Cumulative Voltage Drop - CR4 Discussion Thread

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Selk Associate Join Date: Dec 2016 Posts: 48	Cumulative Voltage Drop 10/09/2018 12:34 AM
	Which is the correct equation for calculating the cumulative voltage drop? For example, the power supply sequence is Transformer --> MDB --> SMDB ---> DP ---> LP ---> 1PHASE FINAL CIRCUIT VD1 = 3 Phase voltage drop from transformer to MDB VD2 = 3 Phase voltage drop from MDB to SMDB VD3 = 3 Phase voltage drop from SMDB to DP VD4 = 3 Phase voltage drop from DP to LP VD5 = 1 Phase voltage drop from LP to FINAL CIRCUIT Cumulative VD = VD1+VD2+VD3+VD4+VD5 (OR) Cumulative VD = (VD1+VD2+VD3+VD4)/1.732 +VD5 If the second equation is correct, why do we have to divide the three phase voltage drops by 1.732? Thanks!
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	Pathfinder Tags: BS 7671 Cumulative voltage drop

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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 32175 Good Answers: 839	#1 Re: Cumulative voltage drop 10/09/2018 3:18 AM
	It depends upon the installation's configuration, which has been withheld from the forum. __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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Rixter Guru Join Date: Apr 2010 Location: About 4000 miles from the center of the earth (+/-100 mi) Posts: 9910 Good Answers: 1141	#2 Re: Cumulative Voltage Drop 10/09/2018 10:06 AM
	It depends on your circuit. If your final single phase circuit is phase-to-neutral and you measure your voltages at the 3 phase points as phase-to-phase, then you divide the 3 phase voltage measurements by the square root of 3 (1.732...) to calculate the voltage drop you would have measured if you had been measuring phase-to-neutral at the 3 phase points.
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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 32175 Good Answers: 839	#4 In reply to #2 Re: Cumulative Voltage Drop 10/09/2018 3:14 PM
	...though this assumes that there is no other load on the circuit. Without the original poster's circuit description, it is impossible to tell. __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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Selk Associate Join Date: Dec 2016 Posts: 48	#6 In reply to #4 Re: Cumulative Voltage Drop 10/09/2018 11:55 PM
	There are other single phase lighting circuits.
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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 32175 Good Answers: 839	#8 In reply to #6 Re: Cumulative Voltage Drop 10/11/2018 7:10 PM
	<...single phase lighting circuits...> 3-phase lighting circuits are somewhat uncommon... __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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TonyS Guru Join Date: Aug 2010 Location: Resting under the Major Oak Posts: 4347 Good Answers: 181	#3 Re: Cumulative Voltage Drop 10/09/2018 11:07 AM
	Your question has been answered on the IET forum. __________________ The reports of my death are greatly exaggerated.
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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 32175 Good Answers: 839	#5 In reply to #3 Re: Cumulative Voltage Drop 10/09/2018 5:52 PM
	Thank goodness for that. NEXT! __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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Rixter Guru Join Date: Apr 2010 Location: About 4000 miles from the center of the earth (+/-100 mi) Posts: 9910 Good Answers: 1141	#7 Re: Cumulative Voltage Drop 10/10/2018 3:58 PM
	What's the point of adding all the drops? The sum of the drops is just the initial voltage minus the final one, correcting for whether it is phase-to-phase or phase-to-neutral of course. You can, however, look at each individual IR drop to show where a possible trouble spot is.
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Users who posted comments:

PWSlack (4); Rixter (2); Selk (1); TonyS (1)

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