Voltage Drop

[jfpe]

I have a question regarding voltage drop calculations. I'm sure it's painfully simple, but it's been troubling me for a little while.

The electric services where I work are predominantly 120/208VAC 3-phase. Apartments and single family homes get 120/208VAC "single phase" or network service, which is two hots and a neutral from the 3-phase distribution system.

When I need to do 3-phase voltage drop calculation, I turn to Chapter 9, Table 9 in the NEC and calculate voltage drop as
VD (line-neutral) =R x Circuit Length x I
To calculate line to line voltage drop, I multiply by 1.73, so VD = 1.73 x R x L x I

When I need to calculate voltage drop in 120/208VAC "single phase" circuit, i.e. two hots and a neutral or commonly two hots and no neutral I want to follow the explanatory text following 215.2(A) in the NEC Handbook. and calculate voltage drop as VD = 2 x L x R x I.

What is the right way to calculate line to line voltage drop for 208VAC? Do I need a factor of 1.73 or 2?

Thanks,
John

 

[trosepe]

Use 1.73 for three phase L-L VD

Use 2 for single phase L-L or L-N VD.

Be sure to use the effective 'Z' of the circuit from table 9 where you are indicating 'R' in your thread.

 

[dpc]

You might want to double check your application of the formulas. Current through a resistance produces the same voltage drop regardless of the magnitude of the voltage source. (e.g. 10 A through 1 ohm = 10 volts, no matter what the source voltage is.)

I don't have an NEC in front of me, so I can't check Table 9.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[tinfoil]

Assuming decent voltage balance and phase angles on the service:

The BALANCED portion of the load (i.e. the smaller of the two 'hot' currents) will flow out and back through the hots, so VD = 2 * R * L * I applies.

The UNBALANCED portion (the difference between the two hot currents, if any) will flow back through the neutral, so you would use the other formula IF THE NEUTRAL is the same gauge wire as the hots.

 

[jfpe]

Thanks.

I'm still having trouble seeing where I draw the line. If I have three hots (120 degrees out of phase )and a neutral, I use VD=1.73*L*R*I. If I have only two hots (still 120 degrees out of phase) and a neutral, do I use the same formula? What if I drop the neutral?

Thanks,

John

 

[jimbofitz]

I'm no expert in home wiring, but when you have 2 hots in a home service isn't it a center tapped single phase source? That means hot-hot would be 240Vrms and hot-neutral would be 120V?

So in your last question John, you won't be 120 degrees out of phase since there is only one phase coming off the grid.

In larger power systems 3-phase is used and you will see the 1.73 factor.

Regards,
Jim

 

[jfpe]

No, around here, an apartment or single family home is 2 hots and a neutral. The hots are 120 degrees out of phase, so its 208VAC between phases. Phase to neutral is 120VAC.

This configuration is common in high density urban areas, but rare else where.

Thanks,
John