ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Recent Posts
Underground phone and coax depth?
by libellis - 02/08/23 08:38 PM
Al Hildenbrand - Minneapolis - obituary
by Bill Addiss - 02/03/23 07:38 PM
by renosteinke - 01/27/23 09:52 PM
Does NEC 551.71 (F) apply to dwellings?
by BigB - 01/20/23 10:46 AM
New in the Gallery:
Burger King crown sillyness
Burger King crown sillyness
by wa2ise, December 11
240/208 to a house
240/208 to a house
by wa2ise, October 9
Who's Online Now
0 members (), 16 guests, and 17 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
#198132 01/13/11 09:36 AM
Joined: Sep 2005
Posts: 153
I am actually dealing with a project in Maine where my German company supplies mechanical stuff but the electrical side is covered by (unfortunately too many different) US guys.

The installation is hooked on something like a 50kVA single phase transformer. I would like to understand more in detail the setup of such an installation.

The engineer that planned the enclosure with the control gear is not really an electrician, but more a programmer so he is not an expert with regard to service hook up.
The manufacturer who assembled the stuff is far away and the guys on site do their job as they have ever done it without really knowing "Why".

Okay there is
- a single phase pole mounted transformer
- three wires going to the meter
- as in conduit unknown wires from the meter to a disconnect
- as in conduit unknown wires from this disconnect to the panel
- in the panel there are 240V and 120V circuit breakers
- in the enclosure there are further motor and circuit breakers

My first basic questions form a European point of view:

1. I did not see any fuses in this setup. Are they hidden in the service disconnect, on the pole or just not present.

2. At least in Germany we are supposed to organize (full) short tripping in a way that only and exactly one breaker trips at a time. We call this "selectivity". This means that in front of a breaker in my enclosure I would have to install either a fuse or a more expensive delayed breaker. To keep fuses selective you will have to keep two steps in size apart. In how far is this standardized in a NA setup?

3. We originally wanted to have an overvoltage protection against damages by lightning. Here the client refused that. How would one integrate that into such an installation.

4. We have to test and prove calculated maximum short current to prove that in case of a hot-to-ground-fault the fuses / breakers will trip within a maximum of typically 5 s. Is there a similar obligation in the US?

5. Same for grounding. It did not look like anything was tested there. Any obligations?

That's it for the beginning. Thanks for some light in the dark.

Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
Here's a typical US arrangement ...

It's fairly common for there to be no disconnects or overload protection at smaller PoCo pole mounted, single phase transformers. At various points in the line, the PoCo will have 'automatic cut-outs' that act like breakers - but also automatically re-set after a few seconds. Only after multiple faults with they stay 'open.'

The winding on the secondary side of the transformer has a connection at either end, as well as a center tap. Thus, there are 240 volts between the end (insulated, "hot") wires, and 120 volts between either 'hot' and the center (bare, 'neutral') wire.

The neutral wire also has a direct connection to a ground rod at the nase of the pole.

The bare neutral wire is aluminum on the outside, and steel on the inside. Thes "ACSR" (Aluminum Cable Steel Reinforced) is intended to support the wires to the service mast. You cannot cut it with most cable cutters without the steel core damaging the cutters.

Your very first overcurrent device is located just AFTER the meter, as part of the service disconnect. This disconnect is the first part of the system that is not sealed, and is accessible to the consumer. The meter is before the disconnect.

The meter typically will have four jaws - two to interrupt each 'hot' wire - and the entire current load will pass through the meter. The neutral (the ASCR was spliced to insulated wire at the top of the mast) will not pass through the meter at all.

At the service disconnect there is also a connection between the neutral, and another ground rod at the building.

What you call 'selectivity' is called 'selective co-ordination' here. We do not always do this. When we are required to have 'selective coordination,' it is necessary to either do an engineering study, considering the various trip curves of the components, or to purchase everything from the same manufacturer, who has had UL evaluate the equipment combinations for selective coordination.

Testing of ground rod impedance is not usually done; if in doubt, we simply add another ground rod at least 6-ft away, and assume that we have a good ground.

There are "surge suppressors" sold that either attach to the main panel, or replace a few of the breakers in the panel. If true lightning protection is desired, it is necessary to install a complete system, with 'electrodes' on the roof, etc. That work is usually done by a specialty contractor.

There is a second sort of surge protection that can be provided at each appliance.

We do not have to demonstrate any trip time. Our product standards require ordinary breakers to trip within 6 cycles (1/10 second). Fuses are typically represented by their manufacturers as clearing a fault within a fraction of a cycle (less than 1/100 second).

Last edited by renosteinke; 01/13/11 11:21 AM.
Joined: Sep 2005
Posts: 153
Hi renosteinke,

thanks a lot for the explanations and sorry for answering late but I had an urgent job over the weekend.

I have understood the following:

In case I short the wires at the entrance of a service panel the overcurrent devices in the service disco will trip.

In case this breaker (?, or are there always fuses?) has welded, there is (often) no other overcurrent protection but on the high voltage side, so the wires will burn definitely?

In case a hot wire before the panel (from disco) has contact to a metal conduit the bonding between panel ground and neutral is the only path to have the disco overcurrent protection tripping. If that fails (welded) one has to wait for the high voltage side? Same for a short before the disco?

If this client side bonding is not done correctly, the installation will remain live in such a case as overcurrent is too small to trip anything? The impedance through the ground will be too high and is not checked anyway.

Did I understand this part of the setup correctly?

What is then the typical maximum short current (kA) you have to calculate with? For comparison: In Germany this would be 10kA at the service fuses and 6kA in a panel.

Thanks for the patience to answer.

Joined: Oct 2000
Posts: 2,722
Broom Pusher and

Let me try to enhance what Reno has previously posted.

Rated Full-Load Amperes (FLA) on the 240/120V Secondary Side of a 1 Phase 50 KVA Transformer will be 208 Amps.
The typical Short Circuit Amperes (SCA) available at the Terminals of a 1 Phase 50 KVA, 240V 3.0 % Z Transformer is apx. 6,250 Amps.
With 50% of the KVA rating as Motor Loads, the Motor Contribution increases the SCA to 6,874 Amps.

The Over Current Protection Devices (OCPD) may be rated at 10KAIC throughout the System.

The Serving Utility should be contacted, and a Fault Level Quote should be requested.
The Minimum SCA at the Service Disconnect will be quoted by the Utility, which should be used for the Base Fault Level to coordinate downstream devices' Fault Ratings (Inrush Capacity / Ratings) from.
Normally, the Building Department will request a Fault Quote from the Serving Utility, to be submitted by the Electrical Contractor.

The Service Feeders between the Transformer and the Line Side of the Meter _MIGHT_ be the Electrical Contractor's responsibility to install.
Some PoCo's require the EC to install both the Ducts and the Feeders; others only require the EC to install Ducts.

In both cases, the Utility's Service Planners / Engineers will specify requirements for Duct size & type; Feeder size, type, etc., and Burial depth + encasement.
The Service Planners will also verify if the Proposed Service Equipment is Compliant per their ESRs (Electrical Service Requirements), and will identify the location to install the Service Equipment at (known as "Meter Spotting")

The Service Equipment will most likely be at least 225 Amps, more likely a 400 Amp Service will be installed (Load Calcs and Panel Schedules will determine the Minimum Service Capacity).

In my area, the quoted SCA for a Single Phase 240/120V Service, with Capacity of 600 Amps and less, would be quoted no higher than 42,000 Amps.
Typically, the same Utility will quote a 225 Amp Service fed from a 1 Phase 50 KVA Transformer at 18,000 Amps, and 22,000 Amps for a 400 Amp Service.

Service Disconnects at the Service Equipment - via Main Circuit Breaker:

If the Service size is 225 Amps, the Main Circuit Breaker will likely be a Frame rated at least 22KAIC @ 240VAC, with a Time-Trip current Curve that extends at least 10X longer than would be found with 100 Amp Frames (types used for Branch Circuits).
Verification of Trip-Current Characteristics may be obtain via Manufacturer's Specifications ("Cut-Sheets").
To confirm proper selective Coordination, verify that the Trip-Current curves for smaller Frames do not overlap with the Larger, upstream frames.

As to Grounding:

A Grounding Electrode System (GES) will need to be created at the Building.
This may consist of a connection to the Cold Water line, where it enters the Building - supplemented by at least one additional Electrode.
Typically, the Supplemental Electrode is Two Driven Ground Rods, separated no less than 6 Feet apart, connected together with a #6 Copper Conductor, then connected to the GES via #6 Copper Conductor.
The Cold Water line will require a Grounding Electrode Conductor (GEC) sized per the Service Feeder Conductors.
If the Service Feeders are 4/0 to 250MCM AL, the GEC would be #4 Copper.

I will stop here, so queries can be made to this information.

-- Scott

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
Thank you, Scott. This topic is more up your alley than mine!

Joined: Oct 2000
Posts: 2,722
Broom Pusher and
No Problem, Reno!!! smile

-- Scott

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Feb 2003
Posts: 939

I will add little more comment with common North American conductor size most of the 200 A serivce typicaly use 120mm˛ { 4/0 AWG } unless speced in NEC code or local code which it will bump up to 150mm˛ { 300 KCM } but the 400 amp service the conductor will work in two way either single 240mm˛ or twinner 120mm˛'s aka parllel set up and this is based on copper conductors with alum conductor it will bump up one size larger.

Also there is other code is the distance of inside the building once it enter the building we have to by X number of feet or meters before we have to add a outside disconnect switch { either breaker or fuse } all it depending on local / state codes.

As far for conductor colour codes it pretty much standarized for majorty of USA { Typically for 120/240 volts single phase will use black , red for phase conductors white is used for netural and green for earth }
Three phase will varies a bit depending on voltage and if wye or delta connection. { there is a colour chart in this fourm I have to find it one way or other }


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: Sep 2005
Posts: 153
Thanks a lot for the high quality answers, I have already tried to read some manufacturer's specifications of the breakers used as service disconnects. I'm just to busy at the moment to answer in a complete way and will come back next weekend.



Link Copied to Clipboard

Tools for Electricians
Tools for Electricians

* * * * * * *
2023 National Electrical Code (NEC)
2023 NEC Now Available!
* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman


Member Spotlight
L.I. New York
Posts: 99
Joined: August 2003
Top Posters(30 Days)
triple 2
Popular Topics(Views)
303,312 Are you busy
232,267 Re: Forum
217,023 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.5