calculation questions

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jstull33

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i currently recieved three calculation questions that i am having trouble with. i think i have figured these out but i want to get another's input to see. can anyone help? :-?


1. a 240v single phse branch circuit has a 200ft run of #4 Al wire. what is the amperage of the circuit? (Use approximate K)


2. Caculate the minimum and maximum size OCPD for an A/C unit with a compressor load of 21A and a condenser load of 1.5A.

3. Can a load of 15.1 kVA be added to an existing commercial service with 400KCMiL THW copper conducters having a maximum demand of 78.4 KVA?
 
Welcome to the forum. You will have to show us your answers and how you came to them before we can help you.

BTW, the questions are not complete the way you posted them.

Roger
 
this is how i calculated these:confused::confused:

1. a 240v single phase branch circuit has a 200ft run of #4 Al wire. what is the amperage of the circuit? (Use approximate K)

Aproximate K Al = 21.2

21.2/1000 x 200 = 4.24 ohms

I= E/R I=240/4.24 = 56.6 amps


2. Caculate the minimum and maximum size OCPD for an A/C unit with a compressor load of 21A and a condenser load of 1.5A.

2008 NEC article 440.22A max ocpd 225%
2008 NEC Article 440.325 min OCPD 125%

21amps x 225%= 47.25amps + 1.5amps = 48.75 amps or 50 amp max ocpd
21amps x 125% = 26.25amps + 1.5 amps=27.75 amps or 30 amp min ocpd

3. Can a load of 15.1 kVA be added to an existing commercial service with 400KCMiL THW copper conducters having a maximum demand of 78.4 KVA?

15100va + 78400va = 93500 watts or va
2008 NEC table 310.16 400 kcmil THW CU = 335 amps allowable ampacity

I=P/E I= 93500watts/ 480 vlts = 194.79 amps. yes it could be added
 
jstull33 said:
this is how i calculated these:confused::confused:

1. a 240v single phase branch circuit has a 200ft run of #4 Al wire. what is the amperage of the circuit? (Use approximate K)

Aproximate K Al = 21.2

21.2/1000 x 200 = 4.24 ohms

I= E/R I=240/4.24 = 56.6 amps
Click to expand...
"K" is a factor for Al. conductors. You need to get the whole formula. Your problem is missing some parameters. Like, are you restricted to a certain % voltage drop?
2. Caculate the minimum and maximum size OCPD for an A/C unit with a compressor load of 21A and a condenser load of 1.5A.

2008 NEC article 440.22A max ocpd 225%
2008 NEC Article 440.325 min OCPD 125%

21amps x 225%= 47.25amps + 1.5amps = 48.75 amps or 50 amp max ocpd
21amps x 125% = 26.25amps + 1.5 amps=27.75 amps or 30 amp min ocpd
Click to expand...
This has always been a weak section for me, but your answer looks reasonable to me.
3. Can a load of 15.1 kVA be added to an existing commercial service with 400KCMiL THW copper conducters having a maximum demand of 78.4 KVA?

15100va + 78400va = 93500 watts or va
2008 NEC table 310.16 400 kcmil THW CU = 335 amps allowable ampacity

I=P/E I= 93500watts/ 480 vlts = 194.79 amps. yes it could be added
Click to expand...
Is this a single-phase circuit? How did you know it was 480-volts (your original question did not state so).
 
these are the questions as i recived them word for word. no limit to VDrop. and i could only assume as to 480vlt existing commercial service. does not say questions are confusing to me doesnt give quite all the info needed i think. but these are as givin?????
 
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