What voltage to use 120 or 240?

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ftex

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Hi, I am confused about what voltage to use to calculate current for cable size of transformer.
for example, single phase given load 140kva 120/240.
if I=140/120v= 1166 A and if 140/240v=583A
I know for fact, we use I=kva/1.73 x V. But for single phase i am not able to find correct solution.
Thank you for your help.
 
For 1Ø use just the line voltage of 240 volts. 1.73 is used for 3Ø voltage.

Welcome to the forum. :)
 
Thank you
OK, when ever i have to choose "cable for service entrance direct from transformer" (before the meter)
if it is single phase 120/240 .. the amp to be consider for cable i =kva/240

if it is 3phase 347/600 .. the amp to be consider for cable i= kva/347

if i got it right than when do we consider 120v for calculation ?
 
What if hypothetically, there are only 120v loads being fed from this panel. Is it possible the cables could be undersized, if 240v was used in the calculations?
 
adamscb said:
What if hypothetically, there are only 120v loads being fed from this panel. Is it possible the cables could be undersized, if 240v was used in the calculations?
Click to expand...

possibly if the loads were not distributed ~ evenly across both legs
if you skipped slots and put all on L1 for example, but why?
if they were evenly, ie, 1/2 the load on each leg, no
i = (load/2) / 120 = load/240
 
ftex said:
if it is 3phase 347/600 .. the amp to be consider for cable i= kva/347
Click to expand...
No, use i= kva/1.732*600, if current in all phases are equal and kva is 3 phase kva. But if kva is single phase kva, then i= kva/347
 
Last edited:
ftex said:
Thank you
OK, when ever i have to choose "cable for service entrance direct from transformer" (before the meter)
if it is single phase 120/240 .. the amp to be consider for cable i =kva/240

if it is 3phase 347/600 .. the amp to be consider for cable i= kva/347

if i got it right than when do we consider 120v for calculation ?
Click to expand...
Welcome, ftex. :thumbsup:

I = kVA ÷ (1.732 × 600V)

...is preferred for formal calculations but it is exactly the same arithmetically as...

I = kVA ÷ (3 × 347V)

...as in 3 legs at 347 volts to neutral.
 
Ingenieur said:
possibly if the loads were not distributed ~ evenly across both legs
if you skipped slots and put all on L1 for example, but why?
if they were evenly, ie, 1/2 the load on each leg, no
i = (load/2) / 120 = load/240
Click to expand...
+1

one generally assumes you will try to achieve best possible balance across the 240 volt coil with the 120 volt loads. If that for some reason can't happen then the actual balance needs further investigation to ensure you don't place too much load on the heavier loaded half of the source.

A single 120 volt 1kVA load is too much to place on just half of a 120/240 volt 1.5 kVA source, but you can balance ten 120 volt .10 kVA loads on the same 120/240 1.5 kVA source.

KVA capacity of what OP likely doesn't have a single 120 volt load though that is more then half the capacity of the source but demand situations from individual loads could still get you into some trouble in some instances with balancing efforts.
 
Thank you...
I was going to create new forum for 3phase...anyways

Lets stay with single phase..
I was told when i want to pick a cable or Bus size .. i have to multiply the Amp(current) by 80% .. is that true?
for example: given load 50kva 120/240 .......then i= 50kva/240v= 208 A ,
then 208 x 80%=166 A I have to pick my cable based on 166A and not 208?
 
ftex said:
Thank you...
I was going to create new forum for 3phase...anyways

Lets stay with single phase..
I was told when i want to pick a cable or Bus size .. i have to multiply the Amp(current) by 80% .. is that true?
for example: given load 50kva 120/240 .......then i= 50kva/240v= 208 A ,
then 208 x 80%=166 A I have to pick my cable based on 166A and not 208?
Click to expand...

No
divide by 80% or 4/5
or mult by 5/4 or 125%
 
ftex said:
Thank you...
I was going to create new forum for 3phase...anyways

Lets stay with single phase..
I was told when i want to pick a cable or Bus size .. i have to multiply the Amp(current) by 80% .. is that true?
for example: given load 50kva 120/240 .......then i= 50kva/240v= 208 A ,
then 208 x 80%=166 A I have to pick my cable based on 166A and not 208?
Click to expand...
Sort of...
You need to select the wire size by the current times 1.25 (which is the same as divided by .80) IF the current is continuous. Use 1.00 if the current is non continuous (or a motor load in which case you use a different calculation entirely.)
But you select the overcurrent device by the calculated load and then use a breaker rated at 1.25 times that unless the breaker and enclosure are 100% rated.
It is frustratingly easy to mix wire size rules and OCPD rules.
 
ftex said:
...
I was told when i want to pick a cable or Bus size .. i have to multiply the Amp(current) by 80% .. is that true?...
Click to expand...
Continuous load and largest motor at 125%, plus noncontinuous load and other motors at 100%... per NEC.

Your location says Toronto... in Canada? Do you even go by the same rules up there?
 
Smart $ said:
Continuous load and largest motor at 125%, plus noncontinuous load and other motors at 100%... per NEC.

Your location says Toronto... in Canada? Do you even go by the same rules up there?
Click to expand...


yes , It is almost same stuff with minor changes.
 
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