When does voltage drop impact any ampacity calculation?

[bjp_ne_elec]

Maybe this is a dumb question - but I'm going to ask it anyway. When does the length of a run and it's subsequent voltage drop start to impact any ampacity caluculations (i.e. derating) - assuming it does.

Thanks,

Brett

 

[jbwhite]

Re: When does voltage drop impact any ampacity calculation?

All the wire size factors should be done first.

VD is optional and therefore done last.

 

[jbwhite]

Re: When does voltage drop impact any ampacity calculation?

Originally posted by jbwhite:
All the wire size factors should be done first.

VD is optional and therefore done last.

To edit my own post... I do all the wire size calculations. then I do the VD calculation seperately..

Last I choose whichever wire size is greater.

I do not include VD in with the other calcualtions.

 

[templdl]

Re: When does voltage drop impact any ampacity calculation?

I doubt if current will be and issue because current will probably go down with less voltage at the load. But, it could also increase with motors. However, I doubt that the conductors would be sized so closely that it would compromise the sizing of the conductors and any derating that may be required.
But here's how I look at it: Resistive loads are simple and motor loads require more attention.
Remember that motor do work and the work is a factor of voltage and current. If that motor is required to put out a give amount of HP it requires more current as the voltage is decreased to provide the HP.
With resistance loads, as the voltage is reduced resistance loads such as lights simply get dimmer.
POCOs who need to reduce power consumption during heavy power usage often reduce the voltage with the intent is reducing the current which in turn reduces power usage.

 

[jbwhite]

Re: When does voltage drop impact any ampacity calculation?

I been thinking.. maybe I misread the OP.
At what distance does voltage drop become a problem?

That depends on amps, volts and wire lenght.
I think the code says it becomes something to consider at 3 per cent.

 

[iwire]

Re: When does voltage drop impact any ampacity calculation?

215.2 FPN No. 2:Conductors for feeders as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, will provide reasonable efficiency of operation.

 

[charlie b]

Re: When does voltage drop impact any ampacity calculation?

Originally posted by bjp_ne_elec: When does the length of a run and it's subsequent voltage drop start to impact any ampacity calculations (i.e. derating) - assuming it does.

Never!

Ampacity is the amount of current that a conductor can handle, under conditions of use, without exceeding its temperature limit. That has nothing whatsoever to do with how much current you are actually putting through the wire. It is just a limit on what it is allowed to carry.

In addition, you never "derate" for voltage drop. The word "derate" has to do with changing the ampacity. A conductor that is rated for 100 amps if it were in the same raceway as two other current carrying conductors would have to be derated to 80 amps if you added one more current carrying conductor to the same raceway. But here again, that is only a declaration of the maximum limit for current. It does not tell you how much current is flowing at any point in time.

If you have a long run with a high voltage drop, and if you are supplying a motor, it is possible that the motor will draw so much current that you could exceed the ampacity of the conductors. This would not change the ampacity of the conductor. Rather, it would involve a current flowing at a value that exceeds the ampacity.

[ January 23, 2006, 06:56 PM: Message edited by: charlie b ]

 

[peter]

Re: When does voltage drop impact any ampacity calculation?

I've been working on this precise question today. My calculations seem to agree with the calculator found on Geralld Newton's website:
http://electrician.com/
Assuming 120 volts, single phase and 3% voltage drop, I get:
#14 31.8' 15 amps
#12 46.7' 20 amps
#10 49.6' 30 amps
#8 57.8' 40 amps
#6 61.1' 60 amps
[Now let's see Mr. Holt's fine forum goober my fine tabulation up.]
These figures seem painfully low, but, as I've said, they seem to agree with the results from the calculator. If anyone sees any errors in these figures, please speak up.
~Peter

 

[sceepe]

Re: When does voltage drop impact any ampacity calculation?

Assuming 120 volts, single phase and 3% voltage drop, I get:
#14 31.8' 15 amps
#12 46.7' 20 amps
#10 49.6' 30 amps
#8 57.8' 40 amps
#6 61.1' 60 amps
[Now let's see Mr. Holt's fine forum goober my fine tabulation up.]
These figures seem painfully low, but, as I've said, they seem to agree with the results from the calculator. If anyone sees any errors in these figures, please speak up.

using same assumptions and 0.9 power factor. I get a 3.42 volt drop at 20A, 120V, #12 awg. 3% of 120V is 3.6 Volts. I calculate you can get to around 49' before you hit 3.6V drop.

The problem is you would never put a continuous load of 20A on a 12 awg circuit fed from a 20 breaker. If you assume that the continuous load in like 80% of the rated current, then you can get a little farther out before worrying about voltage drop. Heres what I calculate for 80% rated current, 120V, 1 phase, 0.9pf
#12 61'
10 68
8 77
6 87
4 105
3 90
2 96
1 103
1/0 114
2/0 113
3/0 120
4/0 124
250 124
300 126
350 128
400 127
500 126


I think the original post may have been asking at what point do have to worry about voltage drop if you have already oversized the wire for the derating factors. In my experience, not very often. If you have been conservative in your load calculations and you have derated the wire (i.e. oversized it) for things like temp, or number of conductors in the conduit, when you go to do the voltage drop calcs, you find you don't have to worry cause your already running #10 125' for a 12A 120V load. The only time I find I have to upsize conductors is when they get up over the 200' length. However, I am pretty conservative with my circuit loads. If you put 13 convenience outlets on a 20A breaker because the code says you can, you may need to be more concerned about the voltage drop. Heck, with diversity you might still be ok....

 

[spsnyder]

Re: When does voltage drop impact any ampacity calculation?

Maybe the post was concidering this scenario... If you assume 120V at the load and calculate an amperage of 10 amps. Then you calculate the voltage drop and you calculate you have a voltage at the load of 12V. Now if you recalculate the amperage you get 100 amps since the voltage at the load is not 120V, but 12V in the second iteration. Recalculating the Voltage drop might yield 6V at the load... Oh no we have an unstable possitive feedback system. . This is obviosly an exagerated scenario, but I think it might have been what the OP was asking.

More commonly, if you perform the second iteration with the new voltage you'd obtain an amoerage very close to the assumed amperage. Who says you can't solve an equation with two unknowns. This problem is almost identical to solving a pump equation where flow is based on head which is based on flow (dynamic head).

 

[sceepe]

Re: When does voltage drop impact any ampacity calculation?

Oh no we have an unstable possitive feedback system.

I don't think so, you basically have two impedances in series. The impedance of the wire and the impedance of the load. Is it your contention that the impedance of the wire changes with current?

Granted the voltage drop equation is current dependant but that is because we are using impedance and current to find the unknown (in this case voltage). The assumption inherent in the voltage drop equation is that the impedance is constant. Anyway, does it physically make sense for 150' of copper wire to have a impedance that varies with current?

 

[hardworkingstiff]

Re: When does voltage drop impact any ampacity calculation?

I find this quote interesting.

"Many inspector members of the IAEI (International Association of Electrical Inspectors),
ASHI (American Society of Home Inspectors) and NAHI (National Association of Home
Inspectors) have reported feeling comfortable with gradually increasing voltage drops as
high as 8 to 10 percent where the line voltage is near the nominal 120 volts."


mikeholt.com/documents/calculations/formulas/5-PercentVoltageDrop.pdf

 

[peter]

Re: When does voltage drop impact any ampacity calculation?

Mr. Jordan,
I don't know what the 0.9 power factor has to so with this.
Of course, I would aim for an 80% capacity but is there anything in the Code that requires this? [Fine HomeBuilding's latest issue doesn't seem to think so.] I am aware of the 125%/80% derating for continuous loads such as lighting.
Anyway, I spent the afternoon with the hideous San Diego sun blazing away drawing up a chart just for #14 THHN. Briefly:
3% VD
15A 39.1'
12A 48.9'
9A 65.1'
6A 97.7'
3A 195.4'
The end result was a couple of those calculus type curves. It seems that around 7 or 8 amps, the curve changes from its steep plunge from the 15 amp point and starts to level out. So I would modestly guess that at about 50% rating of the breaker/wire is about the point at which VD should not be a problem. I hope that answers the guy's question.
~Peter

 

[RCinFLA]

Re: When does voltage drop impact any ampacity calculation?

In engineering terms, most of the NEC specs for wire size is done based on thermodynamic limitation of heat build up in the wire, not on voltage drop. A few watts per foot of wire may not sound like alot but when wrapped with heat insulation (conduit, wire insulation, etc.) can build up a significant rise in temp that over time can degrade insulation material.

For 120 v + wiring usually wire heat build up is first problem unless wire run is very long.

Now if you are dealing with very long runs so within the NEC code amperage specs you have too much voltage drop then the % voltage drop spec comes into play.

You have to be careful about voltage drop for things like induction motors with high starting current. At the run current you might not have too much voltage drop problem but under the heavy starting current there could be significant voltage drop. If this startup voltage drop is bad enough the motor might not spin up thereby stays at or near lock rotor current, at least until circuit breaker or motor heat protector switch trips.

 

[iwire]

Re: When does voltage drop impact any ampacity calculation?

Welcome to the forum RCinFLA.

Originally posted by RCinFLA:
You have to be careful about voltage drop for things like induction motors with high starting current. At the run current you might not have too much voltage drop problem but under the heavy starting current there could be significant voltage drop. If this startup voltage drop is bad enough the motor might not spin up thereby stays at or near lock rotor current, at least until circuit breaker or motor heat protector switch trips.

A question, have you ever seen a motor just sit there and not spin up due to voltage drop?

I will not deny it is possible but it is IMO very unlikely, the mechanical load would have to be one with very high starting torque requirements.

The voltage drop will extend the time until the motor reaches full speed and the current drops down to the running current.

But at the same time it will limit the peak current the OCP will have to hold.

A bit of voltage drop on motor conductors can act like a soft starter and helps prevent the other loads on the same distribution systems from seeing as much of a voltage dip while the motor is starting.

As always everything is a trade off.

 

[ramsy]

Re: When does voltage drop impact any ampacity calculation?

Originally posted by iwire:
The voltage drop will extend the time until the motor reaches full speed.. A bit of voltage drop on motor conductors can act like a soft starter..

Bob, do you have any references to practical applications of soft starting with wire size, or is this mostly experimental?

I'd like to fix my kids electric car (DC) that burns the carpet every time it starts, but without smoking insulation or starting a fire.

 

[iwire]

Re: When does voltage drop impact any ampacity calculation?

Originally posted by ramsy:
Bob, do you have any references to practical applications of soft starting with wire size, or is this mostly experimental?

No I have not every done any calculations for this.

But it is what happens.

A good example is a question asked here a while ago.

A chop saw on a construction site would trip the breaker when plugged into the outlet directly.

Once a 100' cord was used with it the tripping stopped. The long cord reduced the start up current. Of course the saw certainly took longer time to spin up.

 

[ramsy]

Re: When does voltage drop impact any ampacity calculation?

Originally posted by iwire:
A chop saw on a construction site would trip the breaker when plugged into the outlet directly. Once a 100' cord was used with it the tripping stopped. The long cord reduced the start up current.

A few 100' cords got damaged and chucked during trade show work this month. Will try to take one home and fix it, incase needed for a similar purpose. Thanks

Maybe few practical uses exist for soft starting with wire size, since the impedance(Z) can't be removed after the load is started. I'll wait for other ideas to soft start my kids electric car.

 

[iggy]

Re: When does voltage drop impact any ampacity calculation?

VD is optional and therefore done last.

Be careful - many energy codes (including MA) dictate maximum voltage drop, where it has only been a FPN in the NEC.

Also don't forget to increase the size of the EGC when increasing phase conductors for VD.

 

[iwire]

Re: When does voltage drop impact any ampacity calculation?

Nevermind

[ February 03, 2006, 04:16 AM: Message edited by: iwire ]