Voltage Drop

[wirenut46]

I have a 2/0 SER cable 175 feet on a 150 amp breaker i need to know how to figure the voltage drop this is in a multifamily building.:ashamed1::ashamed1::ashamed1:

 

[ActionDave]

Voltage drop is load dependent. You need to find that out first.

 

[Julius Right]

I agree with ActionDave, of course. The voltage drop depends on load: current and power factor [cosfi].
VD=R*I*cosFi+X*I*sinFi the voltage drop from the supply voltage-live to neutral-and the cable end live-to-neutral. For three-phase system, if the referred voltage it is live-to-live then you have to multiply by sqrt(3).
As an example let's say cosFi=0.8 and sinFi=0.6 ?according to NEC 2014 ch.9 Table 9 R=0.16 ohm/1000 ft. [a.c.60 Hz 75oC aluminum]
And XL=0.043 ohm/1000 ft. and considered I=150 A VD=4.2 V [live to neutral].
In order to check the level [NEC recommends a total drop of 5%] you need to know the rated voltage also.
For 480 V [277 V live-to-neutral] 4.2 V will be 1.5%.

 

[Dennis Alwon]

Use this online calculator and see what info you need http://www.elec-toolbox.com/calculators/voltdrop.htm

 

[wirenut46]

The voltage is 240.

 

[wirenut46]

And its feeding a 125 amp subpanel..

 

[wirenut46]

I have never been asked about voltage drop on and ser cable. Its a little confusing.

 

[wirenut46]

Is the maximum 3% or 5%.

 

[John120/240]

Is the maximum 3% or 5%.

Voltage drop is a design consideration only. The NEC is only a recommendation in respect to voltage drop.

 

[kwired]

I have never been asked about voltage drop on and ser cable. Its a little confusing.

voltage drop would be based on size of conductor(s) in the cable. Also remember you have 120 and 208 or 240 volts - so if you have a lot of imbalance and only 120 volts the drop is more severe then it is for 208 or 240.

But as has been mentioned NEC does not specify a specific voltage drop for general applications, the 3 and 5 percent figures given are in an informational note and are nothing more then suggestions. However there can be some AHJ's that have adopted some local amendment that does place limitations on voltage drop.

 

[Smart $]

Is the maximum 3% or 5%.

Voltage drop is a design consideration only. The NEC is only a recommendation in respect to voltage drop.

NEC recommends 5% maximum total for feeder and branch circuit (i.e. from service point to load), and not more than 3% on any one branch circuit. [Paraphrased with speculation... ]

 

[ActionDave]

But as has been mentioned NEC does not specify a specific voltage drop for general applications, the 3 and 5 percent figures given are in an informational note and are nothing more then suggestions. However there can be some AHJ's that have adopted some local amendment that does place limitations on voltage drop.

It should remain nothing more than a suggestion IMO. The available voltage from the power company can vary more than 5%. And the vast majority of electrical equipment can operate just fine from 250V all the way down around 210V, which is what, like 20%?

 

[Dennis Alwon]

NEC recommends 5% maximum total for feeder and branch circuit (i.e. from service point to load), and not more than 3% on any one branch circuit. [Paraphrased with speculation... ]

Yeah but when the branch circuit originates at the service equipment why should it be be figured at 3%

 

[Smart $]

Yeah but when the branch circuit originates at the service equipment why should it be be figured at 3%

Don't ask me...

...I didn't write the recommendation.


(Meaning I would base the recommendation on voltage at the end load)

 

[iwire]

Yeah but when the branch circuit originates at the service equipment why should it be be figured at 3%

Don't ask me...

...I didn't write the recommendation.


(Meaning I would base the recommendation on voltage at the end load)

I wonder how this is dealt with in the energy codes that have voltage drop limitation requirements.