When size matters

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Karl H

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San Diego,CA
Hello gentlemen,

Should I calculate VD for a 117v branch ckt supplying convenience recepticals OCPD @ 20I using the formula; 2 x 12.9 x 16I x L/cm = VD/117, 80% conductor ampacity. Or, 2 x 12.9 x 20I x L/cm = VD/117 with either calc not to exceed 5%? Thanks in advance.
Karl
 
You cannot calculate voltage drop until you know the actual load on a circuit. If the circuit is loaded 80% you may predict the VD but that is a large assumption, few general use receptacle are (or should be) loaded to their capacity. Remember that the 80% restriction on a circuit is ONLY for CONTINUOUS LOADS. Very few things plugged into a convenience outlet would be considered continuous (3-hours or more). Also keep in mind that nominal voltage per the NEC is assumed to be 120 volts - see 220.2(A).
 
When Size Matters (part deux)

When Size Matters (part deux)

Oops, sorry...I was in a rush when I posted the previous entry and didn't give the backstory to the problem which prompted me to ask the question in the first place. We have a couple of guys on our crew, one in particular that will run 12/2 MC for long distances and never even consider VD. On one occasion, he ran a 12/2 ckt for convenience receptacles at least 345'. I mentioned to him that he was going to have a VD, after which and he used his meter to test the last receptacle; the meter read 115v, the supply PNL read 117v (which is where I came up with that voltage btw). His response was that it was only a 2 volt drop and well within the range. I in turn responded that it was a meter, not a load and to apply I is to decrease E. Two weeks later, I receive a call that a ckt wasn't working. The facilities manager tells me that the housekeeping woman complained about her 12I vacuum not working on the East side of the building on the convenience receptacles...the exact receptacle where I had the argument with our guy about VD. My rule is if I'm running convenience ckts that I always make sure that at any point you can plug in a 12I vacuum or dehumidifier in case of flood, etc. and it works...but maybe that's just me.
 
Karl H said:
Oops, sorry...I was in a rush when I posted the previous entry and didn't give the backstory to the problem which prompted me to ask the question in the first place. We have a couple of guys on our crew, one in particular that will run 12/2 MC for long distances and never even consider VD. On one occasion, he ran a 12/2 ckt for convenience receptacles at least 345'. I mentioned to him that he was going to have a VD, after which and he used his meter to test the last receptacle; the meter read 115v, the supply PNL read 117v (which is where I came up with that voltage btw). His response was that it was only a 2 volt drop and well within the range. I in turn responded that it was a meter, not a load and to apply I is to decrease E. Two weeks later, I receive a call that a ckt wasn't working. The facilities manager tells me that the housekeeping woman complained about her 12I vacuum not working on the East side of the building on the convenience receptacles...the exact receptacle where I had the argument with our guy about VD. My rule is if I'm running convenience ckts that I always make sure that at any point you can plug in a 12I vacuum or dehumidifier in case of flood, etc. and it works...but maybe that's just me.
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Karl,

That's a good way to look at an installation. There are other ways also. But, for the above one, just figure a 12amp load at the end of the circuit in your VD formula.

If you want to use just a one way measurement to figure the drop try this on-line site.
http://www.csgnetwork.com/voltagedropcalc.html

Michael
 
When Size Matters

When Size Matters

I don't actually do a 16I calculation; I keep 12I in consideration when running the length of conductor. It's just those other "electricians" in the field who want to debate me and say that since it's a 20I ckt, why not calculate it that way? My response is that it is not a 20I loaded ckt. Some say, "It needs to be calc'd at 16 for 80%" and I respond that it isn't a 16I continuous load ckt. I just want a ckt that can handle any load that a convince ckt would normally see such as; vacuum cleaners, dehumidifiers, reciprocating saws from servicemen, floor buffers and the occasional re-located copy machine. Man I really screwed up that original post! :rolleyes:
Karl
 
Karl H said:
I mentioned to him that he was going to have a VD, after which and he used his meter to test the last receptacle; the meter read 115v, the supply PNL read 117v (which is where I came up with that voltage btw). His response was that it was only a 2 volt drop and well within the range.

Two weeks later, I receive a call that a ckt wasn't working. The facilities manager tells me that the housekeeping woman complained about her 12I vacuum not working on the East side of the building on the convenience receptacles...the exact receptacle where I had the argument with our guy about VD.
Click to expand...

1. Make sure he hears about this complaint.

2. Have him add a hair dryer to his next voltage test.
 
That is so funny Larry because the last sentence i said to him was.Someone will plug in a space heater and you will be the idiot of the day!:wink:
 
I agree that a 345' branch circuit indicates poor design. As a rule-of-thumb, I advise students to consider voltage drop any time the total circuit length exceeds 100'. That is not a hard and fast rule, just a suggestion.
 
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