Comment on the code : Power drop of tension in circuits

Wednesday February 8, 2012

In all the circuits we have to consider the drop of tension due to the impedance of the circuit.

Électricité Québec , Volume 45, Numéro 2 : February 1998
Pierre Liberatore , engeneer and director of the technical services (CEMQ)


In establishing criteria, we insure that the tension used will be acceptable and that the electricians equipment will be fully operational. Otherwise the tension used could be lower than acceptable level which will result by a decrease in heating systems and lighting fixtures motors efficiency.

Quebec`s electrical code has established specific criterias under article 8-102 - 'power drop tension' - , in order to provide correctly used tension between prescribe value. Consequently paragraph 1 of this article requires that the drop of tension of an installation will be based on calculated charge of the arterial or circuit diversion and that the drop does not exceed 5% from the supplier to the consumer (or equivalent) to the point of consumption and do not go over 3% in arterial or derived.

Paragraph 2 of article 8-102, mentioned that the demanding charge on a diversion circuit would be the joint charge, if known. If not, we have to choose the following lower value which is 80% face value of safety device against overload, which 80% of safety deviced against diversion circuits hight intensity.

It requires that the calculation of the power drop tension would be based on calculated charge (section 8). In case of diversion, the demanding charge calculated would be the linked charge. If its unknown, we establish the demand at 80% of safety device value for circuit protection.

According to article 8-102 the percentage allowed for drop tension is based on 2 values: 5% of the whole system of consuming until using point and 3% for artery or derivation circuit which is 240 volts power drop tension of 12 volts (240x5%) would be acceptable for the whole system.

The maximum percentage for all derivation is 3% , the power drop tension allowed in our example would be 7,2 volts (240x3%) if that value is use at the derivation level, the maximum power drop for the rest of the installation connections and arteria would be 4,8 volts (12-7,2). Taking action, we have to choose the conductor consequently and at the lower cost.

In order to make the calculation easier for power drop tension find enclosed two tables on annexe 'D' of the Quebec Electrical Code : table D3 for low tension circuits and D4 for very low tension circuits.

Before using them we recommend that you read the examples and detailed notes at the bottom of the table.

'D3' is calculated for the size of the copper wire 18 to 4/0 AWG and provide approximate distance in meters for each size.

Following a conductive layout for a power drop tension of 1% for a nominal tension of 120 volts for two copper conductor circuits. The inductive reactance was not included considering that it's depended upon size, space and the distance specified have to be corrected according to power drop tension planned, and nominal used tension.

In addition a table of correction factors of distance is included according to nominal temperature of the used conductor and the allowable power charge, percentage established under article 4-004 corresponding with the table and chart. The charts can also be used for aluminum conductor for each length that has an equivalent resistance of copper conductors of which some sections are inferior to both sizes.

Thanks to mister Liberatore and madam Jocelyne Dulude for their valuable collaboration, agreeing to let us publish a online article from the Électricité Québec magazine.

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