T
T.M.Haja Sahib
Guest
Who gave you instruction to select a conductor on voltage drop first, instead of first ascertaining its current carrying capacity?
Voltage Drop Calculation for Conductors larger than 310.16
- Thread starter NorthwestPV
- Start date
- Status
Who gave you instruction to select a conductor on voltage drop first, instead of first ascertaining its current carrying capacity?
kwired
Electron manager
- Location
- NE Nebraska
Who gave you instruction to select a conductor on voltage drop first, instead of first ascertaining its current carrying capacity?
Nobody, but there have been occasions, even on this forum, where that is what people were attempting to do.
It also supports my thoughts that ampacity and voltage drop are not really all that closely related. There are probably a few holes in what I said, but in general they are two completely different things.
T
T.M.Haja Sahib
Guest
Check ampacity of a conductor at 60C, 75C and 90C rating and also the corresponding voltage drop for each. Any relationship there? Are you now convinced that ampacity and voltage drop of a conductor are closely related?
- Location
- Connecticut
- Occupation
- Engineer
I'm not sure what point you are trying to make, but, for example, #1 TW (60deg) has an amcpacity of 110, #1 THWN (75 deg) has an ampacity of 130) and #1 THHN (90 deg) has an ampacity of 150. An 80A load over a distance of 500 feet would have the same voltage drop for all three of these conductors, even though the have different ampacities.
kwired
Electron manager
- Location
- NE Nebraska
Exactly my point when I mentioned that 310.15 ampacity is essentially the rating based on insulation temperature.
Conductors are quite capable of carrying more than what is mentioned in the table, but insulation is not necessarily able to take the resulting heat.
We lost control of the question after post #5. You are installing 4 sets of 250 kcm. Is that cheaper that 2 600 kcm?
However, we need to correct the effect VS affect question.
What Is the Difference Between Affect and Effect?
Before we get to the memory trick though, I want to explain the difference between the two words.
It's actually pretty straightforward. The majority of the time you use affect with an a as a verb and effect with an e as a noun.
When Should You Use Affect?
Affect with an a means "to influence," as in, "The arrows affected Ardvark," or "The rain affected Amy's hairdo." Affect can also mean, roughly, "to act in a way that you don't feel," as in, "She affected an air of superiority."
When Should You UseEffect?
Effect with an e has a lot of subtle meanings as a noun, but to me the meaning "a result" seems to be at the core of all the definitions. For example, you can say, "The effect was eye-popping," or "The sound effects were amazing," or "The rain had no effect on Amy's hairdo."
However, we need to correct the effect VS affect question.
What Is the Difference Between Affect and Effect?
Before we get to the memory trick though, I want to explain the difference between the two words.
It's actually pretty straightforward. The majority of the time you use affect with an a as a verb and effect with an e as a noun.
When Should You Use Affect?
Affect with an a means "to influence," as in, "The arrows affected Ardvark," or "The rain affected Amy's hairdo." Affect can also mean, roughly, "to act in a way that you don't feel," as in, "She affected an air of superiority."
When Should You UseEffect?
Effect with an e has a lot of subtle meanings as a noun, but to me the meaning "a result" seems to be at the core of all the definitions. For example, you can say, "The effect was eye-popping," or "The sound effects were amazing," or "The rain had no effect on Amy's hairdo."
Last edited:
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
There is an issue of semantics in play as to whether the "conductor" is considered to be the whole wire including the insulation or just the metal inside it. The way the Code uses the term implies to me that the Code considers the metal and insulator a single entity. In any event, the amount of current that the metal can carry long after the insulation has vaporized or caught fire is pretty useless information, IMO.
Last edited:
PEDRO ESCOVILLA
Senior Member
- Location
- south texas
ok, now that all the semantics have been hammered out. what is the lenght of your run. that is crucial to the formula, suprised no one asked for it yet. formula is: vd= 2xkxIxL divided by csa of conductor. if you need to determone csa of conductor it is (circular mils) cm=1.732xKXixD DIVIDED BY VD PERMITTED. sorry about caps. someone will complain i'm sure. don't fret , i'm not yelling, just lazy, don't want to go backwards. k is resistivity on ohms/1000 ft. ,I = amps, vd is voltage drop. good luck
PEDRO ESCOVILLA
Senior Member
- Location
- south texas
ggunn , i think you are a bit off base on your above statement. tables 310.15 (B) (16) through 310.15 (B) (20) provide for the maximum allowable current carrying capacity of conductors of specific types (cu or al) with specific insulations, in certain atmosphere in given ambient
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
As I said, it's semantics. For example, the "C" tables refer to how many conductors of different types will fit in various types of conduit, not conductors with different types of insulation. But also as I said, it's what the Code implies to me, i.e., my inferrence. You may infer differently. It makes no difference, the data is the same.
kwired
Electron manager
- Location
- NE Nebraska
I will add, multiplier is 2 for two wire circuit and 1.732 for a 3 phase 3 wire circuit.
I agree, same size and type of conductor has several different ampacities in several different tables - the conductor itself does not change the insulation and possibly ambient temperature is what is different in each table.
Take a look at T 310.15(B)(21) and see how much higher the ampacity is for a bare conductor in free air as compared to same size and type in 310.15(B)(16) for most common insulated conductor applications. Most of them are around twice capacity as 75 deg insulated conductors.
T
T.M.Haja Sahib
Guest
You used conductors with different insulation and so not relevant to the point discussed. So take the conductor of the same size, say #1 and same insulation say THHN and proceed as above.
- Occupation
- Licensed Electrician
Better yet, you do as such and then explain what point you are trying to make.
I understand his. Understanding yours is like clutching a handful of water.
- Location
- Massachusetts
Not at all, from my viewpoint.Forgot your own advice.
The comments you singled out are my attempt, however feeble they be, to alleviate some misconceptions... or at the very least, prevent some.
David did exactly what you asked and you say it is not relevant. Why is it not relevant? What else do you need to prove Davids point?
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
We'll never be able to solve world piece at this rate :lol:
You all do realize you scared the OP'er off, :thumbsdown:
How many amps - 608A
Voltage - 480V, 3 phase
Step 1 - Pick minimum conductor size based on ampacity
608 x 1.25 = 760A, OK, you need 760A worth of cable.
Step 2 - Adjust for 2 sets per conduit, therefore 80% adjustment, i.e. 4 sets total
760A/2 x 1.25 = 475A worth of cable in each conduit
Step 3 - Go to Table 310.15 and select parallel conductors that add up to 475A or more
250KCMIL x 2 = 510A @ 30 deg C (minimum allowable conductor)
Step 4 - Do VD calculation to determine if a larger conductor is required
since a distance is unknown, will calculate the maximum distance you can go with a 250KCMIL loaded with 608A, 75 deg C, assuming max. of 3% VD, using steel conduit.
Dmax = approx 310ft, a longer run will require an increase in conductor size.
You all do realize you scared the OP'er off, :thumbsdown:
How many amps - 608A
Voltage - 480V, 3 phase
Step 1 - Pick minimum conductor size based on ampacity
608 x 1.25 = 760A, OK, you need 760A worth of cable.
Step 2 - Adjust for 2 sets per conduit, therefore 80% adjustment, i.e. 4 sets total
760A/2 x 1.25 = 475A worth of cable in each conduit
Step 3 - Go to Table 310.15 and select parallel conductors that add up to 475A or more
250KCMIL x 2 = 510A @ 30 deg C (minimum allowable conductor)
Step 4 - Do VD calculation to determine if a larger conductor is required
since a distance is unknown, will calculate the maximum distance you can go with a 250KCMIL loaded with 608A, 75 deg C, assuming max. of 3% VD, using steel conduit.
Dmax = approx 310ft, a longer run will require an increase in conductor size.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Ampacity of a conductor is a function of both the insulation material and the metal inside; conductors with less heat tolerant insulation have lower ampacities and conductors with higher cross sectional areas of metal have higher ampacities. Voltage drop is a function of the metal only; conductors with higher cross sectional area of metal have proportionately lower resistance and therefore less voltage drop irrespective of which insulation they have or even if they have insulation. A 1/0 wire has only one voltage drop but it can have more than one ampacity. They are somewhat related because the cross sectional area of the metal affects them both. But you know all that, right?
First choose a conductor size based on ampacity, and then upsize it to reduce voltage drop if needed. Never downsize a conductor for voltage drop. That's pretty straightforward.
- Location
- Connecticut
- Occupation
- Engineer
I'm afraid I don't quite understand his point either. As evidenced by...
But let's try this...
#1 THHN with 60deg terminations has an allowable ampacity of 110, #1 THHN with 75 deg terminations has an allowable ampacity of 130 and, #1 THHN with 90 deg terminations has an allowable ampacity of 150. An 80A load over a distance of 500 feet would have the same voltage drop for all three of these conductors, even though they have different ampacities.
Last edited:
- Status