Undersized Service Conductors in a Renovated Restaurant
A restaurant owner renovates a 1960s building, adding a 60A electric range, a 40A water heater, and additional lighting. The existing 200A service has 2/0 AWG copper conductors. The contractor calculates the new load at 185A and insists the existing service is adequate. However, the panel schedule shows the original service was rated for 180A maximum. The local utility requires a load calculation per Canadian Electrical Code. The contractor argues that since the calculated load is below 200A, no upgrade is needed. The electrical inspector questions the ampacity of the existing conductors, which are run through a conduit with three other current-carrying conductors in a 30°C ambient. The contractor did not apply adjustment factors. The owner is concerned about cost and wants to avoid a service upgrade.
1. What code rules apply to determine the true ampacity of the existing service conductors, and what should the contractor have considered?
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The contractor must apply the Canadian Electrical Code rules for conductor ampacity, including correction factors for ambient temperature and adjustment factors for more than three current-carrying conductors in a raceway. The existing 2/0 AWG copper conductors have a base ampacity of 175A at 30°C (from Table 2). However, since there are four current-carrying conductors in the conduit, Table 5C requires an adjustment factor of 0.80, reducing the ampacity to 140A. Additionally, the ambient temperature of 30°C is the base, so no correction for temperature is needed. The contractor incorrectly assumed the base ampacity without adjustments. The calculated load of 185A exceeds the adjusted ampacity of 140A, so the service is inadequate. The contractor must either upgrade the service or reduce the load. The inspector should enforce this. The owner's cost concern does not override code requirements.
2. If the contractor decides to upgrade the service, what minimum conductor size and service rating would be required, assuming the calculated load remains 185A and the same installation conditions?
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To meet the calculated load of 185A, the service conductors must have an ampacity of at least 185A after applying adjustment factors. With four current-carrying conductors in the conduit, the adjustment factor is 0.80. Therefore, the base ampacity must be at least 185 / 0.80 = 231.25A. From Table 2, a 250 kcmil copper conductor has a base ampacity of 255A at 30°C, which after adjustment is 255 * 0.80 = 204A, sufficient. A 4/0 AWG copper conductor has a base ampacity of 230A, which after adjustment is 184A, just below 185A, so it is not sufficient. Thus, 250 kcmil copper is the minimum. The service rating should be at least 200A, as the next standard size. The contractor must also verify voltage drop and ensure the service equipment is rated for the new load.