Calculate voltage drop/ampacity for three phase?

So say I have a 200’ 3/0 aluminum triplex wire run.
On line calculators are showing roughly a 1 volt drop over that run from the measured 120 volts legs.
Is trying to get 50 amp three phase service out of that cable a no go? 30 amp?
Is the voltage drop to 119 volts per leg a no go for a good quality 5 hp three phase motor?
Is 3/0 more than needed?

Two items you need to know. Ampacity can be from NEC section 310 and the resistance per thousand feet.

Reference table for resistance: American Wire Gauge, AWG Cable Size Description for Aluminum Wire Cable

Reference for ampacity: aluminum electrical wire ampacity - Google Search

From these tables, the ampacity of 3/0 75C not more than 3 wires in a conduit is 155 amps.
The resistance per 1000ft is .101 ohms.

I would say your aluminum wire is more than adequate. I question the 120 volts. That I know of there are no
115 volt 3 phase motors.

Tom

TDegenhart said:

I question the 120 volts. That I know of there are no
115 volt 3 phase motors.

Tom

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As far as I understand it the service I am measuring is four wire wye.
I have voltage of 120 volts from any leg to neutral, and 208 volts between any leg.

This is one of the motors I need to provide power to:



I understand I am way clear on ampacity with that gage wire- I guess I don’t understand what represents brown out condition of delivered power from a cable such as this,
Is that insufficient amperage or that line drop in voltage?

As Tom pointed out there are no 120VAC motors, But I know there is, seen them for sale in Government surplus site but the Government can get anything it wants and probably made special for them. Otherwise they are not available. But why are you calculating voltage to neutral? Did you use 600' feet? 3 phase does not need a return, 120VAC does. Your motor draws 14 amps so as Tom point out it's more than adequate for the motor you listed. What else is on the line?

Trboatworks said:

View attachment 255352

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So to make things clear:
1. Do you want to connect that motor as 220V delta with your measurement as 208V between the three phases.

Hold on a second guys.
Where did I ever say this is a 120 volt three phase motor.
The voltage drop tables I am using are suggesting that I will see a voltage drop on each leg.
They state about 1 volt.
My question at simplest: does that matter and if so how much cable do I need to throw at this problem to minimize it.

And- my experience with three phase motors is limited to hooking them up to VFD’s so if I am missing so basic point about line voltage for them I am not surprised.

My target from this cable run is 30 to 50 A from several small motor loads .

Trboatworks said:

Is the voltage drop to 119 volts per leg a no go for a good quality 5 hp three phase motor?
Is 3/0 more than needed?

Click to expand...

I would think the small voltage drop is not the issue.

I worked out that the run drop will be about 1V, on paper.

What is the issue is that you will a 220V motor with 208 - 1 = 207V between phases. So roughly 208V.

I recall that others have done that with a three phase transformer.
I recall others have just connected a 220V motor to 208V service. I'm not sure if that is good in the long run.

I might hazard the statement that if the motor runs OK at the supply end of the run, the voltage drop across the
run is not going to be a problem with that size conductor. If one were really worried about this a pair of boost transformers
at the far end to get closer to 240 volts is one way to go.

This bucks to 230:

Larson Electronics - 3 Phase Delta Buck/Boost Step-Up Transformer - 208V Primary - 229V Secondary at 41.7 Amps - 50/60Hz

And some info on topic...

240 vs. 208 volts – Do Motors Care? – Brithinee Electric

The "Do motors care" link reflects my experience. I worked in a shop with a lot of 240V three phase motors running on 208. Some would burn your hand if you got careless.

Bill

Trboatworks said:

Hold on a second guys.
Where did I ever say this is a 120 volt three phase motor.
The voltage drop tables I am using are suggesting that I will see a voltage drop on each leg.
They state about 1 volt.
My question at simplest: does that matter and if so how much cable do I need to throw at this problem to minimize it.

And- my experience with three phase motors is limited to hooking them up to VFD’s so if I am missing so basic point about line voltage for them I am not surprised.

My target from this cable run is 30 to 50 A from several small motor loads .

Click to expand...

Your motor plate confirms the Voltage of the motor you are using! 120VAC voltage drop is what your calculating. 120VAV is measured to neutral and there is NO neutral in 3 phase. Don't get involved with 120VAC when working with 3 phase. The calculator for 120VAC probably assumes 600' of wire 300' out and 300' back, I assume it's an online calculator. Forget any 120VAC voltage drop, concern yourself with 3 phase voltage drop. Using 120VAC voltage drop does not apply.

That's less than 1% voltage drop at full load. Your motors won't complain. Prob more important for motor longevity is voltage drop on startup, and the 208/240 issue.

Froneck said:

Your motor plate confirms the Voltage of the motor you are using! 120VAC voltage drop is what your calculating. 120VAV is measured to neutral and there is NO neutral in 3 phase. Don't get involved with 120VAC when working with 3 phase. The calculator for 120VAC probably assumes 600' of wire 300' out and 300' back, I assume it's an online calculator. Forget any 120VAC voltage drop, concern yourself with 3 phase voltage drop. Using 120VAC voltage drop does not apply.

Click to expand...

My 120 volt statement seems to be throwing everyone.
120 volts IS the measured voltage carried in each conductor.
The damn wire and the resultant voltage drop does not care that three conductors are combined with differing phases to be three phase at the other end in a motor to do work.
The only issue is to make sure to calculate at one way not round trip (I am doing that).

I am using this page to define the type of three phase present on site:

Electrical Service Types and Voltages – Continental Control Systems, LLC

That site states the ground as “neutral” so I am using that convention:



This is the stated vocabulary for three phase systems with Wye having a neutral present:



I am using this site to calculate voltage drop:

Voltage Drop Calculator

I am using this site to specify wire size:

Wire Size Calculator

This whole exercise seems to make clear I was assuming some black art with designing three phase circuits.
I am well used to specifying wire size using a 3% voltage drop table.
It appears my question was that simple- state load and run, pick acceptable drop and buy wire..
The only question is I still might be not in sync with the calculator when it is asking for voltage of three phase run.
Ie: “Source voltage- 120, 240, or 480?”

I am using 120 as this is voltage carried by conductor- am I screwing this up?

So running this at 1/0 aluminum and 50 amps I get 2.24% drop taking my line voltage to 117.32

From site above:
“Line-to-line voltages in three phase systems are typically 1.732 times the phase-to-neutral voltages: sqrt{3}=1.732”

So 117.32x1.732= 203.2 volts three phase so I need to buck from there.
Assuming I am adding a three phase buck transformer I can hit nameplate voltage.
I lose some amperage in the transformers I assume.
Am I getting the basics right here?

A 120/208 service by definition has a neutral connected to the transformer, not just ground. The purpose of these miserable services is to be able to run 120V items like lights and computers on the same line as the three phase machines, allowing the builder to get by with only one service instead of two. You can't run devices designed for 120-0-120 power. The person who thought this system up should have been drowned at birth like an unwanted puppy. You can use any voltage you want for the drop calculation, just get the current right.

Bill

Yes I know WYE 3 phase transformers have a Neutral but it also can be ground! You should ground the "Center of the WYE" because with out grounding your power is floating without ground reference, transformer will isolate ground. 3 phase motors have no connection for a neutral though the neutral is grounded you can not use the same wire for ground and neutral. Neutral wire is current carrying, Ground wire is not to be used as current carrying. So when connecting a 3 phase motor the 4th wire is ground and connected to the motor frame. The 1.732 is correct for Wye but not correct for Delta if you have a center tap and worse for open Delta. Your motor draws no current via the neutral connection because there is no connection for it. If you were to compute voltage drop for 120VAC you must use 600' of wire resistance but your motor is not single phase 120VAC and the neutral current is 0 but in 120VAC the neutral current will be the same as the high line. Don't use 120VAC to determine 3 phase voltage! To think 3 phase take 3 120VAC light bulbs and connect one side of each together and connect the other side of each bulb to 3 phase 208VAC line. They will light with nothing connected to the other side except themselves without any neutral wire! Your trying to mix apples with oranges!
What are you trying to do? You mention connecting 5 Hp motor but the nameplate if for 5KW, you calculating 50Amps but name plate is 14Amps.

Froneck, you are bogging yourself down in a simple calculation. If he draws 14 amps in a leg, it will lose .55 volts from one end to the other. If he reads the voltage to neutral, it will read .55 volts lower. If he reads it to another leg, he will need to correct for the vector angle by dividing by 1.732. The voltage drop is the same, the measurement method is just different. He will have the same drop on the other two lines so that needs to be included. In any case, the voltage drop is trivial compared to the problem of running a 240V motor on 208.

Bill

Trboatworks said:

This bucks to 230:

Larson Electronics - 3 Phase Delta Buck/Boost Step-Up Transformer - 208V Primary - 229V Secondary at 41.7 Amps - 50/60Hz

And some info on topic...

240 vs. 208 volts – Do Motors Care? – Brithinee Electric

Click to expand...

You might be fine if the 5Hp motor is not operated for long intervals.

For a voltage drop around/under 1 V, or 1% of power or less, this will have zero effect in practice.
Commercial specs and commercial delivery of power has wider fluctuation.

Corner cases exist.
A VFD will happily run a 3-phase motor at 12 HZ, sometimes less.
The motor heats up a lot more, as the current is much higher at 1/5 rated speed, if You are actually drawing power near rated load.
Say 100 rpm vs 1500 rpm, and using full 100% load, say 1 HP.
The motor is likely to die in minutes/hours/days.
So if you draw / use just the rated power of the motor, say 1 HP, but are using more current than the motor can dissipate, sooner or later leads to motor failure, by running it "hard and slow".

Otoh you can probably/usually run the motor 10.000 hours at 5% load at 100 rpm vs 1500 rpm rated speed, with any VFD.
As long as the current does not overheat anything, and at 5% load it wont, the motor will probably last rated-lifetime and typically 10-20.000 hours or more for industrial motors.

Motors, coils in them, (and (power) electronics) die mostly of heat,
second from lack of use and old age.

If the motor voltage is 10% higher they wont mind, and 10% lower they wont mind, as long as they don´t cook from excessive amps, running at low speed aka high amps/power/rpm.

But the Name plate Voltage is 220 not 240! I would assume that those motors that have a 208/240VAC name plate are actually wound 218VAC or someplace near. That way at 208 they are less than 10% below and at 240 they are less than 10% above.
I see what hes doing with 120VAC, he's using it as if it's 3 phase and the drop is what 14Amps will do if it is one of the 3 phases.
So he has 203VAC volts which is above the 10% limit plus the drop is calculated at Full load. Unless the motors are operating at 5HP the amps will be lower.
Also using Tom's post of .101 ohms/1000' the voltage drop is only 1.5VAC, using different way then 118.5 + 118.5 = 237VAC X .86666 = 205.4 or 118.5 X 1,732 = 205.4 205 or 203VAC should be OK for a 220VAC motor even if both ran at name plate HP load.

You're in a wood working shop. How often is this machine full out continuously. If you want to get really precise, embed a thermocouple in the windings and connect to a recorder. That's how ratings are established. Next, clamp a recording ammeter on the wires and then determine the RMS (Root Mean Square) of the current. That is the average current and consequently the heating value. Or just wire it up and use it. Keeping it clean will do more to keep the temperature down than anything.

Tom