250MCM for 21.7 amps? Voltage drop calc must be wrong?

[winnie]

It is possible (likely even) that the engineer was considering _simultaneous_ starting current and requiring an excessively tight voltage drop.

Before looking at transformers, I second the call to look at _240V_ motors, and taking a careful look at the voltage drop that the motors can tolerate.

It might be the case that the engineer is considering what voltage drop will do to the cameras/security system when the gates operate. Again it might pay to make the system tolerate a larger voltage drop than to try to prevent the voltage drop.

-Jon

 

[Bri22]

Try using the calculator at http://www.southwire.com/support/voltage-drop-calculator.htm
See what you get.

OK, I cheated and used the south wire calculator....

480 single phase
1317 feet
21.7 amps
#4 with a 2.8% voltage drop

When I took my exam like 20 years ago I could figure all this stuff out. Working too much residential for so long I forgot a lot of stuff.

Again, thanks so much for everyones help, and so quickly! You guys are great!

 

[JFletcher]

Thanks for the welcome! Number 8 (or even 4 or 6 for that mater) vs 250MCM for almost quarter mile... It would be silly to consider 250MCM! I have no clue how this engineer got his degree.

Thanks for your help!

You're welcome. In some cases, the cost of the xfmrs is more than larger conductors. 3kva or even 5 (for future loads) are pretty small and not very expensive.

I come up with #4 at 240V.

You guys go out and buy some transformers, disconnects, and some fuses, I'm gonna check the shop and see if there is enough 4AWG if not I'll use 6AWG and take Friday off since I didn't spend two days wiring up transformers.

:?

 

[iwire]

I'll use 6AWG and take Friday off since I didn't spend two days wiring up transformers.

You really suck at transformers, they are just large junction boxes.

 

[GoldDigger]

OK, I cheated and used the south wire calculator....

480 single phase
1317 feet
21.7 amps
#4 with a 2.8% voltage drop

When I took my exam like 20 years ago I could figure all this stuff out. Working too much residential for so long I forgot a lot of stuff.

Again, thanks so much for everyones help, and so quickly! You guys are great!

Note that by selecting single phase you are telling the calculator to use the two way distance as the length of the wire.
If you select three phase the calculator assumes a balanced overall load, whether wired delta or wired wye. That makes the one way wire length appropriate because the voltage drop in the neutral will be zero (zero net current).
In your case with a single load, using a transformer from three phase service to 480Y/277 will still be a two wire drop since the single 120V load cannot be balanced across the three phases (unless you use several transformers at the load end in a strange interconnection.)
Bottom line there: If you go 480, just use a 240/480 three wire secondary at the supply end.

 

[topgone]

The online calculator that I use gives me a drop of 2.5 volts for 21.7 amps at 1371 feet on 250 kcmil copper.

I get a drop of 4.8 volts for 58 amps at 1371 feet using 350kcmil copper

I use this calculator. (you have to scroll down to get to the calculator)

IDK how much difference 1317 feet makes compared with your 1371 feet!

 

[Bri22]

Note that by selecting single phase you are telling the calculator to use the two way distance as the length of the wire.
If you select three phase the calculator assumes a balanced overall load, whether wired delta or wired wye. That makes the one way wire length appropriate because the voltage drop in the neutral will be zero (zero net current).
In your case with a single load, using a transformer from three phase service to 480Y/277 will still be a two wire drop since the single 120V load cannot be balanced across the three phases (unless you use several transformers at the load end in a strange interconnection.)
Bottom line there: If you go 480, just use a 240/480 three wire secondary at the supply end.

Thanks GoldDigger,

I got you until the last sentence. Just to be clear. Your saying use a 240 volt primary, 480 volt secondary transformer at the supply end and then drop back down with a 480 volt primary, 120 volt secondary transformer?

Thanks

 

[GoldDigger]

Thanks GoldDigger,

I got you until the last sentence. Just to be clear. Your saying use a 240 volt primary, 480 volt secondary transformer at the supply end and then drop back down with a 480 volt primary, 120 volt secondary transformer?

Thanks

Close. I was saying to use a 480V secondary with a center tap (240/480) so that you can ground the center tap and keep both lines within 240V of ground. There is, AFAIK no requirement to actually run the neutral to the other end, but you may have to run an EGC (boo, hiss).
And all this is only if you decide to go with transformers in the first place.

 

[Ingenieur]

120:600 xfmr 1-#12/2 w/G uf cable 600:120 xfmr
3-4% drop
2 kva each, running them 'hot' for the duty cycle is no big deal
you can get a 3kva if it concerns you, +$200 each
NEMA 3R enclosure https://m.grainger.com/mobile/product/SQUARE-D-Transformer-22LR49?fc=MWP2IDP2PCP
both xfmrs are the same, spare parts commonality
each xfmr costs about the same as 1 uf run
you would need 5-6 paralleled to get close to 5% drop
xfmrs cheaper

 

[Bri22]

Close. I was saying to use a 480V secondary with a center tap (240/480) so that you can ground the center tap and keep both lines within 240V of ground. There is, AFAIK no requirement to actually run the neutral to the other end, but you may have to run an EGC (boo, hiss).
And all this is only if you decide to go with transformers in the first place.

Got, thanks!

 

[wwhitney]

you may have to run an EGC (boo, hiss).

What about the arrangement shown here:

http://forums.mikeholt.com/showthread.php?t=172543&page=2&p=1683449#post1683449

Cheers,
Wayne

 

[GoldDigger]

What about the arrangement shown here:

http://forums.mikeholt.com/showthread.php?t=172543&page=2&p=1683449#post1683449

Cheers,
Wayne

That probably works too if the OP is willing to go that high above ground for the transmission line.

 

[Bri22]

120:600 xfmr 1-#12/2 w/G uf cable 600:120 xfmr
3-4% drop
2 kva each, running them 'hot' for the duty cycle is no big deal
you can get a 3kva if it concerns you, +$200 each
NEMA 3R enclosure https://m.grainger.com/mobile/product/SQUARE-D-Transformer-22LR49?fc=MWP2IDP2PCP
both xfmrs are the same, spare parts commonality
each xfmr costs about the same as 1 uf run
you would need 5-6 paralleled to get close to 5% drop
xfmrs cheaper

Holly crap, never thought of going to 600 volts. Guess because I never have worked with it. Just did a search and found a 600v primary 120v secondary transformer but not a 120v primary 600v secondary. But you say they are the same, so you can just wire them up in reverse? Makes sense but have never done it. I ran the calculations on the south wire calculator and it came up with a #6 for 1317 feet @ 600 volts and 15 amps. Maybe I did something wrong. Never the less, it's much better than where I started!

Thanks!

 

[JFletcher]

Holly crap, never thought of going to 600 volts. Guess because I never have worked with it. Just did a search and found a 600v primary 120v secondary transformer but not a 120v primary 600v secondary. But you say they are the same, so you can just wire them up in reverse? Makes sense but have never done it. I ran the calculations on the south wire calculator and it came up with a #6 for 1317 feet @ 600 volts and 15 amps. Maybe I did something wrong. Never the less, it's much better than where I started!

Thanks!

You would not have 15A @ 600V; your 21.7A load @ 120V (2604va) would be a 4.34A load @ 600V. Going to 600V would let you use #12cu @ 3.3% VD. #12 isn't a lot cheaper than #10, so if the xfmrs are appreciably more expensive than the 120/480, better to stick to the 480 (or 240 as others mentioned).

I've never pulled wire that distance, but I'd imagine pulling in #10 is easier than #4, and easier/cheaper to splice at the junction boxes. Also, you can go with smaller conduit with the higher voltage/smaller conductors.

 

[wwhitney]

That probably works too if the OP is willing to go that high above ground for the transmission line.

I was referring to texie's idea of using 250.30(A)(1) Exception 2 to permit the feeder to be two wires only, with no EGC or bonding jumper. See also 250.30(A)(2) Exception. That idea applies when one of the two circuit conductors is grounded, regardless of the voltage.

As to 480V vs 600V, I have no information about the cost and availability of transformers, so I'll leave that to others.

Cheers, Wayne

 

[kwired]

Thanks for the welcome! Number 8 (or even 4 or 6 for that mater) vs 250MCM for almost quarter mile... It would be silly to consider 250MCM! I have no clue how this engineer got his degree.

Thanks for your help!

He gets kickbacks from wire and cable companies

Do you really need to be strict on ~3% voltage drop. Some loads maybe, but a motor operated piece of equipment I doubt need to be extremely picky on VD. I bet you could have 5-8% or even 10% drop and have little or no issues with it.

Of course next thing to come up is since power is there, about a 1/4 mile from anything else with power, they will want to add some other electric load, signs, lights, holiday display...

 

[ActionDave]

You really suck at transformers, they are just large junction boxes.

:lol: You're probably right. I'm still taking Friday off.

 

[ActionDave]

It is possible (likely even) that the engineer was considering _simultaneous_ starting current and requiring an excessively tight voltage drop.

Before looking at transformers, I second the call to look at _240V_ motors, and taking a careful look at the voltage drop that the motors can tolerate.....

Thank you. Nice to have another voice of reason in this thread.

 

[iwire]

Thank you. Nice to have another voice of reason in this thread.

For me it is much easier to do transformers than to get another trade to switch motors. Might be different for you, but for me I really don't have a 'change the motor option'.:happyno:


Also, I am really not sure why you think transformers, especially small ones like this are a big deal?

That said, best option might be both, see if the motor can be changed to 480 and send that out there using a transformer at the source if needed.

 

[ActionDave]

For me it is much easier to do transformers than to get another trade to switch motors. Might be different for you, but for me I really don't have a 'change the motor option'.:happyno:

Seasons don't fear the motor, nor do the wind, the rain, or the snow....or something like that.

The size and type of motor under discussion is 99.999% going to be a dual voltage motor.

Also, I am really not sure why you think transformers, especially small ones like this are a big deal?

I don't. I just don't think they are needed for this application.

That said, best option might be both, see if the motor can be changed to 480 and send that out there using a transformer at the source if needed.

That would be most difficult. Single phase 480V motors exist but are quite rare other than HVAC applications.