tx2step
Senior Member
- Location
- North Texas, DFW area
Re: underground service
underground service
- Thread starter sparks1
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sparks1
Senior Member
- Location
- Massachusetts
Re: underground service
If it is a short circuit no!
The main breaker is rated 200 amperes. What do you think?
"
If it is a short circuit no!
The main breaker is rated 200 amperes. What do you think?
tx2step
Senior Member
- Location
- North Texas, DFW area
Re: underground service
If it's supposed to represent some type of fault condition, then why would you worry about the Vd for that condition, since Vd would be the least of your problems?
I guess you can "suppose" anything you like, but if you are teaching students I would think that you would try to remain in the real world?
Consider (for instance) your original question -- how would anyone ever be able to guess what you were asking or what circuit/load conditions and parameters that you had in mind?
I'm sorry, but I think that you (or your students) will have to call the psychic hot-line to get answers to your questions.
No, the worst possible worst case scenario would be a 200 A load on just one phase (120V), and zero load on the other phase. But why you would want to calculate Vd for that improbable scenario (or for the improbable 100 A 120V load that you propose) goes right over my head.
So you are going to calculate Vd for a 100 A 120V load on just one phase, and no other loads on the other phase? Why would you want to do that? What electrician would set up a panel that way? It would be difficult to do intentionally -- I think it would be almost impossible to do accidentally.
If it's supposed to represent some type of fault condition, then why would you worry about the Vd for that condition, since Vd would be the least of your problems?
All on one phase? In a 120/240V 200 A panel, with zero amps on the other phase? Any electrician that would set up a panel like that wouldn't be bright enough to worry about Vd.
I guess you can "suppose" anything you like, but if you are teaching students I would think that you would try to remain in the real world?
I think that you don't make any sense. Are these really the types of questions & conditions that you pose to your students? What is the passing percentage of your students when they take journeyman or masters licensing exams?
Consider (for instance) your original question -- how would anyone ever be able to guess what you were asking or what circuit/load conditions and parameters that you had in mind?
I'm sorry, but I think that you (or your students) will have to call the psychic hot-line to get answers to your questions.
sparks1
Senior Member
- Location
- Massachusetts
Re: underground service
tx2step
Senior Member
- Location
- North Texas, DFW area
Re: underground service
These types of calculators use the "one way" length of the circuit, not the total combined length of the phase wire + neutral wire (for 120V). That's why the calculator specifically says " Enter 1-way circuit length in feet ". If you double the length, then you'll get the wrong answer.
If I were doing a manual Vd calculation (which clearly I wasn't), then I would have to use 2X the 1-way length (for 120V). That 2X multiplier is part of each of the various manual formulas that are commonly used.
Maybe we should also discuss how many angels can dance on the head of a pin? If you throw in that they are all holding a drop light, does that make it a good trick question?
Trick Question: Assume the maximum possible number of angels that can simultaneously dance on the head of a pin. What would the total wattage be if each angel held a drop light that utilizes a 60W lamp?
I guess part 2 of that question would be "What is the maximum distance and wire size for a 200 amp UG residential servive.", since that was your original question?
By the way, what was the correct answer to that original question?
And while you're at it, please show all of your work used in arriving at that answer. Maybe then I'll be able to follow along.
We were specifically addressing the Vd that was calculated from the Vd calculator at the web link: http://www.electrician.com/vd_calculator.html --- not a manual calculation.
These types of calculators use the "one way" length of the circuit, not the total combined length of the phase wire + neutral wire (for 120V). That's why the calculator specifically says " Enter 1-way circuit length in feet ". If you double the length, then you'll get the wrong answer.
If I were doing a manual Vd calculation (which clearly I wasn't), then I would have to use 2X the 1-way length (for 120V). That 2X multiplier is part of each of the various manual formulas that are commonly used.
Sorry, but the rationale (i.e. purpose) of trying to create an exercise based on a very improbable set of conditions "goes over my head". Why not just use conditions and circumstances that the student will really be dealing with, eventually, instead? :roll:
Maybe we should also discuss how many angels can dance on the head of a pin? If you throw in that they are all holding a drop light, does that make it a good trick question?
Trick Question: Assume the maximum possible number of angels that can simultaneously dance on the head of a pin. What would the total wattage be if each angel held a drop light that utilizes a 60W lamp?
I guess part 2 of that question would be "What is the maximum distance and wire size for a 200 amp UG residential servive.", since that was your original question?
By the way, what was the correct answer to that original question?
And while you're at it, please show all of your work used in arriving at that answer. Maybe then I'll be able to follow along.
1793
Senior Member
- Location
- Louisville, Kentucky
- Occupation
- Inspector
Re: underground service
It appears that in order to answer the first post of this thread we need more information. We know the Amps-200 we are looking for the distance and wire. We can plug in all sorts of numbers for wire and distance. If this were a residence, I would think Table 310.15(B)(6) for 200 amps either 2/0 CU or 4/0 AL would be good starting points. I feel we also need to know what is the allowable % of VD 3, 5 etc..
Now all we have to do is start putting in the numbers.
This was a set of Formulas that I saw, I believe on this forum.
"K = R x cm / 1000"
"VD% = 2 x K x D x I / cm"
"D = cm x VD Permitted / 2 x K x I"
"I = cm x VD Permitted / 2 x K x D"
"cm = 2 x K x D x I / VD Permitted"
When using "cm" formula K = 12.9 Cu & 21.2 for AL
"2" Length of wire, there and back Use 1.732 for 3 phase
"D" Length of wire one way
"I" Total Load in Amps
"VD Permitted" Allowable % of Voltage Drop over the circuit
"cm" Circular mil of conductor
"K" Resistance of a cm foot. Use entire # in calculation
"R" Resistance of the conductor Chap. 9 # 8
Always find "K" first
Norb
[ July 23, 2005, 05:26 PM: Message edited by: 1793 ]
It appears that in order to answer the first post of this thread we need more information. We know the Amps-200 we are looking for the distance and wire. We can plug in all sorts of numbers for wire and distance. If this were a residence, I would think Table 310.15(B)(6) for 200 amps either 2/0 CU or 4/0 AL would be good starting points. I feel we also need to know what is the allowable % of VD 3, 5 etc..
Now all we have to do is start putting in the numbers.
This was a set of Formulas that I saw, I believe on this forum.
"K = R x cm / 1000"
"VD% = 2 x K x D x I / cm"
"D = cm x VD Permitted / 2 x K x I"
"I = cm x VD Permitted / 2 x K x D"
"cm = 2 x K x D x I / VD Permitted"
When using "cm" formula K = 12.9 Cu & 21.2 for AL
"2" Length of wire, there and back Use 1.732 for 3 phase
"D" Length of wire one way
"I" Total Load in Amps
"VD Permitted" Allowable % of Voltage Drop over the circuit
"cm" Circular mil of conductor
"K" Resistance of a cm foot. Use entire # in calculation
"R" Resistance of the conductor Chap. 9 # 8
Always find "K" first
Norb
[ July 23, 2005, 05:26 PM: Message edited by: 1793 ]
growler
Senior Member
- Location
- Atlanta,GA
Re: underground service
Tex, you make some good points and seem to be a very reasonable person, but if you don't stop giving sparks idea's the next class of electricians is going to be dumber than the next class of political science graduates. Having said that and probably offending someone, somewhere. I think I see part of the problem. I haven't been around as long as you but I'm not a spring chicken. They have actually taught voltage drop calculations in different ways over the years. Reading this argument got me so confused that I had to pull out the old ugly's, American Electricians' Handbook , a book of electrical Formula's and personal notes. It appears that you can go at the problem in different ways to come to with the same or similar answers. For instance in the AEH they suggest useing 24 instead of 2K. I might ask how 2 X 12.9 = 24. If I were to round off I would think that 26 was a little closer ( I have the 13th Addition ). I think you have a point about more reality based questions. The masters' test isn't going to have anything weird on it and I haven't yet had to calculate a 3 wire single phase VD. Three phase motors loads and I have a nice little calculator for that. Good luck with your argument. If I were a student I think I would just give up and go to barber college.
Tex, you make some good points and seem to be a very reasonable person, but if you don't stop giving sparks idea's the next class of electricians is going to be dumber than the next class of political science graduates. Having said that and probably offending someone, somewhere. I think I see part of the problem. I haven't been around as long as you but I'm not a spring chicken. They have actually taught voltage drop calculations in different ways over the years. Reading this argument got me so confused that I had to pull out the old ugly's, American Electricians' Handbook , a book of electrical Formula's and personal notes. It appears that you can go at the problem in different ways to come to with the same or similar answers. For instance in the AEH they suggest useing 24 instead of 2K. I might ask how 2 X 12.9 = 24. If I were to round off I would think that 26 was a little closer ( I have the 13th Addition ). I think you have a point about more reality based questions. The masters' test isn't going to have anything weird on it and I haven't yet had to calculate a 3 wire single phase VD. Three phase motors loads and I have a nice little calculator for that. Good luck with your argument. If I were a student I think I would just give up and go to barber college.
growler
Senior Member
- Location
- Atlanta,GA
Re: underground service
K=12.9 for copper conductors @ 75 C.
K=21.2 for aluminum conductors @ 75 C.
K=12.9 for copper conductors @ 75 C.
K=21.2 for aluminum conductors @ 75 C.
Re: underground service
Sparks1, instead of going off into left field, you should give your students something in reason as far as a hypothetical exercise.
Roger
These are approximate values used for most calculations and we all use these values but, "K = R x cm / 1000" as posted by 1793 is a little closer to the true value.
Sparks1, instead of going off into left field, you should give your students something in reason as far as a hypothetical exercise.
Roger
growler
Senior Member
- Location
- Atlanta,GA
Re: underground service
Roger, as usual you are right. But that is just another bit of information that would have to be given in the question. If you are to calculate K or use the standard ( lazy man value ), more accurate as opposed to close enough for government work.
Roger, as usual you are right. But that is just another bit of information that would have to be given in the question. If you are to calculate K or use the standard ( lazy man value ), more accurate as opposed to close enough for government work.
tx2step
Senior Member
- Location
- North Texas, DFW area
Re: underground service
You're right, of course. I've learned a number of different Vd formulas, too. There are quite a variety around, and most produce about the same result. So I try to keep it simple -- like using the web based ones. They're usually close enough.
I've only taken (passed) about 6 masters exams so far, but I don't remember any question on any of them that wasn't based on a reasonable real-world situation. I'm trying to bone up again to take the masters test in a couple of states where I may want to relocate to in the future. That's a large part of what I'm doing in this forum.
To all -- sorry if I seemed argumentative. Sometimes I don't know when to give up and shut up. I should have thrown in the towel a couple of rounds ago. My bad. If I start drifting off on a tangent, somebody please just smack me!
Growler -- Well... I started in this business at about 10 years old. Guess who did all of the attic and crawl space work?
You're right, of course. I've learned a number of different Vd formulas, too. There are quite a variety around, and most produce about the same result. So I try to keep it simple -- like using the web based ones. They're usually close enough.
I've only taken (passed) about 6 masters exams so far, but I don't remember any question on any of them that wasn't based on a reasonable real-world situation. I'm trying to bone up again to take the masters test in a couple of states where I may want to relocate to in the future. That's a large part of what I'm doing in this forum.
To all -- sorry if I seemed argumentative. Sometimes I don't know when to give up and shut up. I should have thrown in the towel a couple of rounds ago. My bad. If I start drifting off on a tangent, somebody please just smack me!
tx2step
Senior Member
- Location
- North Texas, DFW area
Re: underground service
Roger -- I agree! That is the essence of what I was trying to say.
tx2step
Senior Member
- Location
- North Texas, DFW area
Re: underground service
By the way, at 16 I certainly wasn't a fully qualified journeyman by today's standards. That was long before state licensing, and the municipal standards set the bar a lot lower. Simpler times back then.
By the way, at 16 I certainly wasn't a fully qualified journeyman by today's standards. That was long before state licensing, and the municipal standards set the bar a lot lower. Simpler times back then.
growler
Senior Member
- Location
- Atlanta,GA
Re: underground service
Tex, I was not trying to criticize you. You could even say I was trying to agree with you. I can tell that you have a world of experience and knowledge. But if I say there is more than one way to skin a cat, PETA will send me a nasty letter. I was trying to say that sparks was maybe stuck on one method of calculating voltage drop. I'm quite sure that I could not pass one of his test, since I couldn't be sure what he's looking for. I hate ambiguous questions. Why anyone would anyone want to get off in the hypothetical world when there are plenty of real world situations to deal with? If it were up to me, training would be centered around what an electrician would use 90% of the time. People only retain a small portion of what they learn in class room. I would rather see someone with the basics drilled into them than someone that might be able to answer a complicated hypothetical question ( given enough time). In my " opinion " , you need to know where to find the formula and how to do the math. You may actually retain that knowledge for when you need it. PS. I have not skinned any cats " lately".
Tex, I was not trying to criticize you. You could even say I was trying to agree with you. I can tell that you have a world of experience and knowledge. But if I say there is more than one way to skin a cat, PETA will send me a nasty letter. I was trying to say that sparks was maybe stuck on one method of calculating voltage drop. I'm quite sure that I could not pass one of his test, since I couldn't be sure what he's looking for. I hate ambiguous questions. Why anyone would anyone want to get off in the hypothetical world when there are plenty of real world situations to deal with? If it were up to me, training would be centered around what an electrician would use 90% of the time. People only retain a small portion of what they learn in class room. I would rather see someone with the basics drilled into them than someone that might be able to answer a complicated hypothetical question ( given enough time). In my " opinion " , you need to know where to find the formula and how to do the math. You may actually retain that knowledge for when you need it. PS. I have not skinned any cats " lately".
tx2step
Senior Member
- Location
- North Texas, DFW area
Re: underground service
I just thought I made myself sound like an old coot, and wanted to clarify that I wasn't ready for a "walker" quite yet.
I've known a lot of people from a lot of different professions (doctors, lawyers, engineers, research scientists, astronomers, educators, politicians, writers, etc, etc. Some of the most intelligent and most well-balanced people that I've known are in the electrical industry.
No problem -- I didn't take it as criticism.
I just thought I made myself sound like an old coot, and wanted to clarify that I wasn't ready for a "walker" quite yet.
I've known a lot of people from a lot of different professions (doctors, lawyers, engineers, research scientists, astronomers, educators, politicians, writers, etc, etc. Some of the most intelligent and most well-balanced people that I've known are in the electrical industry.
sparks1
Senior Member
- Location
- Massachusetts
Re: underground service
"If you double the length, then you'll get the wrong answer."
Exactly my point, a student would need to know this fact to prevent calculation error.
Final answer
Let me explain the scenario, in the formula ED=2kil/CSA the variable (I) is directly proportional to the voltage. A student learning voltage drop calculations for the first time will need to know the formula and understand the correct method to avoid calculation error. Therefore, 240v versus 120v will proportionly decrease the percentage of voltage drop.
[ July 25, 2005, 01:16 PM: Message edited by: sparks1 ]
"If you double the length, then you'll get the wrong answer."
Exactly my point, a student would need to know this fact to prevent calculation error.
Final answer
Let me explain the scenario, in the formula ED=2kil/CSA the variable (I) is directly proportional to the voltage. A student learning voltage drop calculations for the first time will need to know the formula and understand the correct method to avoid calculation error. Therefore, 240v versus 120v will proportionly decrease the percentage of voltage drop.
[ July 25, 2005, 01:16 PM: Message edited by: sparks1 ]
tx2step
Senior Member
- Location
- North Texas, DFW area
Re: underground service
- Status