Three phase feeder with single phase loads; Industrial

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Nitty

Member
Facility voltage - 240 three phase, grounded B (Corner grounded Delta)
Proposed Installation - Ten (10) single phase dehydrators. 240 volt, 46 amps. (just over 11kw each)


The dehydrator will be in 'continuous' operation.

If we were to install a feeder supply 10 dehydrators, being sure to balance them between phases (of course having one phase circuit unbalanced due to the tenth dehydrator), how would we calculate the feeder size?

My thoughts would be a 230 amp feeder (with 250 OCPD) because the following scenario should provide the highest current draw; if the only four dehydrators operating are the ones wired on the same phase circuit (for sake of example A&C) we would calculate as follows;

4 (dehydrators) * 46 (FLC) * 1.25 (continuous) = 230 Amps

As other dehydrators come on line current would be reduced on A&C circuit. If all ten were running the max current across A&C would be around 140 amps. Effectively - 33KVA three phase load + one 11KW single phase load.

Is this the correct way to look at this installation? :thumbsup: or :thumbsdown:
 

Smart $

Esteemed Member
Location
Ohio
With 3 connected A-B, 3 B-C, and 4 A-C, and operating, your line currents would be A&C 279.8A, B 239.0A... but to get these values require doing vector math.

Otherwise you can use the following to shortcut the vector math (for resistive loads only):

IA = sqrt(IAB?+IAC?+IABIAC)
IB = sqrt(IAB?+IBC?+IABIBC)
IC = sqrt(IBC?+IAC?+IBCIAC)
 

Smart $

Esteemed Member
Location
Ohio
The conventional method is to put one-half the phase loads' VA in each of three columns, one for each line (though often called phase). Total each column then divide by the phase voltage divided by square root of 3.

?A?B?C
22kVA22kVA
16.5kVA16.5kVA
16.5kVA16.5kVA

38.5kVA

33kVA

38.5kVA
?(240?sqrt(3))?(240?sqrt(3))?(240?sqrt(3))

277.8A

238.2A

277.8A

As you can see the result is really close, but a little shy.
 
Last edited:

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
On a side note, be sure your two-pole breakers are straight rated (as opposed to slash rated). This often overlooked on delta or corner grounded systems.
NEC 240.85
(The straight-rated are often not an off-the-shelf item as opposed to their slash rated brothers)
 

Nitty

Member
Sorry for the following elementary question but I need to know the difference between the following;

What if we had a piece of equipment that was 240 volt three phase, rated at 33KVA. Would not the current draw per line be 33,000/(240*1.73) = 79 amps?

If so what is the difference between the above scenario and say feeding only nine dehydrators in a balance system; three per line at 11KVA apiece?
 

Smart $

Esteemed Member
Location
Ohio
Sorry for the following elementary question but I need to know the difference between the following;

What if we had a piece of equipment that was 240 volt three phase, rated at 33KVA. Would not the current draw per line be 33,000/(240*1.73) = 79 amps?

If so what is the difference between the above scenario and say feeding only nine dehydrators in a balance system; three per line at 11KVA apiece?
Assuming you meant three per phase (line-to-line), basically little or no difference*.

*reserving the right to retract that, subject to conditions not stated.
 

kwired

Electron manager
Location
NE Nebraska
Sorry for the following elementary question but I need to know the difference between the following;

What if we had a piece of equipment that was 240 volt three phase, rated at 33KVA. Would not the current draw per line be 33,000/(240*1.73) = 79 amps?

If so what is the difference between the above scenario and say feeding only nine dehydrators in a balance system; three per line at 11KVA apiece?
To be truthful that is the approach I would likely take, figure out the balanced load and then add the imbalanced load to two of the phases. May not be entirely accurate, but is close enough most of the time. You could be off just as much in tolerance of the units actually matching their marked rating or even just differences because of voltage tolerances.
 

Nitty

Member
Wait a minute! "Basically little to no difference"!?!:? First I'm being told that I need to perform vector math and I end up with a 350 amp feeder (279 amps*1.25) and then I'm being told that by dropping one 46 amp load I can then install a 175 amp feeder, nine dehydrators balanced with 3 per phase to phase, worst case scenario - only three dehydrators running and all three are on the same phase to phase connection. Looking at smart $ calculations, my question is....why do you divide the voltage by the sq.rt of 3? Shouldn't you multiply the voltage by the sq.rt of 3?
 

Smart $

Esteemed Member
Location
Ohio
Wait a minute! "Basically little to no difference"!?!:?
My bad. I mistook your question to say 11kVA per phase, 33kVA total... and not 3 times 11kVA per phase for 33kVA per phase, 99kVA total


... Looking at smart $ calculations, my question is....why do you divide the voltage by the sq.rt of 3? Shouldn't you multiply the voltage by the sq.rt of 3?
When you are halving the kVA per phase then adding for total per line, you divide by the "wye" line-to-neutral voltage (e.g. on a 208Y/120V 3? system). In the case of 240V 3?, there typically is no "wye" neutral... but if there were, its voltage would be 240 ? sqrt(3) or ~138V.
 

hurk27

Senior Member
Sorry for the following elementary question but I need to know the difference between the following;

What if we had a piece of equipment that was 240 volt three phase, rated at 33KVA. Would not the current draw per line be 33,000/(240*1.73) = 79 amps?

If so what is the difference between the above scenario and say feeding only nine dehydrators in a balance system; three per line at 11KVA apiece?

I think the problem your having is you don't have just 33120VA for a load, that would only be 3 units balanced across all three phases, you have three sets of 33120VA which is 99360VA (9 units) balance across three phases.

Simply if we took 99360/240/1.732 each phase would have 239 amps with just the 9 units balanced, add the 10th unit and two phases will have 285 amps if all 10 units can run at the same time.

Are these dehydrators of the resistance heating element type?
 
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