30amp transformer supplies multiple copiers and other stuff - can it work? - Electrician Talk - Professional Electrical Contractors Forum
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Old 04-14-2018, 08:36 AM   #1
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Default 30amp transformer supplies multiple copiers and other stuff - can it work?

Hallo everyone!

Maybe it's a noob question, but I'd still like to ask. We're currently rewiring a former bakery to let it become a copy shop.

Well, here's what I don't understand:

Since it was a bakery before, the shop is supplied by three phase 220V. The main breaker is 150amps.

Next to the three phase 220V panel sits a 10kVA transformer. On the secondary side are three single phases with each 110V, phase against phase 190V, and one neutral going back. According to the type plate, the secondary side is rated 30amps.

Now I'm concerned about the load on the 110V side. The shop owner wants to supply two copiers, rated 11A and 15A, a laminating machine, which should be around 7A, a paper drill with 5 amps, at least 5 computers, each one rated 4-5 amps, some other stuff which I guess takes another 10amps, and a microwave with 1300W (if I remember correctly).

Now I need to say that I'm relatively new to the trade. My coworker says, 30amps are more than enough to supply all that stuff, because once the machines are heated up and start running, their amperage drops.

But my math says, there's a total of around 80amps, and even if some machines might not use the full amperage all the time, it should be still well above 30. Or am I wrong? Is there something I did not take into concern?

One more thing to think about: The shop owner says the two copy machines are likely to be used together, the computers will be switched on the entire day. The laminating machine and some of the other stuff might join them, too.

So if I'm calculating 11+15+7+5+(5*4)=58amps, that's still 28 above limit, are the machines really likely to drop their current by nearly 50% once they are running? Any ideas on this?

As I guess, it is eventually going to work, because my coworker has a bunch of experience and he'll get it done. But I don't understand why it should work? The maths is against him. Maybe somebody can enlighten me...?

Just in case if anybody's wondering about strange numbers, my location is in Taiwan. So things might be slightly different here than in the USA or in Canada. But of what I know, Taiwan follows the US-rules for electrical installations.

Many greetings and thank you in advance!
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Old 04-14-2018, 08:55 AM   #2
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They don't draw less because they're "heated up". Get that foolishness out of your head.

What you do have going in your favor is that all the equipment isn't running at the same time, at full power, all the time. It's what's called "demand load".
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Old 04-14-2018, 11:49 AM   #3
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Quote:
Originally Posted by MDShunk View Post
They don't draw less because they're "heated up". Get that foolishness out of your head.

What you do have going in your favor is that all the equipment isn't running at the same time, at full power, all the time. It's what's called "demand load".
I thought that with laser printers the inrush was for the fuser to heat up, which causes that delay before it first starts printing. I could be wrong, or could be something that was old technology and no longer applies.

But if there's an inrush, like any inrush, if they're all turned on together at the beginning of the shift or after a power outage I could see that causing trouble.
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Old 04-14-2018, 12:36 PM   #4
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In my experience copiers heat up the ink periodically even while idle. I nabbed a circuit once for some office lighting and got a call back bc the circuit I took power from was for a copier and the lights would dim briefly every few minutes even though the copier in the suite next door was not being used.

The OP has too many weird values for me to comment on other than he should use this experience to ask his journeyman how to do a proper load calc.
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Old 04-14-2018, 01:05 PM   #5
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i don't know if this applies to our side , but transformers can tolerate overload for sometime.so as long as it's not continous it would proably be ok. power companies do it al the time.
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Old 04-14-2018, 02:11 PM   #6
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If all the equipment is operating on 110 volts and you have 3 transformers each rated at 30 amps and the loads are evenly distributed among the 3 transformers then you're ok. Each transformer will see roughly 20 amps.

P.S. Welcome to the forum. Your English is good and you've asked a valid question.
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Old 04-14-2018, 09:12 PM   #7
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Good morning!
@micromind:
There's only one transformer there, output is rated at 30amps. Thanks for the compliment!
@Arrow3030:
I've been asking the journeyman, but he can't exactly tell me how to calculate the load, he just said according to his experience it should work. It's nice that he has this experience to know it should work, but I'd like to find an explanation that doesn't rely on experience.
@MDShunk:
When he told me about this, I thought about an inrush current like you find on a motor, so I thought that makes sense.

Originally the shop owner had even more equipment to run on 110V, such as lighting and one more copier. We've persuaded him to change as much equipment as possible to 220V to lower the current on the 110V side.

On the 220V side there's even more stuff going on, there are 4 A/C, another 4 copiers, a paper cutter, the lighting (shop owner agreed to change the lighting to 220V) and a hot melt glue machine. But the 220V side can take 150amps so I'm not concerned about that.

Anyway, this is a cool experience. Quite different from the usual wiring that we do, which mainly consists of wiring lights and plugs. First time to encounter a shop with so much stuff going on inside there. Actually I'm still wondering if it's going to work. looking forward to see how we're going to wire it.

Thanks for the input guys, I really appreciate it!
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Old 04-14-2018, 11:32 PM   #8
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If the transformer is 10KVA and the 110 volt side is 30 amps then it is a 3Ø transformer with 3 legs at 110 volts to neutral and 30 amps each, effectively giving a total of 90 amps at 110 volts.

Since this is less than the total load of 58 amps, all you need to do is balance the individual loads as evenly as possible across the 3 phases.

Look at it this way, 58 amps at 110 volts = 6.380 KVA. This is less than the 10 KVA rating of the transformer.

If each phase is loaded to 30 amps at 110 volts, the result would be 9.9 KVA. This is pretty close to both amp and KVA ratings of the transformer.
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Old 04-17-2018, 09:56 AM   #9
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So you take the 10kVA-rating as base for the calculation? Makes somehow sense to me.

But maybe I should have written this before, there are four wires going out of the transformer, they are labeled X0-X3 with X0 being the neutral and X1-X3 the phases. On the typeplate is written X0-X3 max. 30A. So that means, the neutral can also take 30A only, right?

Even if you balance all the phases properly, if the sum of the currents exceeds 30A, it's too much, or did I miss something?

Even the coworker isn't that sure about the proper size of the transformer anymore, yesterday he told the client that he might need to acquire a bigger transformer...

Anyway, I'll keep watching and wondering. I'll give you an update once things are up and running.
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Old 04-17-2018, 11:56 AM   #10
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You can have a max of 30 amps on any (or all) of the 3 phases (X1, X2 and X3). The neutral current will never be higher than the highest of any of the phases. In fact, it'll be lower if there's current on all 3 phases.

There's no need to upsize the transformer, if the loads are spread among all 3 phases, it'll be plenty big enough.

The sum of the currents doesn't matter, what matters is that no phase exceeds 30 amps.
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Old 04-18-2018, 11:30 AM   #11
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Huge giant thank you, @micromind!

I've read your post today in the morning and done some more research about transformers in the meanwhile. I've learned that if all three phases have the same amount of current, there is basically no current in the neutral. I've also learned that the neutral equals a vector sum of the line currents and is, like you've stated before, always smaller than the highest single line current.

(Long time ago I've learned and worked as a car mechanic. Guess I'm too used to DC systems, where the neutral/negative current equals the sum of the currents of all lines, but AC is different. When calculating the neutral current, I've followed DC rules... well, there's still a lot to learn out there.)

Thanks again, I really appreciate your help!
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