60A subpanel service - when does it trip?

Question

I am looking at running new 60A subpanel to my shed. According to what I understand from the electrician, there will be two HOT wires and neutral wire running on 6/3 wire.

My question is, since the sub-panel will be supplied with a 60A @ 240V, will each hot wire allow 60A? Will this allow me to load 120amps @ 120V before it trips?

Further information: I will be running 3 x 20A circuits at 120V from the subpanel. I am expecting to have a ~42A continuous load , (14 amp per circuit). If in the future I wish to add additional circuits, I don't want to have to run another wire.

Answer 1

You have two poles, L1 and L2. You can pull 60A down each of them at the same time. If either one exceeds 60, snap

120V feeds between a pole and neutral, so you can pull 60A@120V off L1, and another 60A@120V off L2.

How does this work? Neutral is in the middle of the 240V, so most power goes through one bank, then through the other. Neutral handles only differential current.

Let's say you load one side with 3 servers at 42A and the other side with four air conditioners at 40A. L1 carries 42A, neutral carries 2A and L2 carries 40A.

Don't forget the air conditioners, how else will you remove 5kw of heat from a shed? If it's computers, a better way is "whole room water cooling" like these guys did, and put the disposal radiator outside. But use an SUV radiator from a junkyard (with its own fans and thermostats), don't dingdong a bunch of PC radiators like they did.

How do you rate this? You start with nameplate VA or amps, or preferably measured VA under actual max load. Do not use watts, use VA.* Divide VA by volts to get practical amps. Continuous loads must then be factored by 125%, so your 14A unit needs 17.5A wire capacity.

Try to balance the loads, it will improve wire transmission losses by a factor of 4.

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* AC power comes in a sine wave, but many loads don't use the whole sine wave in the normal way. Watts is the part you use. VA is the whole sinewave the wires must deliver to you. VA is what makes the wires hot.

Answer 2

The double pole common trip breaker will trip when either pole exceeds 60amps. Double pole breakers are common trip, therefore if one side trips then both poles trip.

So drawing .5 amps @120v on one leg, and 61 amps @120v on the other, is a trip condition--and because it's common trip, both sides will go off.

However you could pull up to 60 amps on both lines, as long as 60 is not exceed on either.

Answer 3

Yes, a 60 amp 240 volt breaker is two 60 A 120 V breakers joined together to guarantee tripping both if either trips. Your sub will be able to handle 60 amps of 240 volt load or 120 amps of 120 volt load, excluding things like the 80% reduction for continuous loads.

Answer 4

If you do indeed need 42-Amps continuous, I would run no less than 100-amps of carrying capacity, #3 or #2 Copper, 1/0 Aluminum. This will actually give you only modest expansion capability for the future. With continuous loading, you need significant buffer. If those branch circuit are going to supply high intensity lighting, the circuit wire should be no smaller than #10.

Answer 5

To know when a 60 amp breaker will trip you need the brand and type to look at the trip curves. Most breakers are inverse time trip, if a direct short the trip less than .1 second when the listed value is reached. Most don't know that a standard breaker will hold 3x its value for up to 1 minute depending on the brand and style. They do not trip as you slowly exceed the stamped value until the inverse time / current values are exceeded. I have certified hundreds of breakers Both residential and industrial there is a lot of misunderstanding on how breakers work but if you look at the trip curves you can see what I am talking about.