plus 125% resistance heating loads, plus sum of FLC of rest based on duty cycles of only items in operation at the same time. And that is where my confusion begins.:?

In the end I am really looking at determining that our plan is to add XXX KVA load to the existing XXX KVA building load, putting our new building load at XXX KVA (building load being what the utility transformers see's on its secondary). Having only done limited small load calculations before a production line's worth of industrial machinery is a bit daunting. One thing I guess I could use some additional clarification on is the word load as it pertains to the afformentioned situation. My assumption is that this load would be the listed KVA of the device (in this case the device is a whole machine so nameplate current). Most of the sections of the NEC are relating something (supply conductors or OCPD's) as having min or max values based on the "load" and it drives me crazy that they have not defined "load" in the beginning of the code and the few places they have they use single devices (gets worse with motors because of SF ratings). When we look at strictly outlets for cord and plug you can only connect 16A of "load" to the 20A breaker (80%, but again that is a relationship with an OCPD). That was my reason for asking about the duty cycles (C or NC).
 
IMO load equals minimum circuit ampacity (MCA) on the nameplate...if there is one, and specified as such, not just "AMPS". MCA includes all the 125% factoring, so no additional factoring is required for motors and continuous non-motor loads.

Hard to provide advice on the rest when you present in generalized form...
 
Smart $

Thanks for the replies. Unfortunately the MCA is not listed on the nameplates just the FLC for the machine (I checked and they are not just 125% the largest motor). To give you the extra details:

Building transformer 480/277 Y secondary 1000KVA
To assure transformer is not more than 80% loaded (sized 125% of load) max KVA load on transformer can be 800 KVA
Existing load determined with 30 day study 690A Peak so 480 X sqrt 3 X 690 = 573.655 KVA
So I have estimated a remaining 226.345 KVA for the building transformer

If we just look at one of the machines as an example:

1 X 480V 3ph 42A FLC Machine Nameplate Value (1 VFD 31.2 Input A, 1 VFD 5.7 Input A, 3/4hp MTR, 10A 120V control Circuit 1 phase CPT, 10 amp 24VDC circuit 0.9A per phase input 3 Phase CPT)
Knowing this I can see nameplate FLC is given at the minimum required by NFPA 79.

Supply conductor size per 670.4 would be:
(31.2 X 125%) + 5.7 + 1.6 +2.5 + 0.9 = 49.7A = 50A MCA

OCPD at machine disconnect per 670.4 has a Max value of:
45A (largest branch OCPD in machine) + 5.7 + 1.6 + 2.5 + 0.9 = 55.7 = 55A OCPD MAX

My question is would the added building load be designated at:
480V X sqrt3 X 42A = 34.9 KVA (Nameplate amps only)
480V X sqrt3 X 50A = 41.6 KVA (Supply Conductor rating or MCA)
480V X sqrt3 X 55A = 45.7 KVA (OCPD at disconnect switch will let it draw that much prior to trip)

Hope that provides a better example. Thanks.
 
industrial1003 said:
...Building transformer 480/277 Y secondary 1000KVA
To assure transformer is not more than 80% loaded (sized 125% of load) max KVA load on transformer can be 800 KVA...
Click to expand...
Transformers are rated for 100% continuous loading. However, some considerations lower that value, e.g. heavy motor load?to allow for high starting current, to keep voltage sag above a preferred value, etc. Nonetheless, an 80% cap is not required under the NEC (TTBOMK).


industrial1003 said:
...My question is would the added building load be designated at:
480V X sqrt3 X 42A = 34.9 KVA (Nameplate amps only)
480V X sqrt3 X 50A = 41.6 KVA (Supply Conductor rating or MCA)
480V X sqrt3 X 55A = 45.7 KVA (OCPD at disconnect switch will let it draw that much prior to trip)
...
Click to expand...
I'd say 41.6kVA if the only load on the supply. But in a multi-machine, multi-motor, and other non-motor load(s) environment, it'd be the sum of all true MCA's*, the "pseudo-MCA" if it has largest motor of all loads, FLC of other motors/machines, and all other non-motor loads at NC+125%C.

*I don't know of any NEC calculation allowance for reducing the MCA sum for loads not the largest motor.
 
Smart $,

Thanks again. Looks like I'll be having much fun with this particular calculation set. I wonder if any of this is better spelled out in any of the IEEE color books (not that it supersedes any code requirements, but a full example would be great)? I really wish the code committee would publish a separate book with industrial as it's main focus (though I bet industry is glad they don't).
 
Industrial Machinery

Industrial Machinery

I do not know the contents of NFPA 79, but its title is
something like "standards for industrial machinery"
( apologize if that is not the exact title, but there is
such a standard)
You may be able to look at the standard @ NFPA's
website and see if it is of any help.
paitence and research will get you throuh this

best regards
JR
 
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