Grounding conductor needed in boost/buck system?

PetrosA

Senior Member
I dont know what the actual load is to try and make a judgement, but don't forget to consider transformer cost and losses in your analysis. You will have two transformers with load and no load losses potentially 24 hours a day (depending of course on when this runs and the control scheme if it's not continuous). Two transformers will themselves have non insignificant voltage drop. Sometimes a step up step down doesn't pay for itself. Also for a fixed load like a fountain, perhaps just using a boost transformer if the voltage is too low would be more cost effective.
Cost-effectiveness isn't an issue with this customer. They're already ok with spending many thousands on the materials to have a fountain in the pond...
 
So if I understand correctly, I need to run a bonding jumper from the center tap at the source transformer to ground, correct? Do I need a separate ground rod for this size transformer the same as I'd need for a big one? Seems a little strange... Obviously, there will be a rod installed at the far end when we step it back down.
Yeah you need to ground the center tap (or whichever conductor you are grounding, but you don't just add a rod, see 250.30(A)(4)
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
So if I understand correctly, I need to run a bonding jumper from the center tap at the source transformer to ground, correct? Do I need a separate ground rod for this size transformer the same as I'd need for a big one? Seems a little strange... Obviously, there will be a rod installed at the far end when we step it back down.
You need a GES at the secondary of each xfmr. At or in the house, use the existing. Establish new otherwise.
 

Cow

Senior Member
Location
Eastern Oregon
12.4A@230V
I think you can do this just with oversized wire, with no transformers needed.

I quick off the calc says 1/0 AL would get it done and be right around 3%, but I don't know what that does to your conduit fill with the other wires you need to pull in there too.

We'd probably put a baby vault/large handhole at the ends and midpoint of the conduit run for ease of pulling.
 

Jamesco

Senior Member
Location
Iowa
Occupation
Master Electrician
How big is this pump? I get only 4.3% drop for a 240v 15A load (no transformers) using #4, and that's a huge pump.
12.4A@230V
What's the mains voltage at the house? 240V?

I'm with electrofelon, forget the step up and step down transformers.

You'll get a more efficient system using the #4 THWN cu fed from the panel in the house and use a boost transformer at the pond location to feed the pump motor. I would make sure the buck/boost transformer uses cu windings, not AL.

Size EGC accordingly per 250.122 (B) Table 250.122.
Drive a ground rod and bond it to the EGC

The buck/boost transformer is wired as an auto-transformer so it may not matter if it is wired on the line side or the load side of the GFCI protection for the pump motor. May not?.... I would install the transformer on the line side of the pump motor GFCI protection though.

NEC 682
 
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Jamesco

Senior Member
Location
Iowa
Occupation
Master Electrician
EDIT:
ou'll get a more efficient system using the #4 THWN cu fed from the panel in the house and use a boost transformer at the pond location to feed the pump motor. I would make sure the buck/boost transformer uses cu windings, not AL.
Or increase the size of the branch circuit wire eliminating the boost transformer all together.
Is the 1.5" PVC Schedule 40 or 80?
 
EDIT:

Or increase the size of the branch circuit wire eliminating the boost transformer all together.
Is the 1.5" PVC Schedule 40 or 80?
I didn't run any conduit fill, but there is presumably a limit to what size wire can be run in the existing pipe. I'm thinking the #4 is plenty big enough. 4.3% drop still keeps voltage well within tolerance, in fact that puts it right at nameplate.
 

Jamesco

Senior Member
Location
Iowa
Occupation
Master Electrician
I didn't run any conduit fill, but there is presumably a limit to what size wire can be run in the existing pipe. I'm thinking the #4 is plenty big enough. 4.3% drop still keeps voltage well within tolerance, in fact that puts it right at nameplate.
Agree. You did the math.

Just to add, an aerator/fountain pump, as a rule, runs 24/7 depending on the part of the country, climate, it will be used.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Yes, the first transformer will be in the basement of the house and the secondaries from that will run in a PVC out the driveway 1200 feet to the second transformer. I'm trying to wrap my head around what the EGC would be able to do with the secondary. Would a dead short from secondary to EGC have low enough impedance to trip the breaker feeding the first transformer?
The EGC run with the primary conductors will _not_ provide a low impedance path for the secondary side of the transformer. The EGC that you need to run is for the _primary_ side.

The secondary of the transformer is a new source (a 'separately derived system') so you need to ground and bond the secondary to provide a low impedance return path to it.

-Jon
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Some guys here have mentioned an allowance for no separate EGC if one of the circuit conductors is grounded.
This is something to consider. An outside feeder is permitted to be run using a grounded circuit conductor as the bonding conductor as long as there are no parallel metallic paths (communication circuit cables, water pipes, etc). However this has the side effect that any voltage drop in the grounded conductor shows up as voltage on any bonded metal at the detached structure.

-Jon
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Remember that any method of boosting voltage at the load will result in increased line current, contributing to voltage drop.
 

Jamesco

Senior Member
Location
Iowa
Occupation
Master Electrician
Remember that any method of boosting voltage at the load will result in increased line current, contributing to voltage drop.
Are you saying the auto-transformer will add to the connected load of the branch circuit wiring?

I agree in the OP's case it would make a cleaner looking job to install the transformer in the basement of the house. It could depend on if it hums loud enough to be irritating to a member in the house to hear.

I have installed B/B transformers at the source and at the load end. It depended on the application where it was used.

.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Are you saying the auto-transformer will add to the connected load of the branch circuit wiring?
Yes, in two ways.

1. Magnetizing current. Even an unloaded transformer uses power; that's why they're warm to the touch all the time.

2. The boosted voltage is seen by the source as additional load current; the power has to come from somewhere.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Remember that any method of boosting voltage at the load will result in increased line current, contributing to voltage drop.
Are you saying the auto-transformer will add to the connected load of the branch circuit wiring?
Yes, in two ways.

1. Magnetizing current. Even an unloaded transformer uses power; that's why they're warm to the touch all the time.

2. The boosted voltage is seen by the source as additional load current; the power has to come from somewhere.
An example illustrating the second point that Larry mentioned:
Assume that placing a 10% boost auto-transformer before the branch wiring fully compensates for the voltage drop with a given load. Then a calculation shows that putting the 10% boost transformer at the end of the wiring will result in about a 2% voltage drop at the auto-transformer output. This is due to the additional losses in the wire resistance from the increased current drawn at the transformer input.

Having the transformer at the end also increases the source impedance seen by the load due to wire resistance by the square of the turns ratio or (1.1)² =1.21 times. This makes the voltage vary with load current even more than otherwise. This can be an issue in any case when boosting to compensate for voltage drop when the load is varying. The amount of "overvoltage" if the load turns off also has to be considered.
 
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