NEC Table 9 Alternating-Current Resistance

[K8MHZ]

I give up. :slaphead:

It's this new thing we have about staying on topic, Bob. :roll:

"Hi,

I am trying to calculate voltage drop using the formula: VD=L*R*I/1000 where R values can be found from NEC table 9. "

No one is saying the NEC requires anyone to use his formula, he just wants to.

 

[shoaib10]

Exactly, so why would we expect the table in the NEC be written to acomadate this outside reference? :huh:

That was my point, it's like complaining my Chevys door won't fit my Ford.

NEC doesnt have any where referencing the formula for the voltage drop calcs...Its just stating how much voltage drop should be allowed on feeders and branch circuits..
But,the formula used for VD references to get the value of R from Chapter 9 table 9 which is only valid for 3 single conductors in conduit...OP had a question for more than three...It all started there...

 

[iwire]

NEC doesnt have any where referencing the formula for the voltage drop calcs...Its just stating how much voltage drop should be allowed on feeders and branch circuits..

To be clear the NEC recommends voltage drop it does not tell us or require it except for a few specific cases.

But,the formula used for VD references to get the value of R from Chapter 9 table 9 which is only valid for 3 single conductors in conduit...OP had a question for more than three...It all started there...

Which brings us back to my question to K8, why are we expecting the NEC to be written to match these outside references?

 

[K8MHZ]

Exactly, so why would we expect the table in the NEC be written to acomadate this outside reference? :huh:

It doesn't, and we should have told that to the OP right off the bat. Our bad. :huh:

That was my point, it's like complaining my Chevys door won't fit my Ford.

Sure they will, you ain't seen me with a gas axe. :lol:

 

[shoaib10]

To be clear the NEC recommends voltage drop it does not tell us or require it except for a few specific cases.

Bad choice of word i sued there...recommends is correct..

 

[K8MHZ]

So, AC_DC (Bob, you gotta love that user name) you can't use Table 9, or you can and realize it's just close enough.

Thank you for stopping by, and welcome to the forum.

 

[Ingenieur]

What will change if I don't have three phase? Table 9 shows nothing about that.

Using Table 9 outside its parameters, no matter how much sense it seems to make, is fudging it if you are instructed to use it. I take it the OP doesn't want to fudge it, or he wouldn't have posted his question.

he's not fudging anything

consider a zero sequence CT
run 3 x 100 A
6 x 50 A
9 x 11.11 A

will it read the same?

there is more error introduced by the randomness/geometry of the conductor lay than the change in mutual inductance (which is nil according to Ampere's law)

if you have 3 conductors in a given size conduit, and 6 in a larger one
the effective geometric mean radius is essentially the same, but does not matter if totally enclodes
I = integral(magnetic field H) dl, where dl forms a closed loop as does the conduit
it does not care about qty or orientation

 

[K8MHZ]

he's not fudging anything

Oh, so using a table in the NEC that is specifically worded to be applied to three phase three conductor conduits or cables to a seven wire installation isn't fudging it?

You can do all the algebra you want, but you do have to concede that three does not equal seven. If it did, the OP probably wouldn't have asked his question.

 

[Ingenieur]

Oh, so using a table in the NEC that is specifically worded to be applied to three phase three conductor conduits or cables to a seven wire installation isn't fudging it?

You can do all the algebra you want, but you do have to concede that three does not equal seven. If it did, the OP probably wouldn't have asked his question.

no
As long as the currents are equal

no 3 is ne to 7
but 3 cond x 7 A = 7 cond x 3 A is

 

[K8MHZ]

no
As long as the currents are equal

no 3 is ne to 7
but 3 cond x 7 A = 7 cond x 3 A is

So if you spec'd a job and called for 3 7A conductors and the EC pulled 7 3A conductors, you would be OK with that since you said they were =?

 

[Ingenieur]

So if you spec'd a job and called for 3 7A conductors and the EC pulled 7 3A conductors, you would be OK with that since you said they were =?

Irrelevent, we were discussing the effect on Z
whether increasing the number of conductors made a difference in R + jXl for equal load and ampacity

but if it was code compliant
did not cost more
no I would not care
but I would ask him why lol

 

[K8MHZ]

Irrelevent, we were discussing the effect on Z
whether increasing the number of conductors made a difference in R + jXl for equal load and ampacity

but if it was code compliant
did not cost more
no I would not care
but I would ask him why lol

Yeah, you wouldn't care. Until you tried to push your 7 amp load down a 3 amp conductor.

 

[Ingenieur]

Yeah, you wouldn't care. Until you tried to push your 7 amp load down a 3 amp conductor.

the load is 0.8 x21 A call it 17 A

if I lost 1 of the 3 x 7 A net ampacity is 14A
OC is 17/14 ~ 120%, over-loaded
if I lost 1 of the 7 x 3 A net ampacity is 18 A
OC is 17/18 ~ 95%, ie no OL


what is your point?

 

[pv_n00b]

Hi,

I am trying to calculate voltage drop using the formula: VD=L*R*I/1000 where R values can be found from NEC table 9. However, that table 9 is for three single conductor in conduit. What if I have more than 3 conductors in conduit, where I can find the adjustment factor or how can I calculate for more than 3 single conductors in conduit?

Up to about a #3 conductor skin effect is negligible so you can use R out of the DC table #8 and go from there. You can see this by comparing Tables 8 and 9, up to about #3 the DC resistance is about the same as the AC Z. Remember to use the round trip length of conductor for DC and single phase systems.

If you are over #3, not three phase, and not 3 conductors in a conduit the NEC will not help you get an answer. You need plug and chug software or an EE.

 

[Ingenieur]

Up to about a #3 conductor skin effect is negligible so you can use R out of the DC table #8 and go from there. You can see this by comparing Tables 8 and 9, up to about #3 the DC resistance is about the same as the AC Z. Remember to use the round trip length of conductor for DC and single phase systems.

If you are over #3, not three phase, and not 3 conductors in a conduit the NEC will not help you get an answer. You need plug and chug software or an EE.

Table note 2 says eff Z x current will give a good result for a 2 wire L-N circuit

they give you individual values for Xl and R
try this
using these given values for R and Xl calculate |Z| at pf = 0.85
does it match the effective Z given in the table?
why not?

 

[Skokian]

Include the Reactance in Calculating "Z".

Include the Reactance in Calculating "Z".

Note that on higher power circuits (larger conductor sizes) that the wire reactance actually exceeds the resistance. Hence the complex impedance needs to be calculated rather than just the resistance.