Voltage drop

[Jerseydaze]

Ok can someone tell me how to calculate the voltage drop .I have 85ft of pvc
a 300w pool light at 12 volts and a transformer with taps of 12v 13v and 14v
I need to know what wire size(THHN) and how you found it.

 

[hardworkingstiff]

VD = I*R

Voltage Drop = Current * Resistance (in the wire)

 

[bob]

Ok can someone tell me how to calculate the voltage drop .I have 85ft of pvc a 300w pool light at 12 volts and a transformer with taps of 12v 13v and 14v. I need to know what wire size(THHN) and how you found it.

First if the conductor is underground you will need THHW not THHN.
Look in the back of the NEC at table 9 Conductor Properties to get the AC resistance of the conductor.
Try #10 copper. R = 1.2 per 1000 ft or 0.102 for 85 ft.
amps = watts/volts = 300/12 = 25 amps. #10 is good for 30 amps.
If the lamp is on for 3 hrs or more you will need to increase the wire size to #8.

Volt Drop = amps x resistance = 25 x 0.102 x 2 = 5 volts. You multiply
by 2 because you have a total distance of 2 x 85 ft to cover the hot
leg and the neutral.
I would suggest you locate the transformer closer to the pool to reduce the voltage drop.

 

[Jerseydaze]

what am I allowed 3% or 5%?

 

[Jerseydaze]

Ok can someone tell me how to calculate the voltage drop .I have 85ft of pvc a 300w pool light at 12 volts and a transformer with taps of 12v 13v and 14v. I need to know what wire size(THHN) and how you found it.

First if the conductor is underground you will need THHW not THHN.
Look in the back of the NEC at table 9 Conductor Properties to get the AC resistance of the conductor.
Try #10 copper. R = 1.2 per 1000 ft or 0.102 for 85 ft.
amps = watts/volts = 300/12 = 25 amps. #10 is good for 30 amps.
If the lamp is on for 3 hrs or more you will need to increase the wire size to #8.

Volt Drop = amps x resistance = 25 x 0.102 x 2 = 5 volts. You multiply
by 2 because you have a total distance of 2 x 85 ft to cover the hot
leg and the neutral.
I would suggest you locate the transformer closer to the pool to reduce the voltage drop.

I see where you got 1.2 in table 9 but isn't that chart for uncoated wires

 

[hardworkingstiff]

I believe most building wire is uncoated. I believe the coated column would be used for "tinned" wire or something like that.

 

[iwire]

what am I allowed 3% or 5%?

Your allowed 100% voltage drop unless there is a local code that says otherwise.

The amount you allow is a design choice, with 12 Volt lamps 20 or 3 volts of drop will result in a noticeable reduction of light but in turn the lamp will last longer.

 

[bob]

You have lost a significant amount of light.

Resistance of the bulb 300 w/12 v= 25 amps. R = 12 v/ 25 a = 0.48 ohms
If you add the resistance of the wire R = 0.48 + 0.204 = 0.684 ohms
Amps = 12 v/ 0.684 = Amps = 17.5 amps. Lamp watts = 17.5 x 17.5 x 0.48 = 147 watts.
You have lost 50% of the lamp output. You have a 42% VD.

 

[peter d]

Voltage drop can work to our advantage sometimes. Rather than buy and wire those pesky buck/boost transformers, we can simply run hundreds of feet of wire to achieve the desired "buck" voltage.

Or, let's say you need to run a 120 volt outlet but you only have 277 volts available? No problem! Simply wire the run several hundred feet away from the receptacle location in one direction, then double it back on itself and Presto! No transformer required!


:lol: :lol:

 

[ramsy]

Peter, you might have trouble with code.

Voltages. ..The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.

If the 277vac breaker isn't listed for 120vac, you may also fail this one:

(for) The overcurrent protective devices ..Listed products applied in accordance with their listing shall be considered to meet the requirements of this section.

 

[ramsy]

Your allowed 100% voltage drop unless there is a local code that says otherwise.

Bob, you better move that one decimal to the left. 120v-108v=12 or 10%

AHJ's don't need v-drop rules to fail branch voltages. If the equipment has a nameplate or otherwise lists a voltage, these listings are enforceable: NEC 100.4, 110.3(B), or 100 Voltage, Nominal "..within a range that permits satisfactory operation of equipment."

For 120vac systems, the unavoidable-equipment nameplates found in most inside wiring, include computer, printer, copier, fax, monitor, TV, and other rectified power supplies. While 100vac min-input voltage exists on some nameplates, plenty of rectified nameplates still correspond to the minimum-108vac "utilization voltage" of ANSI C84.1. The NEC enforces the equip. requirements, rather than ANSI C84.1

I would say, NEC rules do regulate voltage drop indirectly, but industry-diverse NEC panel members have historically confined voltage drop complexity to FPN's, perhaps accurately anticipating misapplication by a well represented fraction of authorities having jurisdiction (AHJ).

 

[iwire]

Your allowed 100% voltage drop unless there is a local code that says otherwise.

Bob, you better move that one decimal to the left. 120v-108v=12 or 10%

No, I will stick with my original answer. 8)

By your interpretation dimmers, Soft-starts, VFDs etc would all be unacceptable.

AHJ's don't need v-drop rules to fail branch voltages. If the equipment has a nameplate or otherwise lists a voltage, these listings are enforceable: NEC 100.4, 110.3(B), or 100 Voltage, Nominal "..within a range that permits satisfactory operation of equipment."

Lets look at your code references.

Article 100's "Voltage Nominal" is just a definition, it is not a code section or directly enforceable.

I assume 100.4 was meant to be 110.4

110.4 Voltages.
Throughout this Code, the voltage considered shall be that at which the circuit operates. The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.

Notice that if you read this section it prevents over voltage to the equipment it does not prevent under voltage to equipment.

110.3(B)

(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.

True enough, if you can find a minimum voltage included in the listing or labeling than you may be onto something.

However just because a piece of equipment is tagged 120 volts does not mean suppling it with 50 volts is violation anymore than a suppling it with 119.9 volts.

I am well aware of ANSI C84.1, it is not a code, it is a standard which unless there is a local rule otherwise is unenforceable.

(Many power companies are required to follow ANSI C84.1 by the local power commissions etc.)

I have real doubts that you will find any electrical inspectors enforcing ANSI C84.1.

But lets keep in mind in this thread we are talking about incandescent lamps here and they are designed to run from 0 to 100% of their 'nameplate'.

For 120vac systems, the unavoidable-equipment nameplates found in most inside wiring, include computer, printer, copier, fax, monitor, TV, and other rectified power supplies.

My Dell PC has says it can run fine with voltages from 90 to 136 volts without changing any settings.

I would say, NEC rules do regulate voltage drop indirectly, but industry-diverse NEC panel members have historically confined voltage drop complexity to FPN's, perhaps accurately anticipating misapplication by a well represented fraction of authorities having jurisdiction (AHJ).

We will have to agree to disagree on the NEC regulating voltage drop indirectly.

IMO the reason the NEC has not moved the FPNs to code is simple.

It is not an electrical hazard assuming the rest of the NEC is followed and is outside the stated purpose of the NEC.

90.1 Purpose.

(A) Practical Safeguarding. The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity.

(B) Adequacy. This Code contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance will result in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.

As a final comment I point out that there are a couple spots in the NEC that do directly control voltage drop.

687.4

695.7

If 110.3(B) could be used to control voltage drop than these sections along with the FPNs where unneeded.

Bob

 

[hardworkingstiff]

687.4 ?

 

[iwire]

687.4 ?

Maybe your book is missing that section :lol:

Or more likely I just goofed up.

647.4(D)

 

[hardworkingstiff]

Thanks Bob

 

[ramsy]

You have lost a significant amount of light.

Resistance of the bulb 300 w/12 v= 25 amps. R = 12 v/ 25 a = 0.48 ohms
If you add the resistance of the wire R = 0.48 + 0.204 = 0.684 ohms
Amps = 12 v/ 0.684 = Amps = 17.5 amps. Lamp watts = 17.5 x (17.5 x 0.4&) = 147 watts.
You have lost 50% of the lamp output. You have a 42% VD.

Nice way to see what Volts and Power the equipment is getting.

1) I. = P/E ----------- Get load Amps
2) R. = E/I ----------- Get load impedance
3) Rt = R+R(wire). Get tot. impedance
4) Ic = E/Rt --------- Get choked Amps
5) Pc = Ic*Ic*R ----- Get choked Power
6) Ec = Ic*R -------- Get choked Volts

Here's another method, using either table data or field measurements for the same variables.

1) Plan the install with a target voltage, based on equip. listings.
2) Since Z doesn't vary with Amps, choked Volts = (Nominal - Z*I). Use "what-if" values for I or Z.
3) Start Z within 0.25. Which load fails target voltage depends on install choices below.

____ Target__Voltage ________ Table Data Or Field Measurement
1) I  = P/E ----------------- Get load Amps. or Clamp meter
2) Ec = Nominal-Z*I --------- Pick Target Voltage (choked Volts)
3) Z  = Tot.feet/1k*Ohm@75?C  NEC Tbl.8 or Meter Z=(E-Ec)/I
4) Pc = Ec*I ---------------- Get choked power
5) Ic = Pc/E ---------------- Get choked Amps
____ Check__Wire__Size ______
6) CM   = KIL/VD ------------ Kcu@75?C=12.9, L=Tot.Feet, VD=E-Ec 
7) De   = Ic/Iwire@Max?C ---- Derating Equivalent @90?C or @75?C
8) PF   = Er/E -------------- Tbl.430-150 Motor Spec.or Metered
9) ASCC = E/Z --------------- Available Short Circuit Current

Here's the proof for both methods Z=VD/I and Z=(L/1000)*R

                 Measured --> Proof NEC Tbl 8 or 9
   E  = Nominal 12.0 vac.     12 AWG CM=6530
1) I  = P/E     25.0 Amps     L=72.9 Tot.feet
2) Ec = Nom.-Z*I 8.4 vac.     Ohm@75?C R=1.98
3) Z  = VD/I     0.144 Ohm -> L/1000*R = 0.144 ohm
4) Pc = Ec*I   210.0 Watts    choked Power
5) Ic = Pc/E    17.5 Amps     choked Amps
   VD = E-Ec     3.6 vac ---> KIL/CM=3.6 vac, K=12.9
=========== ================= ====================
6) CM   = KIL/VD          6530.6 Circular Mils
7) De   = Ic/Iwire@Max?C  17.5/30 = 0.583 Max Derating @90?C
8) PF   = Er/E            Get motor Pwr.Fact. or Meter
9) ASCC = E/Z             83.3 Amps

While not NEC enforced, IEEE 1100 (The Emerald Book) may clarify the undefined terms "low impedances" in NEC 250.2, and "circuit impedance", as used by NEC 110.10, to "..clear a fault ..without damage".

"impedance of the equipment-grounding and neutral conductors between equipment and the neutral-to-ground bond should not exceed 0.25 ohm."

For 120vac systems, using Z values within 0.25 should keep end-of-line Target voltages well within 108vac.

 

[ramsy]

Notice that if you read this section (110.4) it prevents over voltage to the equipment it does not prevent under voltage to equipment.

Voltages. ..The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.

Yes, I see. Mike Holt's 99 NEC guide book also shows the code violation if equip. rated 208vac is installed on a 240vac nominal circuit, regardless of voltage sags. 240vac equip. on 208 nomimal is not addressed here.

However just because a piece of equipment is tagged 120 volts does not mean suppling it with 50 volts is a violation anymore than suppling it with 119.9 volts.

Mike Holt uses 110.3(B) against a nominal voltage source that is less than the equipment rated.

Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.

Does Mike's read ignore some practical applications? What issues exist with dimmers, Soft-starts, VFDs etc., if line nominal matches the device listing?

I have real doubts that you will find any electrical inspectors enforcing ANSI C84.1. But lets keep in mind in this thread we are talking about incandescent lamps here and they are designed to run from 0 to 100% of their 'nameplate'.

While 110.3(B) enforces the listing/labels, OEM home/office equip. labels remain close to 108vac / ANSI C84.1. So, wiring that ignores ANSI C84.1 risks the liability of supplying consumer-electronics, or any automation control equip. that designs and labels around that STD.

My Dell PC says it can run fine with voltages from 90 to 136 volts without changing any settings.

My DVD has that range, and my VCR has a 110vac nameplate. If a contractor installed a 90vac branch voltage in my business, I'd ask them to either replace the rest of my consumer electronics with brand new 90vac nameplates, or replace the wiring.

 

[ramsy]

IMO the reason the NEC has not moved the FPNs to code is simple. It is not an electrical hazard assuming the rest of the NEC is followed..

Then we agree, v-drop limits should remain confined to FPN's.

Reasons may exist for direct control of some v-drop issues, but the electrical hazards below, not addressed by the NEC, may be responsive to an impedance STD, such as 0.25 Ohm in ?6.4.1.1.4 of IEEE 1100.

1) A hazard during a voltage sag, without UPS protection, can occur by dipping below the ride-through capability of switch-mode power supplies that reset or lockup during low voltage events. Automation controls with such switch-mode supplies, common to residential, commercial, and industrial property include: burglar, fire alarm, and all associated devices, and emerg. systems, portable life suport, SCR's, rectified lighting, ballasts, etc.. The hazard would be the failure of essential or emergency systems, and maintaining a Z-STD would point to deviices before circuit design issues.

2) Motor imbalance also comes to mind as not enforced by the NEC. A voltage sag single-phasing, > 1% vac can cause a 10% current imbalance. The larger the motor the greater the overcurrent, or hazard in ungrounded delta instalations. A Z-STD would balance circuits at install, and simplify the boundaries of current & voltage markers, since Z does not vary with load.

3) The NEC does not address design issues, such as elevator cars intermitantly starting simultaiusly, regardless of other system failures, from rare sags in voltage. Rapidly ruling out high or low Z-STD values, opens or shorts, rapidly points to more likely trouble spots, such as supply or regulation issues.

As a final comment I point out that there are a couple spots in the NEC that do directly control voltage drop.
687.4 , 695.7
If 110.3(B) could be used to control voltage drop than these sections along with the FPNs where unneeded.

To the extent the nameplate compliance of 110.3(B) includes equipment-voltage limits, branch circuits that fail that minimum-voltage target can not be used with such equipment. 110.3(B) makes building circuits outside of ANSI C84.1 poor advice, and clarifies their is little need for a v-drop standard.

 

[iwire]

.

However just because a piece of equipment is tagged 120 volts does not mean suppling it with 50 volts is a violation anymore than suppling it with 119.9 volts.

Mike Holt uses 110.3(B) against a nominal voltage source that is less than the equipment rated.

Connecting 240 rated equipment to a 208 circuit would be a violation as the nominal voltage would be 208.

Connecting 240 equipment to a 240 volt circuit that is experiencing voltage drop is not a violation as the nominal voltage of the circuit is still 240.

Ramsey as complicated as you like to make this the NEC does not control voltage drop except in the two Sections I posted.

Bob

 

[iwire]

IMO the reason the NEC has not moved the FPNs to code is simple. It is not an electrical hazard assuming the rest of the NEC is followed..

Then we agree, v-drop limits should remain confined to FPN's.

Reasons may exist for direct control of some v-drop issues, but the electrical hazards below, not addressed by the NEC, may be responsive to an impedance STD, such as 0.25 Ohm in ?6.4.1.1.4 of IEEE 1100.............

All that you posted in this post are design issues and not safety issues that are the concern of the NEC.

Automation controls with such switch-mode supplies, common to residential, commercial, and industrial property include: burglar, fire alarm, and all associated devices, and emerg. systems, portable life suport, SCR's, rectified lighting, ballasts, etc..

All those systems have their own specific requirements.

I know little of bugler panels but any Listed Fire alarm panel will deal with voltage sags or switch to battery.

Again you want to write a novel thankfully you do not write code sections, the NEC would be ten volumes :lol:

The simple fact is the NEC does not control voltage drop other than fire pumps and sensitive electronic equipment.

Now that is not to say good design should not be exercised.

FWIW most of our job specs have voltage drop requirements that we must follow. 8)

Also many energy codes have voltage drop restrictions.

Just not the NEC.