RPC output to 3 phase wye

Hi Guys,

I have 208V 3 phase power at my main shop. At my home shop I have 240V single phase. Been running most stuff off of VFD and have been using diesel generators for occasionally used 3 phase equipment. But now have purchased a large format CNC router and I want to run from grid power and not diesel. It's got a multitude of motors and pieces that use three phase. So VFDs aren't a reasonable option for it. And I'm somewhat keen on having plug compatibility between the shops so I can move equipment back and fourth.

So I've purchased an American Rotary 25HP control panel and have a Baldor motor that is going with it.

My understanding is that the output from this RPC will be delta. And that if I want wye, I need a delta wye isolation transformer. Given the size of the RPC, I need the next size bigger than 30kVA. Which should be 45kVA.

Much searching has lead to:
TESTED MGM HT45C3B2SH 45 KVA STEP DOWN TRANSFORMER 240V to 208V 120V 3-PHASE | eBay

Questions:
1) is this the right transformer for what I want?

2) hookup to meet NEC 2017
a) obviously 3 phase delta from RPC in the H1 and H2 and H3 connections

b) obviously 3 phase wye out to panel from the X1 and X2 and X3 connections

c) X0 is my derived neutral. It goes to my neutral bus in my 3 phase wye panel

d) X0 gets bonded to ground. I believe that happens in the transformer and the neutral bus is isolated in the panel. IE no green bonding screw in the panel.

e) additional ground rod required at the the transformer. And bond to building structural steel / metal water pipes in that area. Wood frame building and no metal water pipes, so not much I can do there.

f) output of RPC is protected with 70 amp 3 phase circuit breaker at the RPC. That's about 60 feet from where the transformer and wye panel is. That provides over current protection for the wire. Presumably that also provides protection for the transformer primary.

g) is over current protection required on the secondary of the transformer? IE can I just connect the output to the main lugs on the wye panel or do I need breaker or fuses? Intuitively it seems to me that the 45kVA transformer should be protected fine with the 70A breaker on the primary side. But I'm not at all familiar with the NEC with respect to transformers.

h) Am I missing anything?

Thanks, thanks, thanks!

-Jim

It seems right but why do you need wye?

Just because your service is wye does not mean your load will not be delta.

How much of your 3 phase equipment actually uses a neutral wire? I would just add control transformers to your equipment instead to eliminate that.


The RPC will output wild leg delta, or something like that, which is generally fine but can cause issues with MOVs in very specific scenarios. I doubt it will come up, but it's something to check for.

The output is delta in the sense that it has no neutral. The three wires themselves do not know or care if you hook up a wye load, like a motor, as long as it does not need the neutral.

If you want 208V 3 phase, that transformer will do .

You ground the system at the PANEL, a transformer is considered a "separately derived system", and you treat it like a new service.... bond neutral, and ground to a rod, building steel, etc as required by section 250 NEC.

Transformer is protected by a breaker at 125% of primary full load current rating.

Secondary side gets overcurrent protection for the various loads, through the breakers i the 208V distribution panel. Breaker on primary side is for the transformer, wires to suit breaker rating

One suitable grounding electrode connection should be sufficient, a sel=parate one for the transformer should not be needed, but the transformer case needs an EGC to the panel, as well as the neutral (grounded conductor)..

Your understanding of the connections is mostly correct, but there are variances based on site conditions.

Such as the transformer secondary coil grounding and bonding. It can be done either at the transformer or at the distribution panel. But it is done slightly different for each senario and the details matter.

Secondary protection of the transformer is required in the case of a Delta/Wye transformer.
For a Delta to Delta transformer, primary only protection can be used. But not in your case of Delta to Wye. See 240.4 (F)

(F) Transformer Secondary Conductors. Single-phase
(other than 2-wire) and multiphase (other than delta-delta,
3-wire) transformer secondary conductors shall not be considered
to be protected by the primary overcurrent protective
device. Conductors supplied by the secondary side of a single phase
transformer having a 2-wire (single-voltage) secondary,
or a three-phase, delta-delta connected transformer having a
3-wire (single-voltage) secondary, shall be permitted to be
protected by overcurrent protection provided on the primary
(supply) side of the transformer, provided this protection is in
accordance with 450.3 and does not exceed the value determined
by multiplying the secondary conductor ampacity by
the secondary-to-primary transformer voltage ratio.

Click to expand...

The biggest question I see is the system capacity, what is it?
You don't give any values for the connected load, the nature of the loads or its requirements.
No mention of your service size, or the utility transformer capacity and the length and size of your service conductors.

Case in point is your transformer selection of 45KVA. It has a rating of 108A on the 230V input. Code call for protection at 125% to 250% on the primary. You say it will be protected at your RPC breaker. It will be protected at 70A, but you might not be able to energize it at that value.

Transformers have a high inrush value that is short in duration. Installing too small of an overcurrent device can cause nuisance trips when energizing. That's one of the reasons the NEC allows you to go up to 250% on the input protection. Energy efficient transformers, like motors, have an even higher current inrush. Reverse connected transformers have an even higher inrush yet. You certainly don't want that transformer load connected when starting your RPC.

Looking at the rating of your 25HP American Rotary, They call the output current value for voltage sensitive loads at 30A. Your 45KVA transformer wants at least 108A x 1.25 = 135A minimum input for its rating and to prevent nuisance trips

A 15KVA transformer with an 240V input of 36A x 1.25 = 45A overcurrent protection, might be more doable with your RPC, but then we don't know if that would be suitable for your load. We have no data on that. A 30KVA unit with a 240V input of 72A x 1.25 = 90A might be easier to get energized on your 70A RPC output breaker but not guaranteed.

You would be wise to consider the system capacity and load values before you purchase all of the gear.

Be aware for a 2017 NEC inspection, the rules are a bit more stringent that what works. American Rotary makes note of this fact on their sizing chart. What works on the shop floor may not be code compliant.

Phase Converters 455.6 Conductors.
(A) Ampacity. The ampacity of the single-phase supply conductors
shall be determined by 455.6(A)(1) or (A)(2).

Informational Note: Single-phase conductors sized to prevent
a voltage drop not exceeding 3 percent from the source
of supply to the phase converter may help ensure proper
starting and operation of motor loads.

(1) Variable Loads. Where the loads to be supplied are
variable, the conductor ampacity shall not be less than
125 percent of the phase converter nameplate single-phase
input full-load amperes.

(2) Fixed Loads. Where the phase converter supplies specific
fixed loads, and the conductor ampacity is less than
125 percent of the phase converter nameplate single-phase
input full-load amperes, the conductors shall have an ampacity
not less than 250 percent of the sum of the full-load,
3-phase current rating of the motors and other loads served
where the input and output voltages of the phase converter
are identical. Where the input and output voltages of the
phase converter are different, the current as determined by
this section shall be multiplied by the ratio of output to
input voltage.

Click to expand...

That's enough for one post. Details on the transformer bonding and grounding electrode connection can be covered later. More info on the service and the loads would be beneficial.

SAF Ω

Hi SAF - thanks for the very detailed response. My response inline with your snipped response:

SAF said:

Such as the transformer secondary coil grounding and bonding. It can be done either at the transformer or at the distribution panel. But it is done slightly different for each senario and the details matter.

Click to expand...

As best I can tell, NEC seems to be looking for the grounding at the delta wye transformer. "To prevent objectionable current flow, the grounding electrode conductor connection to the grounded conductor must be made at the same point on the separately derived system where the system bonding jumper and supply-side bonding jumper are connected, as specified in Sec. 250.30(A)(5)." Or at least that is an option and works for me.

SAF said:

Secondary protection of the transformer is required in the case of a Delta/Wye transformer.
For a Delta to Delta transformer, primary only protection can be used. But not in your case of Delta to Wye. See 240.4 (F)

Click to expand...

Awesome. Appreciate the code reference here. Will size and build accordingly.

SAF said:

The biggest question I see is the system capacity, what is it?
You don't give any values for the connected load, the nature of the loads or its requirements.

Click to expand...

The goal is to have a general purpose 208 wye panel and be able to utilize the full potential of the RPC output.

At the moment, my main interest is the Weeke CNC router. It's got control cabinet, vacuum pump, cooling pump, spindle, 3 axis servos, drill bank spindle, etc. It's nameplate rating is 50A.

Another probable machine is a Chiron FS8 CNC mill. Same sort of deal. Spindle. Coolant pump. Fog fan. 3 axis drives. Control cabinet. It's nameplate rating is 30A.

Another machine is a dyno we use for testing equipment we build. It's a 10HP motor with a frequency drive.

Basically all my three phase equipment at work has a 208V wye 5 pin plug on it. I've got neutral and ground to all sockets. Primary plug on our small equipment is 30A 5 wire twist lock.

My goal is to be able to grab something from work with a 30 amp 5 wire twist lock and run it at home. And to be able to run the above mentioned router at home. No need to run multiple machines at the same time. I have zero desire to be re-linking transformers, changing overload heaters or programming, changing VFD programming, figuring out what phases control tranformers run, etc.

Right now I accomplish that with a 30kW 208/120 diesel generator. But I would like to do it with the RPC.

SAF said:

No mention of your service size, or the utility transformer capacity and the length and size of your service conductors.

Click to expand...

Service is 400A 240V. Have 200A fed to the shop. Over about 200ft of 4/0 aluminum conductors.

RPC is fed with a 100A QO2100 breaker on the shop main panel. There is about 3 ft of 2/0 copper conductor to the RPC control panel. RPC control panel to motor is 2 ft of 3 AWG. The 100A breaker and the 3 AWG were dictated because of the complete lack of care in American Rotary's part on wire over current protection. Motors of this frame size all seem to have 1" KO's in their cast pecker heads. Biggest wire I can get through 1" metallic flex (without taking nipple exception) is 3 x 3 AWG + 6 AWG ground. 3 AWG in 75 degree C table gives me 100A of ampacity. I suspect I could take various exemptions and perhaps get to 2 AWG and a 120A single phase breaker, but it's easy to have different reasoning than the AHJ here. And the AHJ here always wins. So I'm trying to play the sizing same in his eyes.

The output of the RPC panel goes about 1 ft to a QO370 enclosed circuit breaker via 3 AWG. From there it is stepped down to 4 AWG where it goes about 70 feet to the new 3 phase panel. Transformer will be located at the end of that 70 foot run. This is because my conduit already embedded in the concrete only had enough fill left for 4 AWG wire and because putting that transformer at the end of the 70 feet of wire would add some resistance and perhaps inductance and make it more likely to start without tripping the breaker. I am also aware that the QO370 breaker may not be the optimal breaker for energizing the transformer. But it is thermal magnetic. And it can be replaced with a more exotic breaker or time delay fuses if it won't do the job.

The utility transformer size is a bit of a sticky wicket and sore spot for me. When I built this place 2.5 years ago, I paid $700 extra and had the biggest transformer they would provision, a 50 kVA, installed for the service. And went with a 400A service, I've got 40kW of interconnected solar permitted and about 20kW of that installed currently. Last year, the cooperative put in place some anti-solar policies. One of those was to make the maximum transformer size 37.5 kVA. Above that is demand billing. And the demand billing is based on either buying or selling demand. So my 20kW of solar would be treated as a 20kW demand. At $15 per kW demand charge, that would be $300 per month demand fees. So my power bill would have gone from being paid $200 per month to somewhere on the order of paying them $400 a month. That wasn't going to happen, so I ended up having to pay them another $200 to have my 1.5 year old 50kVA transformer replaced with a 37.5kVA.

What's the point of selling someone a 400A 240V (96 kVA) service, which they have designed and permitted (40kVA) of continuous loads, and then forcing them into a 37.5kVA transformer? (short answer is because when I get anywhere near 37.5 kVA the voltage will shoot up outside of UL1741 bounds and cause the solar inverters to drop offline and then they won't have to buy as much electricity from me as the inverters cycle off and on all day)

I complained to the state's public utility commission. But the rat bastard legislature exempted cooperatives from PUC oversight. They say you can just vote on members that represent what you want and therefore PUC oversight isn't needed. But my cooperative canceled their last round of elections. There's a lot of funny business going on.

In any case, 37.5 kVA service transformer is all I've got. Hopefully it's got a good service factor or if I catch it on fire, they will come to their senses and put in a bigger transformer.

SAF said:

Case in point is your transformer selection of 45KVA. It has a rating of 108A on the 230V input. Code call for protection at 125% to 250% on the primary. You say it will be protected at your RPC breaker. It will be protected at 70A, but you might not be able to energize it at that value.

Transformers have a high inrush value that is short in duration. Installing too small of an overcurrent device can cause nuisance trips when energizing. That's one of the reasons the NEC allows you to go up to 250% on the input protection. Energy efficient transformers, like motors, have an even higher current inrush. Reverse connected transformers have an even higher inrush yet. You certainly don't want that transformer load connected when starting your RPC.

Click to expand...

American Rotary specified a 45kVA transformer. They said they would sell me a 30kVA, but I would have to sign away the warranty. TEMCO actually said the exact same thing. Although in TEMCO's case, there was no discussion of an RPC feeding it.

I'm planning on starting the RPC with no loads connected. And it won't restart in the event of a power failure. So not worried about starting the RPC under load.

SAF said:

That's enough for one post. Details on the transformer bonding and grounding electrode connection can be covered later. More info on the service and the loads would be beneficial.

SAF Ω

Click to expand...

I did end up purchasing the eBay 45kVA transformer. Negotiated a great price on it. If it doesn't work out, I'll survive.


I think the best course of action for me at this point is to receive that transformer and hook up the primary and see if it will start. If it does, great ... will figure out how to hook it up in a NEC compliant manner. If it doesn't, then I have a new opportunity to decide if I really want wye.

I really appreciate your detailed help.

-Jim

The bonding jumper would be at the panel, in general. Think of it as if it were a service..... you would bond and ground at the panel, and you can do the same here. If you do, then you run neutral and EGC from the transformer to the panel separately as I mentioned.

Iif, as I assume from what you say, you will put in a distribution panel, then that will be the secondary protection, assuming you size the main correctly.

USUALLY the time-current characteristics of the breaker will carry the inrush. Some will not, usually the cheaper ones. But you still need to have a basically adequate protection, and yes, the 70A may not do the trick. It may, however.

An RPC is started with no load. Not even the transformer, but you knew that.

I received the 45kVA transformer yesterday and hooked up the primary to the RPC output. RPC starts it just fine. You can hear the RPC slow down for about 1/4 a second.

I need to fiddle with trying to balance the RPC voltage output. Generated phases are pretty high. As a result the idle current on the transformer is measured at 3.3 amps on A, 3.9 amps on B, and 5.4 amps on C. That's with a fluke clamp on. I'm assuming the power factor is quite low and that it isn't actually drawing 2kW of real power to idle the transformer.

Yeah dont worry about the voltage on the third leg, it will drop under load.

That transformer may only be 200 watts no load.