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Unread 09-13-2019, 07:58 PM   #1
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Default What size transformer do I need?

I dont do this very often...Primary 480 to secondary 208, all 3 phase. Secondary panel is a 200 amp. Also what size primary breaker do i need?
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Unread 09-13-2019, 08:08 PM   #2
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I dont do this very often...Primary 480 to secondary 208, all 3 phase. Secondary panel is a 200 amp. Also what size primary breaker do i need?
My guess would be 45k tranny
Primary breaker 100i

I could be way off on this .
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Unread 09-13-2019, 08:16 PM   #3
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Shouldn’t all electricians be able to do this? Even mid level apprentices?
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Unread 09-13-2019, 08:33 PM   #4
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The 2 standard sizes I've used are 45 and 75 KVA.

The 480 side of the 45 will be 54 amps, the 120/208 will be 125 amps.

On the 75, it's 90 on the 480 side and 208 on the 120/208 side.

You'll need to pick one based on the load.

It's very common to see a 100 amp breaker in a 480 panel feeding a 75 KVA transformer and a 225 amp main in the 120/208 panel.

Just a short note here, the 480 side needs 3 wires and a ground, no neutral. The 120/208 side will need 4 wires and a ground. All grounds tie together. You'll need a neutral to ground bond on the 120/208 side, this can be done either in the transformer or in the panel. Most of us do it in the transformer.

If you bond in the transformer, do not bond anywhere else, there can be only one neutral - ground bond.
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Unread 09-14-2019, 01:06 AM   #5
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Shouldn’t all electricians be able to do this? Even mid level apprentices?
I mean, Marcus transformer makes these cool cards. You can even find them online...

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Unread 09-14-2019, 08:48 AM   #6
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Shouldn’️t all electricians be able to do this? Even mid level apprentices?
I mean, Marcus transformer makes these cool cards. You can even find them online...

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Those can come in handy .
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Unread 09-14-2019, 08:50 AM   #7
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Shouldn’t all electricians be able to do this? Even mid level apprentices?
I see it the opposite way.

I don't want to do it. I think an electrical engineer should be doing this.
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Unread 09-14-2019, 10:18 AM   #8
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I mean, Marcus transformer makes these cool cards. You can even find them online...

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Those are a good reference point but an electrician should still know what size is required.

Like that card says a 75kVA at 600 volts is 72.1a. What does that mean exactly? What size of overcurrent should be used and what is the wire size on both sides? Another thing is we can round up or down on the overcurrent but if we do, now what size wire do we use? What about the size of panel on the secondary? All stuff an electrician should know.

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I see it the opposite way.

I don't want to do it. I think an electrical engineer should be doing this.
Most of the time it is done by an engineer, like on a big job, but a commercial electrician would have to install transformers all the time without a set of prints.

Last one I did was to add a bunch of heat trace for a roof and eves troughing. 75kVA. Should I have hired an engineer for that? Or to install a few car chargers off of the 600 volt distribution. Engineer and stamped prints? I would not have got those jobs if I priced in those costs.
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Unread 09-14-2019, 10:49 AM   #9
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Last one I did was to add a bunch of heat trace for a roof and eves troughing. 75kVA. Should I have hired an engineer for that? Or to install a few car chargers off of the 600 volt distribution. Engineer and stamped prints? I would not have got those jobs if I priced in those costs.
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Should I have hired an engineer for that?
The customer should have, in my opinion.

I feel differently than many people in these situations. Even on small residential work, I want the customer to have plans with all the electric engineered on it, even for a basement renovation. If the customer wants me to do that electrical engineering, I will be happy to, but I will cost as much as the architect or engineer would charge.

I don’t believe in being expected to do this for free so that they don’t have to pay an engineer or architect.
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Unread 09-14-2019, 10:55 AM   #10
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The customer should have, in my opinion.

I feel differently than many people in these situations. Even on small residential work, I want the customer to have plans with all the electric engineered on it, even for a basement renovation. If the customer wants me to do that electrical engineering, I will be happy to, but I will cost as much as the architect or engineer would charge.

I don’t believe in being expected to do this for free so that they don’t have to pay an engineer or architect.
I do it all the time and I do not consider it engineering, it’s just a commercial service call. All that is needed is a service guy with a van and one or two helpers. Again I would not get the job if I tell a customer they will need an engineer to do a one day job installing a simple transformer.
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Unread 09-14-2019, 11:19 AM   #11
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I do it all the time and I do not consider it engineering, it’s just a commercial service call. All that is needed is a service guy with a van and one or two helpers. Again I would not get the job if I tell a customer they will need an engineer to do a one day job installing a simple transformer.
I guess it’s just a difference of opinion. In my opinion, installing a commercial transformer is far different than a service call. It’s something that is planned and permitted and usually specced and handed to the contractor afterwards.

But you come from the land where installing a little residential car charger (something I do on average twice a week as a two hour service call) requires a full load calculation and often a super expensive load shedder.
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Unread 09-14-2019, 11:31 AM   #12
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I guess it’s just a difference of opinion. In my opinion, installing a commercial transformer is far different than a service call. It’s something that is planned and permitted and usually specced and handed to the contractor afterwards.

Sometimes it is, sometimes it’s not, depending on the application. Like adding a car charger to a commercial building from the main 600 volt distribution. Of course there is a permit taken out but no need for an engineer or set of drawings, just a typical commercial service call.

But you come from the land where installing a little residential car charger (something I do on average twice a week as a two hour service call) requires a full load calculation and often a super expensive load shedder.
Adding anything with a large load to a residential service requires some consideration to the size of the main and the amount of load normally put on it. Not very often are houses supplied with a large enough main to accommodate an extra 30 or 40 amp load continuously. A simple fix is for the homeowner to provide proof from their electrical provider that the service doesn’t have much load normally and adding another large load will not overload the service. No extra cost required.

Even adding a large continuous load to a commercial panel requires some load calculations or measurements. It’s done all the time and should be. Again no engineer required, just a typical day for a commercial electrician.
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Unread 09-14-2019, 11:49 AM   #13
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Adding anything with a large load to a residential service requires some consideration to the size of the main and the amount of load normally put on it. Not very often are houses supplied with a large enough main to accommodate an extra 30 or 40 amp load continuously. A simple fix is for the homeowner to provide proof from their electrical provider that the service doesn’t have much load normally and adding another large load will not overload the service. No extra cost required.

Even adding a large continuous load to a commercial panel requires some load calculations or measurements. It’s done all the time and should be. Again no engineer required, just a typical day for a commercial electrician.
What I am saying is that from what I read here, and what I have read in the past, you guys make a simple car charger installation out to be a big thing with more planning and engineering than installing a commercial tranny. My experience of 15 years of full time commercial and 7 years of full time resi, is the opposite.

Just sayin'.
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Unread 09-14-2019, 01:51 PM   #14
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I would never use an engineer for a simple transformer install. Even if a load calc was involved, I'd do it all myself.

Over the last 20 years, I've installed about 2 dozen transformers, from small to 500 KVA, without an engineer, no problems at all.

If you're trained for it and take your trade seriously, you'll have no problem sizing breakers, wire and the transformer itself. The key here is being trained for it. I wouldn't expect a resi guy or a basic commercial installer to be able to handle such a job.
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Unread 09-14-2019, 01:57 PM   #15
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I would never use an engineer for a simple transformer install. Even if a load calc was involved, I'd do it all myself.

Over the last 20 years, I've installed about 2 dozen transformers, from small to 500 KVA, without an engineer, no problems at all.

If you're trained for it and take your trade seriously, you'll have no problem sizing breakers, wire and the transformer itself. The key here is being trained for it. I wouldn't expect a resi guy or a basic commercial installer to be able to handle such a job.
Well this is exactly what I am talking about.

You certainly aren't what I would consider a normal electrician, you are far ahead. You are also trained in this, as you mentioned. And I have to assume that you also charge for the work to engineer it. That's good, that's the way I think it should be. Either engineered, or the contractor paid to engineer it because they have a specialty in that part of the field.

My original comment was based off of this being something that all electrician or even mid level apprentices should just do.
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Unread 09-14-2019, 02:07 PM   #16
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Well this is exactly what I am talking about.

You certainly aren't what I would consider a normal electrician, you are far ahead. You are also trained in this, as you mentioned. And I have to assume that you also charge for the work to engineer it. That's good, that's the way I think it should be. Either engineered, or the contractor paid to engineer it because they have a specialty in that part of the field.

My original comment was based off of this being something that all electrician or even mid level apprentices should just do.
In Canada our license is nation wide and our training is pretty standard with a few differences between the provinces. Union and non union all attend the same school which is mandatory.

We are all trained to size and install transformers
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Unread 09-14-2019, 02:11 PM   #17
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In Canada our license is nation wide and our training is pretty standard with a few differences between the provinces. Union and non union all attend the same school which is mandatory.

We are all trained to size and install transformers
So were we. I still maintain what I said, take it or leave it.
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Unread 09-14-2019, 02:11 PM   #18
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.

My original comment was based off of this being something that all electrician or even mid level apprentices should just do.
My dad and I are designing a system with 600v 3 phase generatoe to 240 delta high leg. I've never done it before, and he hasn't done it in probably 20 years. I've never had to design and install a transformer layout... it isn't that hard though.

This is absolutely something a mid level apprentice can do.

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Unread 09-14-2019, 02:13 PM   #19
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This is absolutely something a mid level apprentice can do.
Jeeze.

A mid level apprentice could do most things if he was instructed and trained prior. Including most of what an electrical engineer does.
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Unread 09-14-2019, 02:15 PM   #20
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So were we. I still maintain what I said, take it or leave it.
So do I.

And if you were bidding the same job as me and wanted to charge extra to spend the 5 minutes it takes to use basic ohms law and code to size a simple 75 kVA transformer you would most likely not get the job.

We do it all the time, it’s not rocket appliances Ricky.
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