30 Jan 2019

Imagine the common scenario where the home owner selects and supplies the all-electric cooking appliance late in the project. The designer had provided them the physical dimensions allowed for the appliance, which the owners had duly used. However, at the finishing stages of the project the cooking appliance arrives but is found to be a much higher amperage than has been allowed for. If the appliance must be used, then what are the ramifications? Well the supply cable and the MCB may have to be replaced by larger ones capable of handling the higher amperage. In extreme cases it could even mean replacing the electrical main into the house (possibly also affecting the meter and DB).

This scenario is not an everyday occurrence, but according to some of our Masterspec users it has happened. And it is not restricted to cooking appliances, it could be high amperage water heaters, space heaters, air conditioning, etc, or the cumulative effect of a number of these types of appliances.

In this article we will give a background of amps, cables and mains for house designers. It won't get too technical as that can get very complicated for the non-specialist. But it may give you a feel as to when you need to consult an Electrical Engineer etc.

What are AMPS?

The electrical terms you may be familiar with are Volts (V), current (I), resistance (R) and electrical power (P). Current (I) is measured in Amps, resistance (R) is measured in ohms and power (P) is measured in Watts. There are a number of mathematical equations that allow these various elements to be manipulated. But with normal power supply voltage is fixed to approximately 240V.

Roughly speaking, electrical cables have resistance which consumes power which increases the current, which produces heat in the cable. Too much heat without the ability to vary the voltage usually leads to the need for bigger (or more) cables to allow increase current and reduce heat.

Relax, that’s the hard bit over!  What it basically means is, a high wattage high amperage appliance may need a bigger cable to squeeze the current through and deal with the heat.

Cable size

The allowable maximum temperature for cables depends on the insulation. Standard thermoplastic insulation (TPS) has a maximum temperature of 75°C while high temperature insulation (like X-90, R-HF-90 etc) can get up to 90°C or 105°C. High temperatures due to Amperage can be made worse by reducing the ability of the cable to lose heat, by enclosing it (particularly in insulation) or exposing it to a high ambient temperature (e.g. a hot roof space). Bundling (bunching) cables also means the cables can add heat to each other.

The Standard AS/NZS 3000:2007 (or 2018), Electrical Installations (known as the Australian/New Zealand Wiring Rules), Table C5 (or C6 in 2018) for example gives the relationship between cable sizes and amperages relative to enclosure situations (for V-90 cable). AS/NZS 3008.1.2 Electrical Installations - Selection Of Cables - Cables For Alternating Voltages Up To And Including 0.6/1 Kv - Typical New Zealand Installation Conditions, provides more examples and detail on cable sizing.

So for example Masterspec Electrical Standard says that the minimum cable sizes for  an average Range (cooking appliance) circuit should be:

  • Single phase: 6mm² high temperature cable on 32 amp MCBs

From AS/NZS 3000:2007 Table 5, a 6mm² cable (V-90) against the back of the lining in an insulated wall would handle 32 amps. AS/NZS 3008.1.2 in its tables for higher temperature cables would allow the appropriate cable to be in the middle of the insulation. If the appliance needed more than 32A a larger cable may be required or where the cable is run may have to be changed. Conversely a lesser amperage may allow a smaller cable, or the type of cable and where it is run may be less critical.


Most houses (and home appliances) use Single Phase supply and distribution with a minimum rating of 60 amps on supply. Heavy commercial and industrial users (and some equipment) often use 3 Phase (or 2 Phase) supply still at 60 amps per phase and single and/or 3 Phase distribution. Except for cable structure and size, the differences between the two are too complex for this article, other than due to the efficiency of 3 Phase you get 3 times the power for 1.5 times more cable.

A few houses with very high demand (over 60 amps) need 2 or 3 Phase supply, usually with (but not limited to) single Phase distribution.

Just over 60 amps is the normal limit for a single phase.

High demand appliances

High demand electrical appliance could be; large cooking appliances, large storage hot water heaters, high volume instantaneous water heaters, electric vehicle fast chargers, large central air-conditioning systems, etc. Although not common, an owner may want 3 Phase appliances or other equipment (e.g. commercial grade workshop tools).

The maximum demand of an appliance on any phase at any one time is complex to calculate, it would have to be obtained from the manufacturer, which is not always easy. Cooking appliance are made slightly easier by AS/NZS 3000:2007 Table 4 (Table 5 2018), which compares the cooking appliance full-load energy rating (Watts) to the maximum Amps demand.

For instance:

  • 1000W to 1300W = 32A
  • Over 1300W = 40A

A normal house is generally not an issue when it comes to total demand and the main supply, but for a high demand house calculating the total demand for the whole house is very complex (it is not a simple addition) and should only be attempted by specialists. Potentially a high demand house may need more than a Single Phase mains supply.

Project requirements

Ensure that you know if selected appliances are low or high demand, and if the client is to supply them, ensure they know the limitations.  If in doubt seek professional advice.

Masterspec Sections

Masterspec has added brief guidance, along the lines of this article, to the Electrical Standard and Basic sections to act as a reminder.