sizing a buck boost transformer

[quinn77]

ok, so i have an existing 250' run of 10-3 uf to a location that homeowner now wants to have an RV power panel. RV is 125 volt / 30 amp. so with a load not to exceed 24 amps, i calculated voltage drop and came up with 15 volts...no wayno, which i had previously made HO aware of. HO tells me if i can make it work and i told him let me calculate so i am thinking boost transformer located at outside loadcenter ( source ) to feed RV. am i correct in thinking this is the right fix? cant find boost txfmr calcs in NEC, so how would i properly size and order this txfmr as i have never designed a circuit containing one.

thanx,
quinn

 

[LarryFine]

Quinn, I recommend running 240v to the load end of the run, and installing a 240-120v transformer. Remember the secondary bonding.

For a given load, the current and resulting voltage drop will be half as much as on 120v.

 

[quinn77]

Quinn, I recommend running 240v to the load end of the run, and installing a 240-120v transformer. Remember the secondary bonding.

For a given load, the current and resulting voltage drop will be half as much as on 120v.

thanks larry...heres more details. 10-3 to a 4 circuit sub. panel. 1- 20 amp power & lighting circuit. 1- 30 amp RV circuit. both 120 volt. both located in new sub panel.

 

[quinn77]

ps...the reason for the sub panel is the RV location is next to a pond in which homeowner wants a light and a plug...RV will not always be there.

 

[iwire]

Boosting the voltage to compensate for voltage drop is usually not the way to go.

You could get a buck boost that would boost the voltage 16 volts, that is no problem. And as long as the 24 amps of load was on the conductors you would be fine. But what happens the load is just a single 100 watt lamp? Now you will have over voltage which is usually worse for equipment than under voltage.

 

[gar]

101215-0951 EST

quinn77:

What is the nominal voltage at the main panel? Suppose it is 125. Then is a 15 V drop a problem? What are the loads at the RV? At what low voltage will these loads have a problem?

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[quinn77]

Boosting the voltage to compensate for voltage drop is usually not the way to go.

You could get a buck boost that would boost the voltage 16 volts, that is no problem. And as long as the 24 amps of load was on the conductors you would be fine. But what happens the load is just a single 100 watt lamp? Now you will have over voltage which is usually worse for equipment than under voltage.

yeah the more i research and calculate...the boost is def not the way. if he plugs in just a radio or such...then we might not like the results. if this was one piece of equipment with dedicated branch then it might work. i think hes just going to have to deal with one 15 amp circuit for now.

 

[quinn77]

101215-0951 EST

quinn77:

What is the nominal voltage at the main panel? Suppose it is 125. Then is a 15 V drop a problem? What are the loads at the RV? At what low voltage will these loads have a problem?

.

the nom voltage is 122 measured. voltage drop calculated is 12 volts. i warned him that if we keep the rv panel he needs to watch his load and i cannot be responsible for anything going faulty...since this was previously installed by a handy man. 110 should be ok, but he wanted 2 ckts, one for rv, one for gfi/light. thats why i am concerned.

 

[gar]

101215-1624 EST

quinn77:

You have to look at the kinds of loads that exist on the circuit.

First, if the wire from the main panel to the load area is sufficiently large for the steady-state load, then from the point of view of said wire voltage drop is of no concern.

This means you only need to study the needs or limitations of the actual loads.

Incandescent light bulbs do not care what the input voltage is so long as it is below their rating. Radios may not care much about voltage so long as it is reasonable. Electric heaters with no fan won't care.

An air conditioner, or other motor device, may be something to check on.

Humans may not like the lights momentarily flickering when an air conditioner turns on.

With 122 nominal I would not boost more than possibly 3 V.

If you use step up and down transformers, then you need to evaluate how their internal impedance, sum of the two transformers, trades off relative to the reduced drop on the transmission line. You could be worse off if you do not use oversized transformers.

#10 copper for a 250 ft distance has about 0.5 ohms loop resistance. 24 A provides the 12 V drop you mentioned. 12 V is about 10%. Double the voltage on the distribution line and the current is 12 A, and the voltage drop is 6 V, or 2.5%.

Next suppose each transformer has a 5% internal impedance. Then the voltage drop will be about 1 - (0.95*0.95*0.975) = 0.12 or 12%.

You would need lower impedance transformers, or oversized transformers.

Instead of changing the distribution line voltage you could at the destination end use a boost transformer, may be 5 V, and have a control with slight hysteresis that at the destination end senses the incoming voltage and switches the boost transformer in and out as needed. Maybe it switches on at 115 V, and off at 120 V.

Another technique is to use a motor driven Variac at the destination end feeding the boost transformer and using appropriate electronics to provide voltage regulation. To adjust for about 15% at 20 A would require a Variac of about 500 VA.

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[sparky59]

Run 240 volts to a sub panel. Put the 20 amp breaker on line 1...put the 30 amp breaker on line 2. Voltage drop will be acceptable.

 

[LarryFine]

Run 240 volts to a sub panel. Put the 20 amp breaker on line 1...put the 30 amp breaker on line 2. Voltage drop will be acceptable.

Agreed, as long as the 10/3 UF really is 10/3 with a red conductor, too.

 

[gar]

101215-2006 EST

Larry:

I believe from quinn77's discussion that one can assume that the RV is the only load at times. This load as a maximum is 120*24 = 2.88 KVA, and has to be supplied from a 120 V circuit. There is no balancing current on the neutral. Thus, 12 V is a good estimate of loop voltage drop.

Supplying 240 from the main panel to a stepdown transformer at the RV probably gains a slight improvement over the current straight 120 V connection.

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[Smart $]

the nom voltage is 122 measured. voltage drop calculated is 12 volts. i warned him that if we keep the rv panel he needs to watch his load and i cannot be responsible for anything going faulty...since this was previously installed by a handy man. 110 should be ok, but he wanted 2 ckts, one for rv, one for gfi/light. thats why i am concerned.

With 10-3, you should be running a 120/240 1? 3W (plus ground) feeder to the subpanel. As mentioned, if you put the the gfi/light on one line and the RV on the other line, your voltage drop will decrease when both circuits are in use. It's a little more complex calculating your voltage for an unbalanced feeder, but if both circuits are conducting at maximum, your voltage drop should be in the neighborhood of 7-8 volts on the RV circuit and about 5-6 on the gfi/light circuit.

 

[LarryFine]

Okay, now I think I grok Gar's last post:

The RV panel appears to be 120v only, not 120/240v, as there's only one ungrounded conductor.

I propose a new solution: rewire the RV.

 

[quinn77]

Agreed, as long as the 10/3 UF really is 10/3 with a red conductor, too.

yessir, 10-3 with a ground

 

[quinn77]

With 10-3, you should be running a 120/240 1? 3W (plus ground) feeder to the subpanel. As mentioned, if you put the the gfi/light on one line and the RV on the other line, your voltage drop will decrease when both circuits are in use. It's a little more complex calculating your voltage for an unbalanced feeder, but if both circuits are conducting at maximum, your voltage drop should be in the neighborhood of 7-8 volts on the RV circuit and about 5-6 on the gfi/light circuit.

this was my original thought process, then realized the 120 volt rv has receptacles outside, potentially eliminating the need for the gfi on the same pole as the rv receptacle. the rv will not always be at the location, so the scenario is rv hooked, gfi probably not in use...rv gone...kids fishing and cleaning fish with electric knife and listening to radio...gfi in use. seems that no matter what id have an unbalanced current ( statistically speaking of course ).

 

[quinn77]

thanks for the input guys...will be calculating the options and submitting to HO.

quinn.

 

[quinn77]

101215-2006 EST

Larry:

I believe from quinn77's discussion that one can assume that the RV is the only load at times. This load as a maximum is 120*24 = 2.88 KVA, and has to be supplied from a 120 V circuit. There is no balancing current on the neutral. Thus, 12 V is a good estimate of loop voltage drop.

Supplying 240 from the main panel to a stepdown transformer at the RV probably gains a slight improvement over the current straight 120 V connection.

.

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seems to be the best option at this point.

 

[gar]

101216-0833 EST

Smart $:

The worst case needs to be considered. That is the RV only and it is 24 A at 120 V. Probably it is much worse than this, inrush to start the air conditioner.

So how well the air conditioner performs for start up at the end of this line is important.

quinn77:

If you put a stepdown transformer at the trailer, and an air conditioner is used, then get a transformer with a low internal impedance and in addition probably oversize the transformer.

.

 

[Smart $]

101216-0833 EST

Smart $:

The worst case needs to be considered. ...

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I realize this.

First, quinn77's post that I replied to made it sound as if having the second circuit would increase the voltage drop. I was just clarifying that with loads connected split-phase, the voltage drop decreases when both circuits are in use.

Second, he reported the supply voltage measured 122. A lot of 120V equipment are listed at a nominal voltage of 115V (including air conditioners, which I doubt is the entire 24A load). If the drop at full 24A usage is only to 110V, this should be within operating tolerance of most all equipment.

Additionally, if the air conditioner is cycling, rather than on continuously, the inrush at startup, or a cycle start, will not suffer the full voltage drop. Calculated voltage drop is indicative of the heating caused by continuous current. There is a lag between current causing the heating and the effect of heating which causes the voltage drop. A basic example of the concept is your toaster's heating element, or practically any heating element which glows after energizing. They do not get red hot instantaneously. Same concept applies to the heating of conductors which effects the voltage drop.