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#625 - 02/15/01 09:53 AM voltage drop
doc Offline
Member
Registered: 11/25/00
Posts: 119
Loc: Texas
how strictly do all of you follow the code on this issue,I have a 250 foot run to make the load at end of run will be roughly 25 amps and it is 3 phase ,now according to my fiquers I need to run some # 6 to stay within voltage drop range .The problem is that the sub panel that I was going to come off of is only wired with # 10 .
Have been in a lot of places where these long runs are made and the wire size is smaller than what is called for. So enlighten me as to if there is some guide lines or do you run wire called for,or am I mis fiquering the wire size
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#626 - 02/15/01 10:13 AM Re: voltage drop
resqcapt19 Offline
Member
Registered: 11/10/00
Posts: 2148
Loc: IL
Doc,
What is the system voltage and how far away from the source is the sub panel?
Don(resqcapt19)
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#627 - 02/15/01 10:18 AM Re: voltage drop
doc Offline
Member
Registered: 11/25/00
Posts: 119
Loc: Texas
Don from the main panel to first sub panel is about 10 feet from first sub to this sub is about 200 feet and from that sub to my equipment is about 250 feet fro sub 1 to sub 2 there is # 10 wire
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#628 - 02/15/01 10:20 AM Re: voltage drop
doc Offline
Member
Registered: 11/25/00
Posts: 119
Loc: Texas
system voltage is 240 volt 3/phase actual voltage may be a little less
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#629 - 02/15/01 01:19 PM Re: voltage drop
sparky Offline
Member
Registered: 10/18/00
Posts: 5303
doc;
So you have about 410' to the MDP....

A #6, 240V run @25A will see 5% around 565'

A #8 , 240V,run @25 Amps will see a 5% drop at about 355'

A #10, 240V run @ 25A will see 5% at 230'
So in reality, Sub #2 is about at that length from source and that's all sub#2 can run as it is.

You could calculate the circuit as a splice, 210' of #10, then 200' of #6 , and probably make out ok. But it would seem simpler to make your run to the MDP.

I'm just going to the MDP, not the utility here.

Hope this doesn't bum you out..
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#630 - 02/15/01 02:03 PM Re: voltage drop
Scott35 Offline

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Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA
Doc,

I'm sure that you know that the voltage drop limits are just suggestions in the NEC, not something that would create compliance issues, but nevertheless they give some helpful and somewhat common to use limits that could be applied in a majority of installations.

OK, with the legal propaganda aside, you are posed with the issue of being able to deliver the proper voltage per KVA to the machine.

If the load is simply a Resistive load [Incandescent lamps or Resistance type element heaters], then the voltage drop will not be too much more than a nuisance. The lower voltage will lower the overall KW that the loads can use. This means dim Incandescent lamps and heaters that are producing less heat than would otherwise.

On the other hand, if the loads are Inductive [Ballasts, Motors, etc.], this can be a real problem. These components will try as hard as they can to draw the amount of KVA as needed. So when the voltage drops down, the current rises up - making the drawn KVA at the load consistent.

Since the subpanel you are planning to branch from is already limited by the #10s, that would be one of the last choices to use.

Sparky suggested to branch from the MDP. This would be better, if possible to get a switch out of it, or if overall possible. Otherwise, the subfeed might need to be upgraded.

Figure out what the voltage will be at the load, running through all points existing. If it's not too bad, or can be low without causing harm, it might be OK. Otherwise, opt to correct.

I definitely design to limit total voltage drop. I use the NEC's suggested levels as maximums for guidelines, unless a manufacturer's specs are more stringent. In some cases, the use of "Boost" transformers is very helpful. If you are unfamiliar with them, they are typical isolated transformers connected as to be Autotransformers in a series additive connection. I am sure you know of them well, but I added the info for those that may have not heard of them, or have not installed one but just heard the name "Buck and Boost"

Good luck!

Scott "S.E.T."
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#631 - 02/15/01 02:29 PM Re: voltage drop
doc Offline
Member
Registered: 11/25/00
Posts: 119
Loc: Texas
THANKS GUYS,
Now bear with me a little longer,
this is a a gas fired washer with a couple of motors and a couple of timers in the control ,on it mfg.tag calls for full load of 22 amps and use a 30 amp fused disconnect ,I went 25 amp because of not knowing if mfg. tag is accurate .
Now my actual voltage is 245 volts across any 2 legs and 122 volts on 2 legs and 217 on high leg. the distance is exactly 425 feet
the reason I question using # 6 or even 8 at this distance is these guys say they have been running it on some # 12 without any problems all they have done is moved it down ,but they did show me where it had been hooked up at and even had a couple of lights tied on with it. distance is the same
Am not sure that they didn't have low voltage problems at the distance we are at.
So at this point am I understanding that I use the # 6 so that there is only the 5 percent drop and not short cut with smaller wire
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#632 - 02/15/01 06:42 PM Re: voltage drop
sparky Offline
Member
Registered: 10/18/00
Posts: 5303
Did you say the same #12 circuit is still there? I would be inclined to take it for a test ride.... , meter it out....

On the job, the scenario that Scott describes is played out quite often. I mean those jobs where it's a good 200 or so feet from the temp service, and all the subs have added 5 times that in cords that are maybe #14 along with a multitude of splitters.

All the motorized tools dim the lights...cords start to melt snow....the carpenter binds up something in his tablesaw....everything functions, but the signs and symptoms are all there

Is there any accounting for motor inrush vs. the VD formula ? If not, would'nt every motor circuit be in VD violation?
( that sounds to obscene to ever be a code!)

Can the formula be done seperately for inductive & resistive parts for your washer?

More Q's than A's again...

I usually do the math,( the figures i posted are from Mike Holt's software) and give it to the customer, tell them to talk it around, especially in possible disputes like you might be headed.


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