We are installing a set of parking lot gates and the distance from our 120 volt single phase panel to the first gate is 450' and the distance to the second gate 545' the load would be 9 amps for each gate. According to our calculations we would need #4 copper conductors. What would be the appropriate size of the bonding conductor?
voltage drop calculations
- Thread starter elecon
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bphgravity
Senior Member
- Location
- Florida
Re: voltage drop calculations
The size of the equipment bonding conductor would be sized per Table 250.122 based on the OCD rating protecting the circuit. However, since you have increased the ungrounded conductors to accomodate for voltage drop, you must meet the requirment of 250.122(B).
Based on you description, the voltage drop may be slightly excessive with #4 conductors.
The size of the equipment bonding conductor would be sized per Table 250.122 based on the OCD rating protecting the circuit. However, since you have increased the ungrounded conductors to accomodate for voltage drop, you must meet the requirment of 250.122(B).
Based on you description, the voltage drop may be slightly excessive with #4 conductors.
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: voltage drop calculations
That question can not be answered with the information given. What size of breaker is protecting the circuit?
That question can not be answered with the information given. What size of breaker is protecting the circuit?
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: voltage drop calculations
The grounding conductor needs to be sized such that a ground fault will trip the protective CB. I would want to have a fault current something like 10X the CB rating to insure it trips. If you use a 20A CB, at 120V the maximum round trip resistance of the hot wire and the ground conductor would have to be low enough to allow 200A of current to flow to trip the CB.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: voltage drop calculations
I infer that the two gates are fed from the same circuit, with 18 amps flowing the first 450 feet to the first gate, and with 9 amps flowing the final 95 feet to the second gate. If that is not correct, then please clarify.
With that configuration, I calculate a voltage drop of 5.01 volts to the first gate, and an additional 0.53 volts to the second gate. The total VD is 5.54 volts, or 4.61%. Since you didn?t give us the details of the feeder to the panel that in turn feeds the gates, then I can only guess that the total VD exceeds 5%. That is not a code violation, but you might want to re-think your choices.
Now to answer your question, let me again presume that one circuit carries both gates. You can use this process with appropriate changes, if your actual configuration is different.
The total current of 18 amps cannot be more than 80% of the branch circuit?s ampacity. So you need 22.5 amps worth of conductor. The next larger overcurrent device is 25 amps. The minimum acceptable conductor for 22.5 amps (before considering VD) is a #12 (here presuming a 75C copper conductor). The required EGC for a 25 amp breaker is a #10 copper.
Now turn to Table 8. Because of voltage drop, you are upsizing from #12 (6,530 CM) to a #4 (41,740 CM). That is an increase in size by a factor of about 6.392. You need to upsize the EGC from a #10 (10,380 CM) by that same factor. 6.392 times 10,380 is 66,349 CM. The smallest size conductor that is at least 66,349 CM is a #2 (66,360 CM). (This one was close!)
So my answer to your question is a #2.
I infer that the two gates are fed from the same circuit, with 18 amps flowing the first 450 feet to the first gate, and with 9 amps flowing the final 95 feet to the second gate. If that is not correct, then please clarify.
With that configuration, I calculate a voltage drop of 5.01 volts to the first gate, and an additional 0.53 volts to the second gate. The total VD is 5.54 volts, or 4.61%. Since you didn?t give us the details of the feeder to the panel that in turn feeds the gates, then I can only guess that the total VD exceeds 5%. That is not a code violation, but you might want to re-think your choices.
Now to answer your question, let me again presume that one circuit carries both gates. You can use this process with appropriate changes, if your actual configuration is different.
The total current of 18 amps cannot be more than 80% of the branch circuit?s ampacity. So you need 22.5 amps worth of conductor. The next larger overcurrent device is 25 amps. The minimum acceptable conductor for 22.5 amps (before considering VD) is a #12 (here presuming a 75C copper conductor). The required EGC for a 25 amp breaker is a #10 copper.
Now turn to Table 8. Because of voltage drop, you are upsizing from #12 (6,530 CM) to a #4 (41,740 CM). That is an increase in size by a factor of about 6.392. You need to upsize the EGC from a #10 (10,380 CM) by that same factor. 6.392 times 10,380 is 66,349 CM. The smallest size conductor that is at least 66,349 CM is a #2 (66,360 CM). (This one was close!)
So my answer to your question is a #2.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: voltage drop calculations
Again using the configuration described in my earlier post, let me now assume the use of PVC conduit. Here is how I address fault current:
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Quite true.
Again using the configuration described in my earlier post, let me now assume the use of PVC conduit. Here is how I address fault current:
</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">545 feet of #4 phase wire (effective Z of .29 ohms per 1000 feet) has an impedance of 0.1581 ohms.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">545 feet of #2 EGC wire (effective Z of .19 ohms per 1000 feet) has an impedance of 0.1036 ohms.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Total impedance of the path = 0.1581 + 0.1036 = 0.2616 ohms.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Fault current is 120 volts divided by 0.2616 ohms, or 458 amps.</font>
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: voltage drop calculations
Do I now correctly understand that there are four gates, and there are two circuits, and each circuit supplies two gates, and each circuit has a total load of 18 amps?
If so, then the voltage drop is 4% to the nearer gate and 5% to the further gate. These do not take into account the VD along the feeder. Again, I would suggest you re-think your choices.
But if you are using a 20 amp breaker, the minimum EGC becomes #12, and the ?upsized? EGC becomes #4 (instead of #2). Going back over my math, I conclude that this will have a lower fault current, but it is still over 300 amps.
Do I now correctly understand that there are four gates, and there are two circuits, and each circuit supplies two gates, and each circuit has a total load of 18 amps?
If so, then the voltage drop is 4% to the nearer gate and 5% to the further gate. These do not take into account the VD along the feeder. Again, I would suggest you re-think your choices.
But if you are using a 20 amp breaker, the minimum EGC becomes #12, and the ?upsized? EGC becomes #4 (instead of #2). Going back over my math, I conclude that this will have a lower fault current, but it is still over 300 amps.
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: voltage drop calculations
Anytime a 20 amp breaker is used the EGC must be as large as the ungrounded conductor. 250.122(B)
Anytime a 20 amp breaker is used the EGC must be as large as the ungrounded conductor. 250.122(B)
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: voltage drop calculations
Yep. Nothing unusual there. For a 20 amp circuit the phase conductor is #12 and the EGC is #12. Upsize the phase conductor by any amount and the EGC will have to be upsized by the same amount. A #4 would work for both. VD to the farthest gate is 3%.
Yep. Nothing unusual there. For a 20 amp circuit the phase conductor is #12 and the EGC is #12. Upsize the phase conductor by any amount and the EGC will have to be upsized by the same amount. A #4 would work for both. VD to the farthest gate is 3%.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: voltage drop calculations
A thought has occurred to me. I wonder if it would be reasonable to run 230V out to the gate and then install a small transformer and whatever else is needed to bring it back down to 120 to power the gate. Would this be more cost effective? How about if you boosted it up to 480 and then back down?
Re: voltage drop calculations
120/240 1phase the power you have? if so, wouldnt it be a lot cheaper to run a feeder out to the first gate location at 450 ft with #2, and installed an outdoor sub panel, then run your 4 separate circuits with #12 ...seems a little less money verses 4 separate circuits going 450 and 545ft. using #4....probably about 1000bucks cheaper.... just in wire cost.
120/240 1phase the power you have? if so, wouldnt it be a lot cheaper to run a feeder out to the first gate location at 450 ft with #2, and installed an outdoor sub panel, then run your 4 separate circuits with #12 ...seems a little less money verses 4 separate circuits going 450 and 545ft. using #4....probably about 1000bucks cheaper.... just in wire cost.
jimwalker
Senior Member
- Location
- TAMPA FLORIDA
Re: voltage drop calculations
Why not use aluminum ? far cheaper
Why not use aluminum ? far cheaper
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