Sizing Double Neutrals

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W6SJK

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Informal poll: When sizing a double neutral, do YOU size the neutral based on twice the overcurrent protection, or on twice the phase wire ampacity?

For example, with a 100A panelboard feeder using 2AWG phase wires, do you use a 3/0AWG neutral (2x100A=200A @75deg) or do you use a 4/0AWG (2x115A=230A wire ampacity)?

From what I have read, either will be sufficient. Just curious...
 
Re: Sizing Double Neutrals

General, we are responding to an owner, or engineer, and if they say double, we would double the wire. For 100A, you can't parallel #2, so would use 3/0.
 
Re: Sizing Double Neutrals

I consider the idea of "doubling the neutral" to be doubling the cm size of the required grounded conductor, or just simply sizing equal to the ungrounded conductors.

For example, lets say you have a 200A service with two 3/0 ungrounded conductors and say a #1 grounded conductor. #1 = 83690 cm X 2 = 167,380 = 3/0.

If the required neutral is only a #4, #4 = 41740 cm X 2 = 83480 = a #1 conductor.
 
Re: Sizing Double Neutrals

My situation is not a service with reduced neutral. It is a feeder to a panelboard serving computers.

Doubling the cm does not double the ampacity. That's why the ampacity of 500kcmil is not double that of 250kcmil. The conductor is less efficient in radiating the heat as it becomes larger, which is one of the reasons for paralleling smaller conductors.

If it was a 3/0AWG feeder I would be tempted to just run two 3/0 neutrals. But since this is a 100A 2AWG feeder I cannot parallel the neutral.
 
Re: Sizing Double Neutrals

Ah, I missed the context of your question.

Even though the ungrounded conductors are good for up to 115A, the OCD limits the loads to 100A. So, if the neutral load is expected to be up to twice the load due to non-linear harmonics, I would base it on the OCD. So for your example, a conductor good for 200A, or the 3/0.
 
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