3 phase subpanel

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jbfan74

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Newnan Ga
I have searched and can not find the answer, so here goes.
I have 16, 20amp, 250,volt receptacles, and two 30 amp, 250 volt receptacles to wire into a subpanel. the 20 amp receptacles have a load of 16 amps each. I can not find the formula to figure the size wire and ocpd needed.
This panel will be feed from a main panel that comes from a 45kva transformer.

If someone could post the formula so I can figure this out, I would be greatful.
 
Something does not make sense here. First of all, which wire(s) and OCPD(s) are you trying to size? The secondary of the transformer? The feeder to the sub-panel? The branch circuits themselves?

My problem is that you are (apparently) talking about 16 receptacles, each drawing a load of 16 amps at 250 volts. Multiplying those numbers, and I get a total of 64 KVA. That exceeds the transformer rating all by itself, not to mention any other existing loads on the main panel.

What is the basis of your saying each 20 amp circuit will have a load of 16 amps? Is that just the 80% limit, or will all 16 circuits really be drawing that much power at the same time?
 
I am trying to size the subpanel and conductors.
I am getting the 16 amps from the paperwork for the cisco switche power supply.
The I T deparment said 4 power supplies, and 2 outlets each for a total of 16.
The paperwork shows an input voltage of 100-240 vac, and input curreent of 16 amps each.
That is all the info I have.
I need to prove to my boss, and his boss, that the current transformer from the ups will not handle this extra load.
 
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as Charlie points out, on the surface, you possibly have an immediate problem on the laod vs the transformer capacity. Are the 240 v loads single phase or 3 phase? waht is the load on the 30 amp circuits ?
are all loads coincidential ?
If you transformer secondary is truly 240v (as opposed to 208v)
then, if I did my math correctly) your 240v volt panel main would need to be no greater than 150 amps for transformer protection, although 125 amp would IMHO be better)
Can you tell us more about the load ?
 
again, assuming my math is correct, your 16 amp load, 208 single phase would be about 30 kw on your 3 phase system (53248 single phase 208 on 3 phase), add your two 30 amp outlets and your 45 kva transforemr should handle the load.
The secondary protection of the 45kva transformer would probably be a 150 amp device.
 
OK Let me restate what I have.
I have a 45 kva tramformer feed from a ups. This transformer feeds a panel , with then feeds several small panels. The load on this panel is about 50 amps.
I need to install a 3 phase panel with the following loads
8, 20amp, 250 volt receptacles, and 2, 2 30amp, 250 volt receptacles.
The paperwork for the cisco switches state that each input has a 16 amp draw.
So 16X8=128 amps.
I do not know if each input pulls the full load.
What formula do I use to figure the 3 phase load.

I know that the 45kva transformer will have to be replaced.

Hope this clears some of the confussion.
 
Thanks for the clarification. Here is where you are going wrong:
jbfan74 said:
So 16X8=128 amps.
Click to expand...
As an old boss of mine was fond of saying, ?True, but irrelevant.?

Three phase currents do not add up that way. The 16 amps that leave the panel on Phase A will pass through one of the cisco switches, then pass into another one (let us say one that is connected to Phase B), and will therefore return to the panel on the Phase B wire. So adding the 16 amps seen by the Phase A switch to the 16 amps seen by the Phase B switch is double counting the same amps.

There isn?t a ?formula? that I can offer to solve the whole problem, though a formula will come into play to address part of the problem. What I can offer is a ?process.? It starts with converting everything to VA, then adding all the loads, and only converting back to amps as the final step. So here goes.

Start with the existing load.
You have 50 amps on the panel already. 50 amps, multiplied by 208 volts (the phase to phase value of the source), and multiplied again by the square root of three (this factor will always come into a 3-phase calculation) gives you an existing load of 18KVA.

Now let?s add load:
16 amps per switch, times 120 volts, times 8 switches, equals 15,360 VA.
30 amps times 250 volts times 2 receptacles, equals 15,000 VA.

Total VA added is therefore 30,360 VA.
Add that to the existing 18,000 VA, and your new total is 48,360 VA. You just exceeded the capacity of the transformer by about 7%.

To complete the calculation, divide the 48,360 VA by 208 volts, and divide again by the square root of three, and the new current will be 134 amps.

One more thing: I assumed, since you gave me no other information, that the two 30 amp receptacles will draw all 30 amps. That is not likely. Also, there is no allowances here for the possibility that not all loads will be running at full capacity at the same time. So it is not impossible that this proposed installation would be acceptable.
 
Thanks Charlie. The 2 30 amp receptacles are for other switches, so I am not sure of the load. As it stands now only one is in use.

When you added the 16 amps per switch, you use 120 volts. Should I use the 208volts that the switch will be using?
Again, thanks for your help.
 
081125-1014 EST

Switching power supplies today tend to be designed to work over a large input voltage range. The actual input current will depend upon input voltage for a constant ouput load.

Your power supply may require 16 A at 120 V, but would only require 9.3 A at 208 V and 8 A at 240 V.

Meaning power input is less than 1920 W and VA input is 1920 VA.

.
 
Why does Cicso mark their power supplies this way....120-240 volts, 16 amps? The nameplate implies that it pulls the same current no matter what voltage it is connected to and that is not correct. They should give the voltage range and the VA for load calculating purposes.
 
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