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Discussion Starter · #1 · (Edited)
Be aware that code rules change and some info could be out of date

Electrical Theory & Training
Basic Theory and Meter Usage -- Just The Cowboy (Harvey)
Navy Electricity & Electronic Training Series
Equipment Grounding Conductor Paradox
Why grounds and neutrals are tied only at the main service- By Charlie Beck

Raceway & Wire Info
Greenlee Pipe Bending Guide
Another Pipe Bending Guide
Klein Pipe Bending Guide
Sizing the equipment grounding conductor by Dennis Alwon w/ Mike Holt graphics
Angle Pulls Illustrated by Mike Holt

Motors & Transformers
Motor Protection- Voltage Unbalance- Single Phasing by Cooper-Bussman
Transformer Installation by Mike Holt

General Code Info
Branch Circuits Pdf-- by Mike Holt
Afci & Gfci Pdf -- by Mike Holt
Explanation of Range Tables with examples-- by Dennis Alwon
Swimming Pools
Equipotential Bonding by Erico
Diving Board Clearance Graphic

Csst Gas Piping
Csst & Lightning Protection
Csst Bonding Requirements- by Dennis Alwon

Tap Rule
10' Tap Rule
 

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Discussion Starter · #6 · (Edited)
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An Interesting Paradox When
Upsizing the Equipment Grounding Conductor (EGC)



Suppose we need to feed a sub panel that will have a calculated load of 55 amps. We will be using PVC conduit with Thhn wire. Table 310.16 says that for 55 amps we could use #6 THHN copper and we could fuse it at 70 amps since the 75C rating of the wire is 65 amps. Article 240.4(B) allows us to use the next higher sized breaker as long as the calculated load does not exceed the 65 amp rating of the wire.

Let us explore why we can use the 75C rating of #6 Thhn and not the 60C or 90C rating. First we need to look at article 110.14 (C) of the NEC. It states that if we know the rating of the terminals in the circuit then we may use that rating but if the terminal ratings are not know( old equipment may not have the info available), then we must use 60C for circuits 100 amps or less and 75C for circuits over 100 amps. Most all of today’s terminals and breakers are rated 75C.

Since there are basically no terminals rated 90C we must work with the weakest link in the circuit. Even when we have conductors rated 90C we cannot use the 90C rating of the wire for the final ampacity. So what good is 90C wire? It may be used for derating purposes such as excessive heat or conductor fill. As long as the final ampacity is not more than the weakest part of the circuit then we are good to go.

So, back to the example, the calculated load is 55 amps and we are allowed to protect the wire at 70 amps using #6Thhn. If we protect the panel at 60 amps then we only need a #10 wire for the EGC. However, if we protect it at 70 amps, then we would need a #8 awg wire for the EGC. Ref. T. 250.122. For this example let’s say we want to protect the panel at 70 amps so a #8 EGC is required.

Table 250.122 states the minimum size EGC would be #8 for 70 amps overcurrent devices. But suppose we decide to upsize the ungrounded conductors to #3Thhn because of voltage drop on the circuit. This wire has an ampacity of 100 amps. Article 250.122(B) requires that we must upsize the EGC proportionally to the upsize in the ungrounded conductors.

A primary reason for the requirement of Article 250.122(B) is that when you upsize the ungrounded conductors, you increase the available fault current at the same time. The fault current is determined by the size of the conductors supplying the fault as well as other factors such as impedance on the wire. The longer the length the greater the impedance.
If we look at Table 8 in Chapter 9 of the NEC we see that #6 is 26240 cir.mil and
#3 is 52620 cir.mil. This means that the circular mil of #3 is twice that of #6.

52620/26240= 2

This means that our EGC must also be increase by a factor of 2. We see from the same tables that #8 is 16510 cir.mil. So now we have 16510*2= 33020 cir.mil

Working backwards in the table we would need a #4 EGC.

Here is the paradox. If, instead, we decide to fuse the sub panel at 100 amps instead of the 70 amps discussed, I can keep my #8 EGC and be compliant. This seems a bit unusual but it is one of the many oddities of the Nec.

If this were not a sub panel but a piece of equipment that demanded 70 amp max., then we would have no choice except to increase the EGC.


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Why grounds and neutrals are tied only at the main service, and not at a subpanel.

Reference: NEC article 250.42.

We need to start by noting two things: (1) Current is always seeking a path back to its source, and (2) Current will take every available path it can find.

The function of the equipment grounding conductors (EGC), that ones that connect to the ground bar in the panel, is to carry fault current. If a fault occurs with a piece of equipment, such that a hot conductor comes into contact with the case or other external metal part, any person who touches that equipment is going to get a shock. The shock can be enough to kill, but the current will not be high enough to cause the breaker to trip.

However, with the EGC creating a path from the case back to the ground bar, then via the ground screw or bonding jumper to the neutral bar, the current in this path will be high enough to trip the breaker. This will terminate the event before the person can receive a fatal shock. That is why the ground and neutral buses are connected at the main service disconnecting means – to complete the current path from the fault point back to the source. In this context, I am treating the main panel as the "source." Once the current gets to that point, it has nowhere else go.

If you also connect the ground and neutral at a subpanel, then there will be two paths for current to flow back to the source during normal operation. Current will be flowing in the neutral most of the time (unless the loads running at the moment are perfectly balanced among the phases). But with the ground and neutral tied together both at the main panel and at the subpanel, the EGC will be in parallel with the neutral wire. Therefore, the EGC will carry current. This will cause the external metal parts of each and every component that has an EGC its to become energized. You could not safely touch anything in the facility.
__
Charles E. Beck, P.E., Seattle


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Discussion Starter · #10 · (Edited)
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Transformer Installation

By Mike Holt for EC&M Magazine

Installing transformers in accordance with the NEC requires the designer and installer to be up-to-date with the current Code requirements. Most installations can be a NEC challenge and transformers can extend that challenge to a new level. A properly designed, installed and safe installation will ensure that the conductors and equipment are properly sized and protected. In addition, grounding is also an overriding issue. So let’s get started.

For this exercise, let’s assume we are installing a 45 kVA and a 112.5 kVA 480V delta primary to a 208Y/120V 3-phase wye secondary. Each transformer supplies a lighting and appliance branch circuit panelboard with continuous nonlinear loads, typically found in today’s office buildings. The length of the conductors from the transformer secondary to the lighting and appliance branch-circuit panelboard is not in excess of 10 ft, and all terminals are rated 75�C.

Step 1. Determine Transformer Current Ratings: Determine the primary and secondary current rating of the transformers:

Primary CurrentSecondary Current
45 kVA45,000 VA/(480 x 1.732) = 54A45,000 VA/(208 x 1.732) = 125A
112.5 kVA112,500 VA/(480 x 1.732) = 135A112,500 VA/(208 x 1.732) = 312A
Step 2. Primary Protection [450.3]: The primary winding of transformers shall be protected against overcurrent in accordance with the percentages listed in Table 450.3 and all applicable notes. Where 125 percent of the primary current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker as listed in 240.6(A), the next higher rating can be used [Note 1].

45 kVA54A x 1.25 = 68A, next size up 70A
112.5 kVA135A x 1.25 = 169A, next size up 175A
Step 3. Size Primary Conductor: Feeder conductors supplying continuous loads shall be sized no less than 125 percent of the continuous loads based on the conductor ampacities as listed in Table 310.16, before any ampacity adjustment in accordance with the terminal temperature rating [110.14(C) and 215.2(A)(1)].

45 kVA54A x 1.25 = 68A, 4 AWG rated 85A at 75�C, Table 310.16
112.5 kVA135A x 1.25 = 169A, 2/0 AWG rated 175A at 75�C, Table 310.16
Sizing Equipment Grounding Conductor (when required) 250.122(A):
The size of the equipment grounding (bonding) conductor for the transformer primary is based on the primary protection device.

45 kVA70A Primary Protection (Step 2), Table 250.122, 8 AWG
112.5 kVA175A Primary Protection (Step 2), Table 250.122, 6 AWG
Step 4. Size Secondary Conductor: Secondary conductors can be run without secondary overcurrent protection at the point of supply for 10 ft, if the ampacity of the conductor is not less than the rating of the overcurrent protective device at the termination of the tap conductors. This means that the next size up rule contained in 240.4(B) does not apply.

Secondary overcurrent protection is not required, but overcurrent protection is required for lighting and appliance branch-circuit panelboards, and this protection is required to be located on the secondary side of the transformer in accordance with 408.16(A) and (D).

Overcurrent Protection Device Size: Where a feeder supplies continuous loads, the rating of the (secondary) overcurrent device shall not be less than 125 percent of the continuous load [215.3] as listed in 240.6(A).

45 kVA125A x 1.25 = 156A, 175A protection
112.5 kVA312A x 1.25 = 390A, 400A protection
Author’s Comment: Secondary overcurrent protection is not required for the transformer, but overcurrent protection is required for the lighting and appliance branch-circuit panelboard. Since secondary overcurrent protection is provided in this example, the primary protection device can be sized up to 250% of the primary current rating in accordance with Table 450.3(B) and 240.21(B)(3).

Secondary Conductor Size: Secondary conductors must have an ampacity rating not less than the rating of the overcurrent protective device at the termination of the conductors in accordance with Table 310.16 based on 75�C terminal rating [110.14(C)]. This means that the next size up rule contained in 240.4(B) does not apply.

45 kVA125A x 1.25 = 156A, 175A protection = 2/0 AWG, rated 175A
112.5 kVA312A x 1.25 = 390A, 400A protection = 600 kcmil, rated 420A
But…. Where the number of current-carrying conductors in a raceway or cable exceeds three, the allowable ampacity shall be reduced in accordance with Table 310.15(B)(2)(a). For our examples, there are four current-carrying conductors on the secondary [neutral considered current carrying 310.15(B)(4)(c)], therefore the conductor ampacity after adjustment [based on 90�C ampacity [110.14(C)], must be no less than 175A for the 45 kVA transformer and 400A for the 112.5 kVA transformer.

45 kVA3/0 AWG, rated 225A x 0.80 = 180A, greater than 175A protection
112.5 kVA600 kcmil, rated 475A x 0.80 = 380A, therefore we must use the next size larger conductor
112.5 kVA700 kcmil, rated 520A x 0.80 = 416, greater than 400A protection
Step 5. Grounding and Bonding [250.30(A)]. Transformer secondarys that operate at over 50 V [250.20(A) and 250.112(I)] must be bonded to an effective ground-fault current path to ensure that dangerous voltage from ground-faults will not remain [250.2(A)(3)]. In addition, separately derived systems shall be grounded to the earth to stabilize the system voltage to earth during normal operation [250.4(A)(1)].

250.30(A)(1) Bonding – Effective Fault Current Path. To provide the low impedance path necessary to clear a ground-fault on a separately derived system, the metal parts of electrical equipment must be bonded together (equipment grounding conductor) and connected to the system grounded conductor (X0 Terminal). The bonding jumper used for this purpose must be sized in accordance with Table 250.66, based on the total area of the largest ungrounded (hot) conductor as follows:

45 kVA Secondary Conductors 3/0 AWG = 4 AWG Bonding Jumper
112.5 kVA Secondary Conductors 700 kcmil = 2/0 AWG Bonding Jumper

The neutral-to-case bond can be made at the source of a separately derived system or at the first system disconnecting means or overcurrent device.

When there is no secondary side disconnecting means or overcurrent device(s), the neutral-to-case bond is made at the source of the separately derived system.

DANGER: Failure to provide a low impedance ground-fault path (no neutral-to-case bond) for the separately derived system can create a condition where a ground-fault (line-to-case fault) cannot be removed. The result is that all electrical metal parts, as well as the building structure, will remain energized with dangerous line voltage if a ground-fault (line-to-case fault) occurs.

CAUTION: The neutral-to-case connection for a separately derived system cannot be made at more than one location if so doing would result in a parallel path for neutral current flow [250.30(A)(1) Exception No. 1]. Such multiple neutral current return paths to the grounded (neutral) conductor of the power supply can create fire and shock hazards, as well as power quality problems from electromagnetic interference. See 250.6 and 250.142(A).

250.30(A)(2)(a) Grounding Single Separately Derived Systems. A grounding electrode conductor for a single separately derived system must be sized in accordance with 250.66, based on the total area of the largest secondary ungrounded (hot) conductor. This conductor shall connect the grounded conductor of the derived system to the grounding electrode as specified in 250.30(A)(4). The grounding electrode conductor must terminate at the same point on the separately derived system where the neutral-to-case bonding jumper is installed [250.30(A)(1)].

45 kVA Secondary Conductors 3/0 AWG = 4 AWG Bonding Jumper
112.5 kVA Secondary Conductors 700 kcmil = 2/0 AWG Bonding Jumper

Author’s Comment: The grounding electrode conductor must connect directly to the grounded neutral conductor terminal. It cannot be terminated to the case of the transformer.

250.30(A)(4) Grounding Electrode. The grounding electrode conductor must terminate to a grounding electrode that is located as close as practicable to, and preferably in the same area as the nearest:

(1) Effectively grounded metal member of the structure.
(2) Effectively grounded metal water pipe, within 5 ft from the point of entrance into the building.

Exception: The grounding electrode conductor can terminate at any point on the water pipe system for industrial and commercial buildings where (1) conditions of maintenance and supervision ensure that only qualified persons service the installation, and (2) the entire length of the interior metal water pipe that is being used for the grounding electrode is exposed.

(3) Where an effectively grounded metal member of the building structure or an effectively grounded metal water pipe is not available, one of the following electrodes must be used:

  • An electrode encased by at least 2 in. of concrete, located within and near the bottom of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 20 ft of one or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than � in. in diameter, or consisting of at least 20 ft of bare copper conductor not smaller than 4 AWG. See 250.52(A)(3).
  • A ground ring encircling the building or structure, in direct contact with the earth, consisting of at least 20 ft of bare copper conductor not smaller than 2 AWG. See 250.52(A)(4).
  • Rod or pipe electrodes not less than 8 ft in length. See 250.52(A)(5).
  • A plate electrode that exposes not less than 2 sq ft of surface to exterior soil. See 250.52(A)(6)
  • Other local metal underground systems or structures such as piping systems and underground tanks. See 250.52(A)(7).

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Csst and Bonding Requirements- by Dennis Alwon

There are 2 types of csst made by many different manufacturers.
Standard or Original csst and
Counter Strike (omegaflex brand)

Since counterstrike does not require extra bonding we will talk about the original csst. By the way, the original csst was yellow and the counterstrike has a black jacket.

CSST (corrugated stainless steel tubing) is a flexible gas line material that has been used in many homes around the world. The idea behind csst was a noble one. It was going to cut job labor in half while insuring flexibility in the gas line. This flexibility was a great idea for areas where earthquakes or other phenomenon caused building to move.

The problem became apparent after many homes were nearly destroyed by fire. If lightning would strike a house and travel along a grounded object and if the csst was nearby it would burn a hole in the csst causing a torch effect which would set the home ablaze.

What we learned from this is that csst must be bonded. The fire code also requires bonding of csst.

How do we bond csst?

There are 2 ways we can bond csst
One can attach our copper wire to the brass fitting, not to the csst itself, or


156692



One can attach to the black iron pipe where the csst originates. If you attach to the black iron pipe then you must sand the black paint off the pipe so that there is good contact to the pipe


156693



In either case you must use a listed pipe clamp for csst (UL467). Any pipe clamp with a UL 467 marking is suitable for bonding csst or gas pipe.

One other note is that the fire code only requires a #6 bare copper wire run back to the service and connected to the grounding electrode conductor or into the panel. Some manufacturers state that bonding should be sized based on the service conductor sizing using T250.66 which is now T250.102(C)(1) for bonding. In larger services the grounding electrode conductor would be larger than #6 however I have not seen that enforced.

Here is the gas code as of 2021

CSST gas piping systems, and gas piping systems containing one or more segments of CSST, shall be electrically continuous and bonded to the electrical service grounding electrode system or, where provided, lightning protection grounding electrode system.
7.12.2.1

The bonding jumper shall connect to a metallic pipe, pipe fitting, or CSST fitting.
7.12.2.2

The bonding jumper shall not be smaller than 6 AWG copper wire or equivalent.
7.12.2.3*

The length of the jumper between the connection to the gas piping system and the grounding electrode system shall not exceed 75 ft (22 m). Any additional grounding electrodes installed to meet this requirement shall be bonded to the electrical service grounding electrode system or, where provided, lightning protection grounding electrode system.
7.12.2.4

Bonding connections shall be in accordance with NFPA 70.
7.12.2.5

Devices used for the bonding connection shall be listed for the application in accordance with UL 467, Grounding and Bonding Equipment.

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Q. Please explain the 10-ft tap rules?

A. Except as permitted by Sec. 240.21(A) through (H), overcurrent devices must be placed at the point where the branch circuit or feeder conductors receive their power. Taps and transformer secondary conductors aren’t permitted to supply another conductor (tapping a tap isn’t permitted) (see Fig. 1).

Ecmweb Com Sites Ecmweb com Files Uploads 2015 02 Nec Code Quandaries 1
Fig. 1. Except as permitted by (A) through (H), overcurrent devices must be placed at the point where the conductors receive their power.


Branch circuit taps are permitted in accordance with Sec. 210.19 [240.21(A)].

Conductors can be tapped to a feeder as specified in Sec. 240.21(B)(1) through (B)(5). The “next size up protection rule” of Sec. 240.4(B) isn’t permitted for tap conductors [240.21(B)] (see Fig. 2).

Ecmweb Com Sites Ecmweb com Files Uploads 2015 02 Nec Code Quandaries 2
Fig. 2. Conductors can be tapped to a feeder as specified in Sec. 240.21(B).


Feeder tap conductors up to 10 ft long are permitted without overcurrent protection at the tap location if the tap conductors comply with the following [240.21(B)(1)]:

The ampacity of the tap conductor must not be less than [240.21(B)(1)(1)]:

a. The calculated load in accordance with Art. 220, and

b. The rating of the overcurrent device supplied by the tap conductors.

Exception: Listed equipment, such as a surge protection device, can have their conductors sized in accordance with the manufacturer’s instructions.

The tap conductors must not extend beyond the equipment they supply [240.21(B)(1)(2)].

The tap conductors are installed in a raceway when they leave the enclosure [240.21(B)(1)(3)].

The tap conductors must have an ampacity not less than 10% of the rating of the overcurrent device that protects the feeder [240.21(B)(1)(4)].

Note: See Sec. 408.36 for the overcurrent protection requirements for panelboards.

Here’s an example problem that helps better explain how to apply these rules.

A 400A breaker protects a set of 500kcmil feeder conductors. There are three taps fed from the 500kcmil feeder that supply disconnects with 200A, 150A, and 30A overcurrent devices. What are the minimum size conductors for each of these taps?

200A disconnect: 3/0 AWG is rated 200A at 75°C, and is greater than 10% of the rating of the overcurrent device (400A).

150A disconnect: 1/0 AWG is rated 150A at 75°C, and is greater than 10% of the rating of the overcurrent device (400A).

30A disconnect: 8 AWG is rated 40A at 60°C. The tap conductors from the 400A feeder to the 30A overcurrent device can’t be less than 40A (10% of the rating of the 400A feeder overcurrent device).

Holt is the owner of Mike Holt Enterprises, Inc. in Leesburg, Fla. He can be reached at www.mikeholt.com.


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