PG&E Distribution System and Arc Flash Assessment Letter

Date:

PG&E Representative: Telephone #:707 449-5755

Dear Customer,

PG&E’s distribution system is dynamic and due to the many variables in the distribution system a range of impedances must be provided to assist in your arc flash assessment. The dynamics of the system include:

The distribution circuit that is feeding your transformer. The distribution circuits are continually being reconfigured due to system needs and general maintenance. These changes can lead to a significant change in the system impedance.

The transformer size (including any future replacement size). When a transformer is replaced the size is reassessed based on the expected loading.

Impedance of the transformer (PG&E specifies a minimum and maximum impedance range)

PG&E protective devices on the high side of the service transformer are intended to provide protection to the primary system and should not be relied upon to protect the service panel or customer systems.

PG&E recommends that all electrical work on the main breaker to the service panel should be done in a de-energized condition to eliminate arc flash hazards at this location.

The following data should be used for the system impedance:

Maximum: 100 MVAMinimum: 4.5 MVA

Maximum X/R ratio: 40

PG&E reassesses the transformer load whenever a transformer is replaced. PG&E does NOT notify the customer when a new transformer is installed. Each

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PG&E Distribution System and Arc Flash AssessmentAttachment 1 - Impedance Ranges for PG&E Transformers

table below lists the full range of transformer sizes for each transformer type. At the time of replacement, any size listed in the table could be installed.

For Example: Referring to Table 3a the possible replacement transformer sizes for a 3-phase 300kVA 208Y/120V pad-mount transformer range from 75 kVA to 1000 kVA.

Table 1: Single-Phase Pad-Mount Transformer Impedance Range

Transformer kVA

Secondary Voltage120/240

Minimum Maximum25 and 50 1.4% 2.4%

100 1.8% 3.0%167-500 3.0% 5.0%

Table 2: Duplex Pad-Mount Transformer Impedance Range

Transformer kVASecondary Voltage

120/240Minimum Maximum

25-10 and 50-10 1.4% 2.4%100-25 and 100-50 1.8% 3.0%

Table 3: Three-Phase Pad-Mount Transformer Impedance Rangea)

Transformer kVA

Secondary Voltage208Y/120

Minimum Maximum75 1.4% 3.5%

150 1.9% 4.5%300 2.8% 4.5%

750-1000 5.3% 6.2%

b)

Transformer kVA

Secondary Voltage480Y/277

Minimum Maximum75 1.4% 4.5%

150 1.4% 4.5%300 1.9% 4.5%

750-2500 5.3% 6.2%

Table 4: Single-Phase Subsurface Transformer Impedance Range

Transformer kVA

Secondary Voltage120/240

Minimum Maximum25 and 50 1.4% 2.4%

100 1.8% 3.0%167 and larger 3.0% 5.0%

Attachment 1 - Page 2 of 3

PG&E Distribution System and Arc Flash AssessmentAttachment 1 - Impedance Ranges for PG&E Transformers

Table 5: Three-Phase Subsurface Transformer Impedance Rangea)

Transformer kVA

Secondary Voltage208Y/120

Minimum Maximum150 1.9% 4.5%300 2.8% 4.5%

750-1000 5.3% 6.2%

b)

Transformer kVA

Secondary Voltage480Y/277

Minimum Maximum150 1.4% 4.5%300 1.9% 4.5%

750-1000 5.3% 6.2%

Table 6: Single-Phase Overhead Transformer Impedance Rangea)

Transformer kVA

Secondary Voltage120/240

Minimum Maximum10-75 1.4% 2.4%100 1.8% 3.0%

167-500 2.0% 5.0%

b)

Transformer kVA

Secondary Voltage240/480

Minimum Maximum10-75 1.4% 2.4%100 1.8% 3.0%

167-500 2.0% 5.0%

Table 7: Three-Phase Overhead Transformer Impedance Rangea)

Transformer kVA

Secondary Voltage208Y/120

Minimum Maximum45 – 112.5 1.4% 3.5%

150 1.9% 4.5%225 2.1% 4.5%300 2.8% 4.5%

b)

Transformer kVA

Secondary Voltage480Y/277

Minimum Maximum75 1.4% 4.5%

150 1.4% 4.5%300 1.4% 4.5%

750-2500 1.9% 4.5%

Attachment 1 - Page 3 of 3