Panelboards and Wiresizing

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jwalker

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I am a new electrical engineer and I am having trouble finding literature on how to determine wire sizing.

How would I determine the wire size needed for a 120/208V Lighting Panel that is rated for 225A (estimated load is approx 200A) 3-phase 4wire. The run is 500ft and I assume I am aiming for less than 3% voltage drop. I don't know whether this run is on the primary or secondary side of the 480/120-208v transformer but I am again assuming secondary.

I would like to be able to do this by hand if possible because i have tried 4 websites and have recieved 4 different answers with the same data. Seems fishy. I have one website telling me 1000kcmil one telling me 350kcmil and another telling me 700kcmil. Any help would be appreciated.

Thank you.
 
Welcome to the Forum.

It is very important to know which side of the transformer you are on. The amount of voltage drop, in terms of volts, will be the same. But the percentage will not. With a run that long, I would prefer to design the system with the transformer nearer the load. Bur for this discussion, let?s presume it is the secondary run that is 500 feet.

Start with the load. You say you have an estimated load of 200 amps. That will not do. You need to calculate the load, per Article 220. But for the moment, let?s assume the results of the calculation is 200 amps. Are you planning to design the system for the 200 amp load, or for the capacity of the panel (i.e., 225 amps)? That is a design choice, and is yours to make, unless the client has a specific requirement one way or the other. Let?s work with 200 amp load.

Using a spreadsheet calculator that I downloaded from this site but that is no longer available (other sites have similar calculators), I came up with a answer of 350 MCM wire.

If you want to do this by hand, it is a simple application of Ohm?s Law. You know the voltage at the ?sending end,? and you know the voltage you want at the ?receiving end.? That gives you a delta-v. You know the current. Ohm?s law will give you the resistance of the wires. Be careful to apply the ?square root of three? factor where applicable, since this is a three phase system.

Once you have the resistance, you divide by length, and put the results into terms of ?ohms per 1000 feet.? Then you look up the resistance, per 1000 feet, in Table 8, and pick a conductor with a resistance value no higher than the one you calculated.
 
For low voltage system (<600V) three phase applications
Voltage drop formula and calculation example

Z(effective) = ((VD x 1000)/(Length x Amps x 1.732))

Table 8 gives equivelant Z Effective select cable as appropriate

Example:
Motor 50hp, 3 phase, 480V motor located 550 ft from Motor Control center with a 3% Voltage drop at motor, conductors routed in aluminum conduit.

VD = 480V * 3% = 14.4
From Table 430.250; 50hp motor = 65FLA
Length = 550 ft

Z eff = (14.4*1000)/(550*65*1.732) = .233
From Table 8 Effective Z at .85pf for uncoated copper wires
#3 wire = .24
#2 wire = .19

phase wire size for this application would be #2
 
Ok I just got the call back from the rig and the calculated load is 193A @ 120V and the 500ft wire is from the secondary side of the transformer. The formula being used is:

Vd(%) = 1.732 * I * R * D

R= (120*.03)/(1.732*193A*500)*1000
R=.0215 Ohm/Kft

From Table 8 copper = 600 kcmil

Its alot easier than i was making it out to be thanks again.
 
I did get a response from the rig and they have the 500ft run on the secondary side of the xfmr and a calculated load of 193A. So from this I calculated my resistance of .0214 Ohm/kFT corresponding to 600 kcmil. I am not familiar with MCM but it doesn't seem our numbers match up. Its a simple formula so I must be missing something.
 
also

also

jwalker,
if I may. let me also add that if part of your duty is to selet the panelboard, once you have determined the wre size please make sure the panelboard and the feeder overcurrent device you specify have lugs that will accept that size wire.
 
jwalker said:
. . . corresponding to 600 kcmil. I am not familiar with MCM . . . .
Click to expand...
"kcmil" and "MCM" are two ways of expressing the same unit of measure. Both mean, "thousand circular mils." MCM is the older of the two versions, and some would call it obsolete. I am not among them.

Your error was in using 120 volts, instead of 208 volts. This is a three phase load, and you need to use the phase-to-phase values. When I make that correction in your solution, I get a result of 0.0373 ohms. A 350 MCM copper conductor will work here. I also get an answer of 350 MCM, when I use the calculator I alluded to earlier.
 
jwalker said:
I did get a response from the rig and they have the 500ft run on the secondary side of the xfmr and a calculated load of 193A. So from this I calculated my resistance of .0214 Ohm/kFT corresponding to 600 kcmil. I am not familiar with MCM but it doesn't seem our numbers match up. Its a simple formula so I must be missing something.
Click to expand...

The use of M for 1000 originated from the fact that M is the Roman numeral for 1000. I have even seen cases where MM was used for million. As there is greater use of the metric system, and use of the term mega for million, people have come to use k for 1000 and M for million. That is the reason for changing from MCM (1000 circular mils) to kcmil.
 
Just for S&G, have you tried to calculate the cost of running this feeder high voltage with a step down @ the panel location? 2000' of 350 mcm isn't cheap.
 
jwalker said:
I am a new electrical engineer and I am having trouble finding literature on how to determine wire sizing.

How would I determine the wire size needed for a 120/208V Lighting Panel...
Click to expand...

As a new engineer, I would like to help you get off on the right foot on a few things:

#1 Become familiar with IEEE standards as they dictate the world of engineering.

#2 Always be aware that the NEC is not a design guide. It is intended to aid in installing a safe system, and it is only MINIMUM requirements, which are most often insufficient in providing a good system.

#3 Properly state voltages e.g. 120V/208Y is not the same as 208Y/120V (although some will tell you it's understood to be the same, they are wrong)

#4 Know that when people use MCM instead of KCMIL, they are not following the conventions of IEEE or the NEC (Changed what, 10 years ago) They are probably the same folks that still use "PT" instead of "VT". They will be old and gray, profess they used slide rules at some point in time, and think the pocket protector was the greatest invention ever, besides the Twinkie.

#5 Don't use this forum to replace thinking for yourself.

BTW: why didn't you ask this question to your supervising P.E.?
 
I know the NEC says the feeder VD needs to be 3% but since the small size of our drill floor I am to understand that the branch circuits will hardly use any of their 2% Vd; so to address the issues of the cost of the cable, the cable termination on the panel lugs and the large wire (equivalent to 777 kcmil for shipboard cable) would it be a good idea to calculate the wire for a 5% Vd?

Curious that I should use 208V instead of the 120V I used in the calc. I thought that conversion factor 1.732 (sqrt[3]) would give me 208.

Also, I do work closely with my supervisor but its Turkeyday, he is not in and I am alone in the office (not enough vacation time yet). I do have another PE that I talk to regularly that currently currently does that work that I will be taking over when my training completes in a few years.

Thanks again
 
kingpb said:
#4 Know that when people use MCM instead of KCMIL, they are not following the conventions of IEEE or the NEC (Changed what, 10 years ago) They are probably the same folks that still use "PT" instead of "VT". They will be old and gray, profess they used slide rules at some point in time, and think the pocket protector was the greatest invention ever, besides the Twinkie.

quote]

I resemble that remark :smile: , but, rest assured, there are still a few tricks he can learn from an ole dog (even one that still speaks in MCM terms)

oh, and J Walker, this forum is a good place to pick up on those "tricks"..welcome aboard
Click to expand...
 
jwalker said:
I know the NEC says the feeder VD needs to be 3%
Click to expand...

Then you 'know' something the rest of us do not. :wink:

The NEC does not require anything about voltage drop except in a few specific installations, fire pumps for instance but there the voltage drop is required not to exceed 15%.

(see 695.7)

kingpb gave some you some great info, take look at some of it again.

kingpb said:
#2 Always be aware that the NEC is not a design guide. It is intended to aid in installing a safe system, and it is only MINIMUM requirements, which are most often insufficient in providing a good system.
Click to expand...

Then take a moment to look at three short NEC code sections 90.1(A), (B) and (C).

90.1(B) really lays it on the line.

What I am trying to point out that if your interested in providing the best for your future clients you can not count on NEC requirements to get you there.
 
ray cyr said:
kingpb could you expand a bit on the difference between 120V/208Y and 208Y/120V? Thank you.
Click to expand...
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