Re: Transformer Neutral Size

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Lightninarrester

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Re: Transformer Neutral Size

Guys,

1500KVA 13.8/480Y/277, how do you size the neutral conductor if the unbalanced load amps are unknown? lets assume full neutral. we will have 2255A secondary FLA, 6x500KCM per phase. Thanks
 
Re: Re: Transformer Neutral Size

How did you come up with 2255 amps? 1500 KVA divided by 480 and divided again by the square root of three gives me 1805 amps. 5 sets of 500 MCM would do that job.

But to answer your question, I would simply use the same conductors (5 sets of 500 MCM) for the neutral. If you wanted to use a smaller conductor, then you need to go about it from the other direction: calculate the load first, as opposed to stating the transformer size first.
 
Re: Re: Transformer Neutral Size

The easiest way is to provide a full size neutral that would also be 6x500KCM.

To be more cost consertive, subtract some of the balanced 3 phase loads from the 2255 Amps. For example, if you know the 2255 includes 500 amps for 3 phase motor loads, you could reduce the neutral to 6x350KMC.

Steve
 
Re: Re: Transformer Neutral Size

The NEC does not require you to size a transformer's secondary conductors on the basis of 125% of the rated secondary current. Rather, it requires, in certain specific circumstances, that you protect the secondary of the transformer at no higher than 125% of the rated secondary current.

What the conductors have to be size for is the calculated load. The 125% factor comes into the load calculation, in that you count 125% of the largest motor and 125% of any continuous loads, before arriving at the final calculated load.

Your selection of 6 sets of 500 MCM is in compliance with code. You may or may not need to go that high. It's your design choice.
 
Re: Re: Transformer Neutral Size

Size neutral for the assumed unbalanced load. We usually size teh neutral at 30% of the phase conductors becasue 90% of our load is 3-phase motors and well balanced panels. We could probably get by with 20% or even 15% but the 30% gives some margin for unusual future loads.
 
Re: Re: Transformer Neutral Size

2002 NEC 220.22 allows 70% D.F. for that over 200A.
Also, "neutral" load is only drawn by loads that are connected to the "neutral". A 3-phase motor does not draw "neutral" load, whereas 277V. lighting does.
 
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