EXAM PREP question

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jatindall82

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Hello all! My name is Jason and I reside in Texas. I am studying for my masters exam and am in dire need of some help. I know this question should be a simple calculation but I just am missing something. its extremely frustrating that I don't know this but im very eager to learn and have no one to help me out. Here is the question:

A 3 phase 120/230 Volts Multifamily dwelling building has a total demand load of 220 KVA, What is the minimum size of service conductor that can be used for building?

I know the answer is 600 kcmil aluminum. it also gave me the calculation, but I just cant make it make sense to me.

Full load current I = P / E * 1.732 = (220 * 1000) / 230 * 1.732 = 552 Amperes

Amps = watts divided by the voltage, times 1.732

Am I reading this right? PLEASE HELP!
 
We must assume 75C on these questions so 600 kcm al is equal to 545 amps so I don't see how that can be used for a load of 552 amps however according to art. 220.5 (A) I believe we have to use 240V and not 230V ?????. In that case the load would be 530 amps and then 600 kcm would work.
 
Dennis Alwon said:
We must assume 75C on these questions so 600 kcm al is equal to 545 amps so I don't see how that can be used for a load of 552 amps however according to art. 220.5 (A) I believe we have to use 240V and not 230V ?????. In that case the load would be 530 amps and then 600 kcm would work.
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??

I'm more inclined to think there is something missing from the data like info we can use in relation to Table 220.84
 
Pracctice questions

Pracctice questions

Hey thanks so much for your input. The question came from a free online study site. the fact that yall think it may not be right kinda scares me enough to not want to use that site. Any idea on another site too get some practice questions from?
 
jatindall82 said:
Hello all! My name is Jason and I reside in Texas. I am studying for my masters exam and am in dire need of some help. I know this question should be a simple calculation but I just am missing something. its extremely frustrating that I don't know this but im very eager to learn and have no one to help me out. Here is the question:

A 3 phase 120/230 Volts Multifamily dwelling building has a total demand load of 220 KVA, What is the minimum size of service conductor that can be used for building?

I know the answer is 600 kcmil aluminum. it also gave me the calculation, but I just cant make it make sense to me.

Full load current I = P / E * 1.732 = (220 * 1000) / 230 * 1.732 = 552 Amperes

Amps = watts divided by the voltage, times 1.732

Am I reading this right? PLEASE HELP!
Click to expand...

It's mathematically impossible for these two voltages to exist in a normal system. It should be 240/120 or 208/120.
 
120/230 three phase

120/230 three phase

I never heard of a 3 phase 124/230 service for a multi family dwelling. the service would have a HIGH leg.
 
domnic said:
I never heard of a 3 phase 124/230 service for a multi family dwelling. the service would have a HIGH leg.
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Agree that it's supposed to be 240/120 three phase and not "230"/120. So, definitely a stinger leg. I've seen this service voltage on single family dwellings via an open delta bank more than you might expect in the Deep South. I've also seen it on an older multi-tenant commercial facility where all the spaces were single phase and the three phase served a small elevator only. In such a case you surely can't assume balanced currents on all three phases. Playing devil's advocate, couldn't that be the case here? All the units have single phase services and the three phase is for an elevator or the like only? Then 220 kva/240 volts = 917 amps.
 
domnic said:
I never heard of a 3 phase 124/230 service for a multi family dwelling. the service would have a HIGH leg.
Click to expand...

Iron_Ben said:
Agree that it's supposed to be 240/120 three phase and not "230"/120. So, definitely a stinger leg. I've seen this service voltage on single family dwellings via an open delta bank more than you might expect in the Deep South. I've also seen it on an older multi-tenant commercial facility where all the spaces were single phase and the three phase served a small elevator only. In such a case you surely can't assume balanced currents on all three phases. Playing devil's advocate, couldn't that be the case here? All the units have single phase services and the three phase is for an elevator or the like only? Then 220 kva/240 volts = 917 amps.
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Can someone explain how any of these voltages mentioned would indicate a high leg, 3Ø, 4W Delta system?
 
infinity said:
Can someone explain how any of these voltages mentioned would indicate a high leg, 3Ø, 4W Delta system?
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May be a brain cloud night.. I don't understand your question. The OP originally stated 120/230 three phase. Assuming 120/240 Three Phase, is there any other way to obtain that other than high-leg ?
 
augie47 said:
May be a brain cloud night.. I don't understand your question. The OP originally stated 120/230 three phase. Assuming 120/240 Three Phase, is there any other way to obtain that other than high-leg ?
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Sorry my quotes were confusing. I was asking about the voltages listed 124/230 and 230/120 the latter of which we've assume to be incorrect.
 
infinity said:
Sorry my quotes were confusing. I was asking about the voltages listed 124/230 and 230/120 the latter of which we've assume to be incorrect.
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Regardless of the order, 120V and 230V do not fit together in one system.
It is either 120 and 240 or 115 and 230.
And in a three phase system those cannot be the only available voltages.
 
Are we to assume a misprint or trick question on the 230V part? 120/230 doesnt exist, 120/208 does, 120/240V high leg delta (and single phase) does. Has 230V 3ph ever existed in the US (or even overseas)?
 
JFletcher said:
Are we to assume a misprint or trick question on the 230V part? 120/230 doesnt exist, 120/208 does, 120/240V high leg delta (and single phase) does. Has 230V 3ph ever existed in the US (or even overseas)?
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230/400V is a very common 3-phase voltage in the rest of the world.

115/230V is within the IEEE tolerance of a 120/240V system. 115/230V probably used to exist, and still does to some extent, in the USA. At one point it was 110/220V, then it moved to 115/230V to the 120/240V of today. Nominal voltage is a difficult specification to maintain. This is often the result of utility upgrades improving the capacity of substation level transformers, in order to expand the utility feeder. As a result, existing customers get an increase in voltage. This is why it is a good idea to have taps available on the transformer, so you can adapt to the moving target.

120/230V, like others have said, is mathematically impossible for a three phase system. The ratio between specified voltages on three phase needs to be sqrt(3) for the standard WYE, or 2 for the high leg delta.

I haven't explored higher order phase systems to know whether this could be a possibility, but although systems like "4-phase square/cross" and "5-phase pentagon/star" may be interesting in theory, they are not practical.
 
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